Sodium Hyperhydration Improves Performance With No Change in Thermal and Cardiovascular Strain in Female Cyclists Exercising in the Heat Across the Menstrual Cycle

Click name to view affiliation

Lilia Convit Centre for Sport Research, School of Exercise and Nutrition Sciences, Deakin University, Geelong, VIC, Australia

Search for other papers by Lilia Convit in
Current site
Google Scholar
PubMed
Close
https://orcid.org/0000-0001-5656-9106
,
Liliana Orellana Biostatistics Unit, Deakin University, Geelong, VIC, Australia

Search for other papers by Liliana Orellana in
Current site
Google Scholar
PubMed
Close
https://orcid.org/0000-0003-3736-4337
,
Julien D. Périard Research Institute for Sport and Exercise, University of Canberra, Bruce, ACT, Australia

Search for other papers by Julien D. Périard in
Current site
Google Scholar
PubMed
Close
https://orcid.org/0000-0002-6266-4246
,
Amelia J. Carr Centre for Sport Research, School of Exercise and Nutrition Sciences, Deakin University, Geelong, VIC, Australia

Search for other papers by Amelia J. Carr in
Current site
Google Scholar
PubMed
Close
https://orcid.org/0000-0003-3855-2540
,
Stuart Warmington Institute for Physical Activity and Nutrition, School of Exercise and Nutrition Sciences, Deakin University, Geelong, VIC, Australia

Search for other papers by Stuart Warmington in
Current site
Google Scholar
PubMed
Close
https://orcid.org/0000-0002-2414-7539
,
Mégane Beaugeois School of Life and Environmental Sciences, Deakin University, Burwood, VIC, Australia

Search for other papers by Mégane Beaugeois in
Current site
Google Scholar
PubMed
Close
https://orcid.org/0000-0002-5030-4059
,
Anju Abraham School of Life and Environmental Sciences, Deakin University, Burwood, VIC, Australia

Search for other papers by Anju Abraham in
Current site
Google Scholar
PubMed
Close
, and
Rhiannon M.J. Snipe Centre for Sport Research, School of Exercise and Nutrition Sciences, Deakin University, Geelong, VIC, Australia

Search for other papers by Rhiannon M.J. Snipe in
Current site
Google Scholar
PubMed
Close
https://orcid.org/0000-0002-3754-6782 *
Free access

This study investigated the effect of sodium hyperhydration on thermal and cardiovascular strain and exercise performance in unacclimatized endurance-trained females exercising in the heat and whether effects differ between menstrual cycle (MC) Phase 1 (low estrogen and progesterone) and MC Phase 4 (moderate estrogen and high progesterone). Twelve female cyclists/triathletes completed four trials in a randomized, double-blinded, crossover design. Participants consumed 30 ml·kg−1 fat-free mass fluid with either sodium chloride (7.5 g·L1) or placebo (sucrose) 2 hr prior to 75 min of steady-state cycling (60% V˙O2peak) followed by a 200-kJ time trial (TT) in 34 °C and 60% relative humidity, with both interventions completed during MC Phase 1 and Phase 4. Rectal temperature and heart rate were measured at baseline, every 5 min during steady state, every 50 kJ of TT, and TT completion. Body mass was measured every 30 min preexercise and pre and post steady state and TT to assess hydration status. Linear mixed models were fitted to estimate intervention and MC phase effect. There were no significant sodium hyperhydration or MC phase effects on rectal temperature or heart rate (p > .05). Body mass increased with sodium versus placebo (0.38 [0.02, 0.74] kg; p = .04), with a greater increase in MC Phase 4 (0.69 [0.17, 1.2] kg; p < .001). TT performance improved with sodium versus placebo (−1.55 [−2.46, −0.64] min; p = .001), with a greater improvement in MC Phase 4 (−1.85 [−3.16, −0.55] min; p = .005). Sodium hyperhydration is a promising heat mitigation strategy for females undertaking prolonged exercise in the heat, especially during MC Phase 4 and when fluid access is limited.

Prolonged exercise in hot and/or humid (>35 °C and ∼80% relative humidity [RH]; Havenith et al., 1995, 1998) environments increases thermal (e.g., core and skin temperature) and cardiovascular (e.g., heart rate [HR]) strain, which can impair endurance capacity and performance (Périard et al., 2021). The elevation in thermal and cardiovascular strain can be exacerbated by dehydration in response to excessive body water loss through sweating if fluid replacement is not adequate (Casa, 1999). Hyperhydration with an osmotically active ingredient (e.g., sodium) prior to exercise can increase total body water above euhydration (Goulet et al., 2018) and delay the onset of dehydration (Jardine et al., 2023). The increased total body water with hyperhydration can decrease cardiovascular and thermal strain during exercise in the heat (Goulet et al., 2018; Jardine et al., 2023). Hyperhydration can also improve endurance capacity by 1.2%–21.6% in cycling and running time to exhaustion (TTE) tests, respectively (Greenleaf et al., 1997; Sims, van Vliet, et al., 2007), and performance by 7.8% (Coles & Luetkemeier, 2005) to 13.0% (Morris et al., 2015) in cycling time trials (TT) in hot conditions.

The majority of hyperhydration research has been performed in males and may not directly translate to females due to lower maximal sweat rates in females in uncompensable ambient conditions, which may reduce heat dissipation (Gagnon & Kenny, 2012; Yanovich et al., 2020). To date, only one study (Sims, Rehrer, et al., 2007) has investigated sodium hyperhydration (7.5 g·L−1 of sodium) in female athletes (high hormone midluteal phase of menstrual cycle [MC Phase 4] and active triphasic oral contraceptive pill), demonstrating improved fluid balance, attenuated rate of rise in core temperature during cycling (70% V˙O2max in 32 °C and 50% RH), and increased TTE (∼21%) compared with hyperhydration with low sodium (0.23 g·L−1 of sodium). The benefits observed in this study may have been enhanced due to the thermogenic effect of progesterone (i.e., the thermoregulatory set point is increased; Janse de Jonge, 2003), which elevates core temperature by ∼0.2–0.3 °C at rest (Janse de Jonge et al., 2012; Kolka & Stephenson, 1997; Pivarnik et al., 1992) and ∼0.2–0.7 °C during prolonged endurance exercise. This thermogenic effect can impair TTE in warm–hot conditions (Hashimoto et al., 2016; Janse de Jonge et al., 2012; Kolka & Stephenson, 1997; Pivarnik et al., 1992) when compared with the low-hormone early follicular phase of the MC (MC Phase 1; Elliott-Sale et al., 2021). Estrogen is known to promote fluid retention by increasing sodium reabsorption in the kidneys, whereas progesterone has a diuretic effect that encourages sodium and water excretion, collectively regulating body fluid balance throughout the MC (Stachenfeld, 2008). Furthermore, altered renal function and electrolyte balance with differing hormone profiles across the MC may influence hydration status (e.g., plasma volume and extracellular fluid volume; Stachenfeld, 2008) and the effectiveness of sodium hyperhydration as a heat mitigation strategy for female athletes (Sims, Rehrer, et al., 2007). However, research comparing the effects of sodium hyperhydration across different phases of the MC is lacking. Moreover, it is currently unknown whether sodium hyperhydration can improve prolonged self-paced exercise performance of females exercising in the heat.

Therefore, this study aimed to determine the effect of sodium hyperhydration on thermal and cardiovascular strain and exercise performance in unacclimatized endurance-trained females exercising in the heat and whether effects differ between MC Phase 1 (low estrogen and progesterone) and MC Phase 4 (moderate estrogen and high progesterone).

Methods

Participants and Recruitment

Thirteen endurance-trained female cyclists and triathletes were recruited via social media and sporting organizations, but only 12 (n = 9 Tier 2 and n = 3 Tier 3 athletic caliber; McKay et al., 2021) finished the study and were included in the analysis. Participants completed an online consent form and screening questionnaire in REDCap (version 14.0.29, Vanderbilt University) and were required to (a) have a cycling history of ≥2 years; (b) cycle >3 times per week (>5 hr per week; Decroix et al., 2016); (c) be healthy (e.g., free of asthma, diabetes, cardiovascular, renal, and gastrointestinal conditions, etc.); and (d) have a regular natural MC (21–35 days in length; Elliott-Sale et al., 2021). Participants were excluded if they had any heat exposure (temperatures > 25 °C) within the previous 2 months, were pregnant or breastfeeding, or had recent cessation (previous 6 months) of hormonal contraceptives. Ethical approval for this study was granted by the Deakin University Human Research Ethics Committee (2022-033).

Experimental Design and Procedures

Participants completed one familiarization session and four experimental sessions in a double-blinded (for the supplement but not the MC phase), quasi-balanced, crossover, random order factorial design during the cooler months in Melbourne, Victoria (April–October ∼6–16 °C), avoiding natural heat acclimatization. The four sessions consisted of hyperhydration with sodium or placebo in MC Phase 1 (3–6 days after onset of menses) and MC Phase 4 (7–9 days after positive urinary luteinizing hormone test; Elliott-Sale et al., 2021). MC phases were identified by calendar-based counting and confirmed via serum [17-β-oestradiol] and [progesterone]. Progesterone <16 nmol·L−1 (Elliott-Sale et al., 2021) was classified as potential luteal phase deficiency (LPD) in MC Phase 4 due to inability to clinically confirm LPD, defined as a short luteal phase (<9–10 days; Mesen & Young, 2015; Schliep et al., 2014) and [progesterone] <16 nmol·L−1 over several days (∼3 days; Elliott-Sale et al., 2021; Schliep et al., 2014). All participants (n = 12) were included in the main analysis, with a sensitivity analysis performed in the no-LPD subgroup (n = 8) to exclude participants with potential LPD (n = 4) during MC Phase 4 and identify whether effects differed based on [progesterone].

Familiarization Session

Participants’ height (in meters; Holtain Limited), body mass (BM in kilograms; TANITA), body fat (in percentage), and fat-free mass (lean mass + bone mineral content; in kilograms; Lunar iDXA, GE Healthcare) were measured. Fat-free mass was used to determine fluid/sodium dosage. Participants completed a V˙O2peak test involving a progressive incremental cycling protocol, starting at 100, 125, and 150 W for 5 min each (self-selected cadence >90 rpm), followed by 25-W increments each minute until exhaustion (∼21 °C; ∼40% RH; Tanner & Gore, 2012) on a LODE bike (Excalibur Sport) using a calibrated Moxus Modular Metabolic cart (AEI Technologies, Inc.). The criteria for establishing V˙O2peak were a plateau in V˙O2 with a workload increase, a respiratory exchange ratio >1.10, and rating of perceived exertion (RPE) >19 at voluntary termination of the test (exhaustion). Power at 60% V˙O2peak for the steady-state (SS) component of the experimental session (Tanner & Gore, 2012) was estimated using the V-slope method (Schneider et al., 1993). Participants then completed a 200-kJ TT and all surveys and scales for perceptual responses to familiarize them with the protocol and increase test reliability for TT exercise in the heat (Che Jusoh et al., 2015).

Experimental Sessions

Participants were required to abstain from alcohol, high-sodium food and fluids, and strenuous exercise 24 hr before each session. A standardized frozen meal was provided for participants to consume the night before each session (2,236 kJ, 13 g protein, 15 g total fats, 60 g carbohydrates, 883 mg sodium). A food and exercise diary using a smartphone app (Easy Diet Diary, Xyris Software Pty Ltd.) was completed 24 hr before the first session and confirmed on testing day by visual inspection. Food, fluid intake, and exercise were replicated for the subsequent sessions. Dietary analysis software (FoodWorks Professional, v10.0, Xyris Software) and a post hoc one-way analysis of variance were used to determine compliance across sessions (fluid [in milliliters; p = .953], energy [in kilojoules; p = .843], carbohydrate [in grams; p = .920], protein [in grams; p = .700], total fat [in grams; p = .960], and sodium [in milligrams; p = .965]).

Participants attended the laboratory rested and overnight fasted (∼10 hr) at the same time of day to minimize diurnal variations in core temperature and hormonal profiles. A standardized breakfast with 1 g·kg BM−1 of carbohydrate (∼1,857 kJ, 11 g protein, 13 g total fats, 66 g carbohydrates, and 306 mg sodium) and 250 ml of water was provided, and habitual caffeine drinkers consumed a standardized coffee sachet (∼50 mg caffeine) with the water to avoid withdrawal symptoms (Pickering & Kiely, 2019). The hyperhydration protocol at rest commenced 2 hr before exercise, consisting of 30 ml·kg−1 fat-free mass of water with regular (sucrose-containing) cordial (∼17%) as per manufacturer’s instructions (Cottee’s Cordials, Cadbury Schweppes) with either 7.5 g·L−1 of sodium chloride (sodium) in gelatine pills (Melbourne Food Depot) or placebo (sucrose), with matched number of pills and sucrose-free cordial (same flavor) to match carbohydrate intake across the protocols (0.1 g protein, 0 g total fats, 10.2 g carbohydrates, and 25.5 mg sodium; Figure 1). The hyperhydration solution and pills were divided equally into four 20-min time points for consumption within 5 min.

Figure 1
Figure 1

—Schematic of the experimental sessions. FFM = fat-free mass; RH = relative humidity; TT = time trial; SS = steady state; HR = heart rate; BM = body mass.

Citation: International Journal of Sport Nutrition and Exercise Metabolism 35, 2; 10.1123/ijsnem.2024-0125

Blood samples (∼16 ml at baseline and 12 ml at other time points) were collected with an aseptic cannula inserted in the antecubital vein (McFarlane Medical). One milliliter of blood was drawn into a safePICO syringe (Radiometer) and analyzed in duplicate for blood glucose using an ABL800Flex blood gas analyzer (radiometer). The residual blood was centrifuged (3,600 rpm, 4 °C, 10 min, and serum [6 ml] was left to clot for 30 min), and 50 μl of plasma was analyzed in duplicate via freezing point depression (Osmometer K-7400S, Knauer) for plasma osmolality (POsm). The remaining plasma and serum were stored at −80 °C until analysis of [17-β-oestradiol] and [progesterone] using ELISA kits (Abcam) as per manufacturer’s instructions. Nude BM was measured every 30-min preexercise and pre and post SS and TT using calibrated scales to the nearest 0.1 kg (A&D Weighing) to estimate change in hydration status (Figure 1). Rectal temperature (Tre) was measured using a rectal probe (Mon-a-therm Probe 400TM, Covidien) self-inserted 12 cm beyond the anal sphincter connected to a data logger (Squirrel SQ2020, Grant, Instruments Ltd.), and HR was measured using a chest monitor (Polar). Skin temperature was measured using four wireless iButtons fixed on the upper arm (biceps brachii), calf (gastrocnemius lateral head), thigh (rectus femoris), and chest (pectoralis major; Thermochron iButton, Thermodata) to calculate mean skin temperature (Tsk; Ramanathan, 1964). Gastrointestinal symptoms (GIS) and thirst were measured using an 11-point Likert rating scale (0–10; Gaskell et al., 2019) at baseline, every 30 min during preexercise hyperhydration, every 10 min during SS, and at the end of the TT. RPE (Borg, 1998) and thermal sensation (TS; Young et al., 1987) were measured at baseline, every 5 min during SS, and every 50 kJ during the TT. The exercise protocol consisted of 75-min SS cycling at 60% V˙O2peak power output, with no additional fluid ingested. A 10-min period was allocated to obtain blood samples and a nude BM measurement and for participants to consume 324 ± 32 ml of temperate water (equivalent to 7 ml·kg−1 fat-free mass). This was followed by a 200-kJ TT in an environmental chamber set to 34 °C and 60% RH (Sims, Rehrer, et al., 2007) with an airflow of 4.5 m·s−1 (Kestrel Instruments). No external encouragement was provided, only feedback for time during the first 75 min and a countdown of work performed every 50 kJ during the TT.

Statistical Analysis

A sample size of 12 participants was required to achieve 80% power to detect a mean difference in the rate of rise in Tre of 0.4 °C with an SD of 0.2 for both groups (α = .05, two-sided test, 2 × 2 crossover design), calculated from a previous study in females (Sims, Rehrer, et al., 2007; PASS software, version 16, NCSS, LLC.). Physiological and exercise performance outcomes and their changes from baseline (noted as Δ) were analyzed using linear mixed models with participants as a random effect to account for repeated measures (Gelman, 2007). For outcomes with one observation per session, the linear mixed models included “intervention,” “phase,” and their interaction as fixed effects. From these models, we reported the marginal means for intervention and placebo (a) overall (both MC phases) and (b) within MC phases. Longitudinal outcomes were analyzed considering two time periods, SS (i.e., including preexercise hyperhydration [T-120 to T0 minutes] and submaximal exercise [T0 to T+75 min]) and TT (i.e., T0kJ to T+200 kJ of work completed) due to different measurement units (minutes and kilojoules). The linear mixed models included “intervention,” “phase,” and “time” (categorical) and all interactions as fixed effects. From these models, we reported the marginal means for intervention and placebo averaged across the time period (a) overall (both MC phases), (b) within the MC phases, and (c) plots of the estimated means across time for each combination of “intervention” and “phase” (Stata/SE, version 18, StataCorp). Total GIS is reported as the accumulative total of all symptoms (Gaskell et al., 2019) during preexercise, SS, and TT. Intervention and phase differences in total GIS and thirst were determined via a Friedman test due to data being not normally distributed (SPSS Statistics, version 26). A Wilcoxon signed-rank test post hoc analysis was conducted for total GIS and thirst and a Bonferroni correction applied, with statistical significance defined as p < .017. All results are reported as estimates and 95% confidence interval, except for participants’ characteristics, which are reported as mean ± SD, and GIS and thirst, which are reported as median and interquartile range due to skewed distribution. Statistical significance is defined as p ≤ .05 unless otherwise specified. A sensitivity analysis for all outcomes was repeated as described earlier excluding participants (n = 4) with potential LPD.

Results

Participants

The exercise sessions were completed on MC Day 5 ± 1 for Phase 1 and Day 22 ± 3 for Phase 4. Eight of the 12 participants had progesterone concentrations >16 nmol·L−1 (no-LPD). There were no significant differences in participant characteristics between no-LPD and LPD participants except for progesterone concentrations (Table 1).

Table 1

Participants’ Characteristics and Hormonal Concentrations Compared Between No Luteal Phase Deficient and Potential Luteal Phase Deficient Participants

Baseline characteristicsaAll (N = 12)No-LPD (n = 8)LPD (n = 4)
Mean ± SDMean ± SDMean ± SD
Age (years)33 ± 535 ± 531 ± 1
Height (cm)165 ± 7167 ± 7160 ± 1
BM (kg)62.3 ± 6.361.7 ± 6.663.5 ± 6.1
Fat-free mass (kg)46.3 ± 4.446.2 ± 4.946.6 ± 4.0
Body fat (%)27.2 ± 3.927.1 ± 4.827.3 ± 1.2
BSA (m2)6.9 ± 0.56.9 ± 0.56.8 ± 0.5
V˙O2peak (ml·kg−1 BM·min−1)53.5 ± 7.856.6 ± 5.647.2 ± 8.7
Hormonal concentrationsbMean [95% CI]Mean [95% CI]Mean [95% CI]
Progesterone (nmol·L−1)
 Phase 14.4 [3.3, 5.6]5.3 [4.2, 6.4]2.7 [1.1, 4.3]*
 Phase 454.9 [38.1, 71.6]73.6 [63.4, 83.9]17.5 [3.5, 31.5]**
Estradiol (pg·ml−1)
 Phase 120.6 [15.1, 26.0]20.7 [14.1, 27.4]20.2 [10.8, 29.5]
 Phase 469.3 [50.1, 88.6]62.7 [39.7, 85.7]81.9 [50.4, 113.4]

Note. No-LPD = no luteal phase deficiency group; LPD = potential luteal phase deficiency subgroup (excluded from the sensitivity analysis); BM = body mass; BSA = body surface area; CI = confidence interval.

aCompared with a T test. bOvarian hormones concentrations compared with a linear mixed model with participant as a random effect.

*p < .05 and **p < .001 compared with the no-LPD subgroup.

Steady State

The interaction Intervention × MC phase × Time was not significant for all outcomes during SS. Therefore, the intervention effect for all participants, the intervention effect within each MC phase, and the p value for the Intervention × Phase interaction are reported (Table 2).

Table 2

Physiological Outcomes at Baseline and in Response to a 2-hr Hyperhydration Intervention With Sodium and Placebo and 75-min SS Cycling Across Phases 1 and 4 of the MC

  Baseline of SSSS
SodiumPlaceboDifference

p
Interaction

p
SodiumPlaceboDifference

p
Interaction

p
Hydration outcomes
 BM (kg)Overall62.53 [59.30, 65.76]62.38 [59.15, 65.61].44562.86 [59.63, 66.09]62.49 [59.26, 65.72].040
Phase 162.38 [59.14, 65.62]62.52 [59.28, 65.76].617.09462.72 [59.48, 65.96]62.65 [59.41, 65.89].787.094
Phase 462.68 [59.44, 65.92]62.25 [59.00, 65.49].11163.01 [59.77, 66.25]62.32 [59.08, 65.56].009
  POsm (mOsm·kgH2O−1)Overall289 [286, 292]289 [286, 292].959293 [291, 295]289 [286, 291]<.001
Phase 1288 [285, 292]292 [288, 295].233.571295 [292, 297]290 [288, 293].020.571
Phase 4290 [286, 294]286 [282, 290].198292 [289, 295]287 [284, 289].002
 Fluid retention (ml)Overall616.1 [443.5, 788.8]109.2 [−65.4, 283.7]<.001
Phase 1568.3 [362.7, 774]153.1 [−52.6, 358.8]<.001.252
Phase 4666.0 [460.3, 871.7]63.3 [−149.0, 275.7]<.001
 Ingested fluid retained (%)Overall44.4 [31.8, 57.0]7.6 [−5.1, 20.4]<.001
Phase 140.5 [25.5, 55.6]10.8 [−4.2, 25.9]<.001.230
Phase 448.5 [33.4, 63.5]4.3 [−11.2, 19.9]<.001
 Sweat rate (L·hr−1)Overall0.6 [0.5, 0.7]0.7 [0.6, 0.8].001
Phase 1

0.6 [0.5, 0.7]0.7 [0.6, 0.8].003.457
Phase 40.6 [0.6, 0.7]0.7 [0.6, 0.8].062
Thermal strain outcomes
Tre (°C)Overall36.94 [36.75, 37.13]36.90 [36.71, 37.10].62037.88 [37.70, 38.06]37.87 [37.69, 38.06].953
Phase 136.90 [36.68, 37.11]36.82 [36.60, 37.03].415.91437.84 [37.65, 38.03]37.84 [37.65, 38.03].972.110
Phase 436.98 [36.77, 37.20]36.99 [36.77, 37.22].91037.92 [37.73, 38.11]37.91 [37.71, 38.10].907
Tsk (°C)Overall34.23 [34.08, 34.38]34.02 [33.86, 34.17].02435.50 [35.39, 35.61]35.30 [35.19, 35.42]<.001
Phase 134.30 [34.10, 34.50]34.11 [33.90, 34.32].151.59835.51 [35.37, 35.64]35.28 [35.14, 35.42].004.598
Phase 434.16 [33.96, 34.35]33.92 [33.72, 34.13].07735.49 [35.36, 35.63]35.33 [35.19, 35.47].033
Cardiovascular strain outcomes
 HR (beats·min−1)Overall69 [59, 80]70 [59, 80].911137 [127, 147]137 [127, 148].634
Phase 168 [57, 79]69 [57, 80].835.602137 [127, 147]137 [127, 147].974.602
Phase 471 [60, 82]71 [59, 82].958136 [126, 147]138 [128, 148].487
Perceptual outcomes
 RPE (au)Overall11.75 [11.14, 12.36]12.06 [11.44, 12.67].144
Phase 111.61 [10.94, 12.29]11.94 [11.27, 12.62].251.888
Phase 411.90 [11.22, 12.57]12.17 [11.48, 12.86].357
 TS (au)Overall4 [4, 5]5 [4, 5].1575 [5, 6]5 [5, 6].924
Phase 14 [4, 5]5 [4, 5].085.7995 [5, 6]5 [5, 6].909.799
Phase 45 [4, 5]5 [4, 5].7885 [5, 6]5 [5, 6].807

Note. n = 12. Data are presented as model estimates and 95% confidence intervals for overall and phase effects and were analyzed using a linear mixed model including “intervention,” “phase,” and “time” (categorical) and all possible interactions as fixed effects. Baseline of SS corresponds to T-120. Overall: Comparison between sodium and placebo regardless of phase; Phase 1: Low estrogen and progesterone; Phase 4: Moderate estrogen and high progesterone. Bold values denote significant differences between sodium and placebo. MC = menstrual cycle; SS = steady state; HR = heart rate; BM = body mass; POsm = plasma osmolality; TS = thermal sensation; RPE = rating of perceived exertion; Tre = rectal temperature; Tsk = skin temperature.

SS Baseline

There were no significant differences between intervention groups or between MC phases for any outcome at SS baseline time (T-120), except for a higher Tsk (0.21, 95% confidence interval [0.03, 0.40] °C; p = .024) with sodium compared with placebo (Table 2).

Hydration Outcomes

Fluid retention (volume [in milliliters] and percentage of ingested fluid) was higher during the preexercise hyperhydration period (T-120 to T0) with sodium compared with placebo (both p < .001), with no MC phase effect (both interactions Intervention × Phase, p > .23). During SS, the BM increase was higher with sodium compared with placebo (p = .040), although this increase was only observed during MC Phase 4 (Table 2). However, relative to baseline, ΔBM was higher with sodium compared with placebo in both MC phases (Supplementary Table S1 [available online]). POsm was higher with sodium compared with placebo (p < .001), and this effect was observed in both MC phases (Table 2). Sweat rate was lower with sodium compared with placebo (p = .001), and this effect was observed in MC Phase 1 (p = .003; Table 2).

Thermal and Cardiovascular Strain

During SS, Tsk was higher with sodium compared with placebo (p < .001). The intervention effect for Tsk was observed in both MC phases (Figure 2B, Table 2). However, there was no intervention or phase effect for ΔTsk (Supplementary Table S1 [available online]). There was no overall intervention or intervention within MC phase for effect for Tre, HR, RPE, and TS (Figure 2A, 2C2E, Table 2) or their change from baseline (ΔTre, ΔHR, ΔRPE, ΔTS; Supplementary Table S1 [available online]).

Figure 2
Figure 2

—Physiological responses during the SS cycling (75 min) and the 200-kJ cycling TT in the heat with sodium and placebo hyperhydration across Phases 1 and 4 of the MC (n = 12). (A) Tre, (B) Tsk, (C) HR, (D) RPE, and (E) TS. Linear mixed model included “intervention,” “phase,” and “time” (categorical) and all possible interactions as fixed effects. MC = menstrual cycle; TS = thermal sensation; RPE = rating of perceived exertion; TT = time trial; SS = steady state; HR = heart rate; Tsk = mean skin temperature; Tre = rectal temperature.

Citation: International Journal of Sport Nutrition and Exercise Metabolism 35, 2; 10.1123/ijsnem.2024-0125

Time Trial

The interaction Intervention × MC phase × Time was not significant for all outcomes during TT; therefore, the same estimates described for SS were reported (Table 3).

Table 3

Physiological and Performance Outcomes at Baseline of TT and in Response to 2-hr Hyperhydration Intervention With Sodium and Placebo and 200-kJ TT Across Phases 1 and 4 of the MC

  Baseline of TTCycling TT
SodiumPlaceboDifference

p
Interaction

p
SodiumPlaceboDifference

p
Interaction

p
Hydration outcomes
 BM (kg)Overall62.15 [58.93, 65.38]61.42 [58.19, 64.65]<.00161.92 [58.69, 65.15]61.14 [57.91, 64.37]<.001
Phase 162.03 [58.79, 65.27]61.58 [58.34, 64.82].109.12061.78 [58.54, 65.02]61.30 [58.07, 64.54].087.120
Phase 462.28 [59.04, 65.52]61.25 [58.01, 64.50]<.00162.06 [58.83, 65.30]60.97 [57.73, 64.21]<.001
 POsm (mOsm·kgH2O−1)Overall302 [299, 305]295 [292, 298]<.001302 [299, 305]294 [291, 297]<.001
Phase 1304 [300, 308]296 [292, 300].005.983304 [300, 307]296 [292, 300].001.983
Phase 4300 [296, 304]293 [289, 297].016300 [297, 304]293 [289, 297].002
 Sweat rate (L·hr−1)Overall2.1 [1.8, 2.4]2.1 [1.8, 2.4].739
Phase 12.2 [1.8, 2.5]2.1 [1.8, 2.4].788.479
Phase 42.0 [1.7, 2.3]2.1 [1.8, 2.5].468
Thermal strain outcomes
Tre (°C)Overall38.28 [38.07, 38.48]38.19 [37.99, 38.40].30238.52 [38.32, 38.71]38.40 [38.20, 38.60].093
Phase 138.28 [38.06, 38.51]38.17 [37.94, 38.40].304.99538.51 [38.29, 38.72]38.39 [38.17, 38.61].230.995
Phase 438.27 [38.04, 38.49]38.22 [37.99, 38.45].66938.53 [38.32, 38.75]38.42 [38.19, 38.64].240
Tsk (°C)Overall35.09 [34.79, 35.39]34.60 [34.30, 34.90]<.00135.57 [35.28, 35.86]35.13 [34.83, 35.42]<.001
Phase 135.12 [34.78, 35.46]34.44 [34.10, 34.79]<.001.10835.60 [35.27, 35.92]34.98 [34.65, 35.31]<.001.108
Phase 435.06 [34.73, 35.40]34.76 [34.41, 35.10].06435.54 [35.22, 35.87]35.28 [34.95, 35.61].078
Cardiovascular strain outcomes
 HR (beats·min−1)Overall113 [103, 122]111 [102, 120].510158 [149, 166]155 [147, 164].282
Phase 1114 [104, 123]107 [98, 117].103.183158 [149, 167]153 [144, 162].080.183
Phase 4111 [101, 121]114 [104, 124].456157 [148, 166]158 [149, 167].860
Perceptual outcomes
 RPE (au)Overall14 [13, 15]14 [14, 15].125
Phase 114 [13, 15]14 [13, 15].556.475
Phase 414 [13, 15]15 [14, 15].117
 TS (au)Overall5 [4, 5]5 [4, 5].9866 [5, 6]6 [5, 6].295
Phase 15 [4, 5]5 [4, 5].519.1366 [5, 6]6 [5, 6].751.136
Phase 45 [4, 5]5 [5, 6].4956 [5, 6]6 [6, 6].077
Performance outcomes
 TT completion time (min)Overall22.41 [19.52, 25.31]23.95 [21.06, 26.85].001
Phase 122.75 [19.79, 25.72]23.99 [21.03, 26.96].054.001
Phase 422.06 [19.10, 25.02]23.91 [20.93, 26.89].005
 TT average power output (W)Overall155 [136, 174]145 [126, 164]<.001
Phase 1155 [136, 174]144 [124, 163].003<.001
Phase 4156 [136, 175]147 [128, 167].031

Note. n = 12. Data are presented as model estimates and 95% confidence intervals for overall and phase effects and were analyzed using a linear mixed model including “intervention,” “phase,” and “time” (categorical) and all possible interactions as fixed effects. Baseline of TT corresponds to T0kJ. Overall: Comparison between sodium and placebo regardless of phase; Phase 1: Low estrogen and progesterone; Phase 4: Moderate estrogen and high progesterone. Bold values denote significant differences between sodium and placebo. MC = menstrual cycle; SS = steady state; HR = heart rate; BM = body mass; POsm = plasma osmolality; TS = thermal sensation; TT = time trial; Tre = rectal temperature; Tsk= skin temperature.

TT Baseline

At TT baseline, BM was higher with sodium than placebo (p < .001). However, BM increased significantly with sodium only during MC Phase 4 (Table 3). POsm was higher with sodium compared with placebo (p < .001), with the effect observed in both MC phases (Table 3). Tsk was higher for sodium compared with placebo (p < .001), with the effect observed only in MC Phase 1 (Figure 2B, Table 3). There were no intervention effects for Tre, TS, and HR (Table 3).

Hydration Outcomes

After the TT, BM was higher with sodium compared with placebo (p < .001). However, this effect was only observed for BM and ΔBM in MC Phase 4 (p < .001 and p = .040; Table 3 and Supplementary Table S1 [available online]). POsm was higher with sodium compared with placebo (p < .001), with this effect observed in both MC phases (Table 3). There were no intervention effects in sweat rate during TT.

Thermal and Cardiovascular Strain

During TT, Tsk was higher with sodium compared with placebo (p < .001), and this effect was only observed in MC Phase 1 (p < .001; Figure 2B, Table 3). There were no intervention effects for Tre, HR, RPE, and TS (Figure 2A, 2C2E, Table 3) or their change from baseline (ΔTre, ΔHR, ΔRPE, ΔTS; Supplementary Table S1 [available online]).

Exercise Performance

Time trial completion time was faster with sodium compared with placebo (−1.55 [−2.46, −0.64] min; p = .001). This effect was significant in MC Phase 4 (−1.85 [−3.16, −0.55] min; p = .005) and in the same direction but not significant in Phase 1 (−1.24 [−2.51, 0.02] min; p = .054; Table 3). Average power output was higher for sodium compared with placebo (10 [5, 15] W; p = .001), with this effect observed in both MC Phases (Table 3).

Gastrointestinal Symptoms and Thirst

There was a statistically significant difference in GIS and thirst (both p < .001) based on intervention period (preexercise, SS, and TT; Table 4). GIS were higher with sodium compared with placebo preexercise (p < .001), with no differences during SS (p = .086) or during TT (p = .045). Thirst was not different preexercise (p = .613) or during TT (p = .325) but was lower during SS (p = .004) with sodium compared with placebo. There was no MC phase effect for GIS or thirst at any period.

Table 4

Comparison of Total GIS and Thirst Responses Between Sodium and Placebo Interventions

 PreexerciseSteady stateTT
SodiumPlaceboSodiumPlaceboSodiumPlacebo
GIS8 (2–12)3 (0–4)**0 (0–0)1 (0–8)0 (0–0)0 (0–3)
Thirst4 (1–9)3 (1–9)39 (32–47)48 (38–58)**8 (7–10)9 (8–10)

Note. n = 12. Data are presented as median (interquartile range). Maximum possible scores for all GIS is 180; Total GIS; sum of all accumulative upper (belching, heartburn, bloating, stomach pain, urge to regurgitate, regurgitation, and vomiting), lower (flatulence, intestinal pain, urge to defecate, defecation, loose stools, diarrhea, and blood in stools), and other (nausea, dizziness, and stich) symptoms; differences in total GIS and thirst were determined via Friedman test and post hoc Wilcoxon signed-rank tests with a Bonferroni correction, **p < .017 between sodium and placebo interventions. TT = time trial; GIS = gastrointestinal symptoms.

Sensitivity Analysis

A sensitivity analysis was performed for all outcomes that only included participants with progesterone concentrations >16 nmol·L−1 (no-LPD group) in both MC Phase 4 sessions (n = 8; Supplementary Tables S2–S3 [available online]). The only differences observed in the no-LPD group compared with the full participant analysis included MC phase effects for fluid retention (ml) and percentage of ingested fluid (both interactions Intervention × Phase, p < .04); an intervention effect within MC Phase 4 for ΔHR, which was significantly lower with sodium compared with placebo (−7.97 [−14.25, −1.70] beats·min−1; p = .013) during SS (Supplementary Table S3 [available online]); and Tsk significantly higher with sodium compared with placebo at TT baseline (0.46 [0.07, 0.85] °C, p = .020) and during TT (0.44 [0.08, 0.79] °C, p = .015; Supplementary Table S2 [available online]). Nonstatistically significant differences in the no-LPD group compared with the full participant analysis were observed for BM only, with no difference between sodium and placebo during SS either overall (0.3 [−0.05, 0.66] kg; p = .096) or within MC phases (Supplementary Table S2 [available online]).

Discussion

The aim of this study was to determine the effect of sodium hyperhydration on thermal and cardiovascular strain and cycling performance in females exercising in the heat and differences between MC Phases 1 and 4. Key findings were that sodium hyperhydration improved cycling performance, reducing TT completion time by ∼5%, with a significant improvement in both MC phases when compared with placebo. Although Tsk was higher with sodium hyperhydration during both SS and TT compared with placebo, this did not appear to significantly influence autonomic thermoregulation as there were no differences in Tre, TS, or cardiovascular strain between sodium hyperhydration and placebo. MC phase interactions with sodium hyperhydration included increased BM and faster TT completion in MC Phase 4 and increased Tsk in MC Phase 1. These findings suggest that sodium hyperhydration may be a useful heat mitigation strategy for females completing endurance cycling in the heat, particularly in MC Phase 4 and when fluid access is limited.

Thermal and Cardiovascular Strain

There were no differences in Tre, HR, TS, or RPE or their changes from baseline between sodium and placebo hyperhydration during the SS or the TT in this study. These findings contradict previous studies wherein sodium hyperhydration attenuated the rise in Tre and HR at constant work rates (70% V˙O2max; 155 ± 7 W) in the heat (32 °C and 50% RH) by ∼ 0.4 °C·hr−1 and ∼9 beats·min−1 in females (Sims, Rehrer, et al., 2007) and by ∼0.57 °C·hr−1 in males but with no differences in HR (Sims, van Vliet, et al., 2007). Sodium hyperhydration also attenuated the Tre increase by ∼0.3 °C and lowered HR (∼5 beats·min−1) compared with euhydration at the end of an 18-km treadmill TT (28 °C and 25%–30%RH) in males, although TT completion times were the same (Gigou et al., 2012). Differences in Tre and HR findings could be attributed to differences in the type (e.g., cycling vs. running), duration, and intensity (e.g., self-paced TT vs. fixed intensity TTE) of exercise, ambient temperature (e.g., higher in this study), and/or the hyperhydration protocol and hydration responses across studies. For example, in this study, during the SS, participants cycled at a lower average power output (111 ± 32 W) than in previous research, followed by a short TT (∼23.3 [14.28–35.97 min]; Figure 3), allowing a higher power output (156 vs. 146 W), likely resulting in increased metabolic heat production with sodium compared with placebo hyperhydration but similar Tre, HR, TS, and RPE responses. Although there were no differences in Tre, HR, TS, and RPE, the higher power output and faster completion times with sodium compared with placebo suggest potential beneficial thermal, cardiovascular, and perceptual responses during the TT (e.g., sodium hyperhydration attenuated the rise in Tre, HR, and perceptual responses that would typically be observed with higher power outputs).

Figure 3
Figure 3

—200-kJ cycling TT completion time (minutes) in the heat with sodium and placebo hyperhydration across Phases 1 and 4 of the MC (n = 12). *p < .05 between sodium and placebo within the MC phase. #p < .05 between sodium and placebo estimates independent of MC phase. Linear mixed model included “intervention,” “phase,” and their interaction as fixed effects with participant as a random effect. TT = time trial; MC = menstrual cycle.

Citation: International Journal of Sport Nutrition and Exercise Metabolism 35, 2; 10.1123/ijsnem.2024-0125

In this study, Tsk was higher with sodium compared with placebo hyperhydration during SS (0.19 [0.09, 0.3] °C) and TT (0.44 [0.23, 0.65] °C). This may be partly due to elevated baseline measures with sodium compared with placebo (0.21 [0.03, 0.41) °C), potentially due to sodium having induced thermogenesis and imperceptibly increased body temperature (Wu et al., 2023) and/or other unknown mechanisms. However, the elevated Tsk with sodium hyperhydration may not be of clinical importance as there was no increase in other measures of thermal or cardiovascular strain, and TT performance improved compared with placebo. Additional research is required to elucidate the physiological responses behind increased Tsk with sodium and establish whether reductions in the benefits of sodium on Tre and HR are limited to lower intensity/ambient conditions and/or fixed-intensity exercise only.

Cycling Performance

This is the first study to investigate the effect of sodium hyperhydration on endurance exercise performance in female athletes exercising in the heat. Compared with placebo, sodium hyperhydration significantly improved cycling performance, reducing completion time by 1.55 min (∼5%). Throughout the TT, sodium hyperhydration enabled a higher power output (∼10 W [5–15 W]) without differences in thermal or cardiovascular strain or perceptual responses compared with placebo. Similar performance benefits have been observed in male cyclists, where sodium hyperhydration (60 mg·kg−1 BM of sodium in capsules) with unlimited fluid access reduced TT completion time (200-kJ TT after 1-hr cycling at 50% maximal power) compared with placebo (aspartame in capsules; ∼1.3 min) and no treatment (∼1.7 min), with similar physiological responses to the interventions (Morris et al., 2015). Conversely, in male runners, sodium hyperhydration (26 ml·kg−1 BM, 130 mmol·L−1 sodium in solution) compared with euhydration improved thermal (Tre < 0.3 °C) and cardiovascular (HR < 5 beats·min−1) strain but not performance (18-km treadmill running TT at ∼28 °C and 25%–30% RH; Gigou et al., 2012). This may be due to preexercise BM gains because of fluid retention (∼59%) and its potential negative effects on running economy (Jardine et al., 2023). Our findings suggest that acute BM gains from sodium-induced fluid retention do not impair female cyclists’ performance in controlled laboratory conditions. However, this requires further investigation in females engaged in endurance weight-bearing sports, such as running or uphill cycling. To date, only one study in males has shown no significant effects on running economy after glycerol hyperhydration at moderate intensity (60% ·V˙O2max; Beis et al., 2011). However, given that running economy is generally impacted by additional external mass (Jardine et al., 2023), it is crucial to explore how BM gains from sodium hyperhydration affect exercise performance in females.

Effect of Sodium Hyperhydration Across MC Phases

At baseline, there were no significant differences in Tre between MC phases for all and no-LPD participants. This finding contradicts previous literature with reported higher Tre (∼0.2 °C–0.3 °C) at rest during MC Phase 4 (Janse de Jonge et al., 2012). However, these differences in Tre are most evident during sleep, or upon waking and before any activity (Baker et al., 2020), which differs from the conditions of our study and may explain the lack of difference in Tre observed at baseline. The MC phase interactions with sodium hyperhydration in this study included increased Tsk in MC Phase 1 and increased BM and faster TT completion time in MC Phase 4. This suggests an improved hydration status in MC Phase 4 with sodium that likely contributed to the lower ΔHR during SS and increased TT power output in the no-LPD participants as well as reduced TT completion time in all participants (−1.85 min) and the no-LPD participants (−2.73 min). These findings are supported by a study showing improved hydration, cardiovascular strain, and TTE in females during MC Phase 4 and in oral contraceptive pill users (Sims, Rehrer, et al., 2007). This indicates that higher progesterone concentrations potentiate the effects of sodium hyperhydration on fluid dynamics (e.g., increased renal sodium absorption, increased plasma volume, and maintenance of extracellular fluid volume; Stachenfeld, 2008), which may have additional endurance exercise capacity and performance benefits for females in the heat during MC Phase 4 compared with Phase 1. It remains to be determined whether similar responses across MC phases can be achieved using other hyperhydration agents, such as glycerol. A recent systematic review (Jardine et al., 2023) on hyperhydration agents highlighted that only 5% of the participants in hyperhydration studies are female, and none of these studies report female data separately from that of males or with appropriate MC phase verification to enable comparisons with different (e.g., sodium vs. glycerol) hyperhydration agents. Future research should aim to compare the responses to different hyperhydration agents in females to establish which agent most effectively supports improved hydration status and exercise performance in the heat. Further investigation is warranted to understand the underlying mechanisms of these responses, including fluid dynamics (e.g., plasma volume change, sodium ingestion and excretion, urine excretion) and arginine vasopressin hormonal release differences between MC Phase 1 and Phase 4.

Gastrointestinal Symptoms and Thirst

In this study, sodium hyperhydration led to higher GIS preexercise but no differences during SS or TT. However, GIS scores with sodium hyperhydration were generally low (21 from total of 180) with most participants (75%) reporting mild symptoms (severity of < 5/10). No previous study has evaluated the incidence and severity of GIS with sodium hyperhydration. From a practical perspective, the dose and timing of sodium hyperhydration used in this study did not contribute to GIS during exercise and was, therefore, well tolerated. This suggests that sodium hyperhydration is a viable strategy for female athletes competing in hot conditions. However, further research is needed to explore its efficacy across other exercise modalities, such as running and uphill cycling.

This study demonstrated no differences in thirst sensation during preexercise hyperhydration and TT and reduced thirst during SS with sodium compared with placebo. This contradicts a previous study in males where thirst was increased preexercise after ingesting sodium capsules (6 mg·kg−1 BM) with ad libitum fluid intake compared with no treatment and placebo (Morris et al., 2015). However, that study did not measure thirst during exercise, and the increased preexercise thirst may be attributed to ad libitum rather than prescribed fluid intake. The reduced thirst during SS with sodium in our female participants may be due to sufficient fluid intake and retention delaying the thirst stimulus until at least 1% BM loss (Greenleaf, 1992). In practical terms, the reduced thirst postsodium hyperhydration may assist with avoiding hyponatremia but may be inadequate to maintain the improved hydration status with sodium hyperhydration, especially during long-duration (>90 min) and high-intensity exercise in the heat with high sweat losses (Kenefick, 2018). Therefore, advocating for individualized hydration plans is crucial to prevent hypohydration in prolonged endurance events in the heat.

There are a few limitations to this study that must be acknowledged. First, constraints related to financial resources, time commitment, and narrow collection periods limited participant recruitment, resulting in a small sample size of individuals with no LPD. A larger sample size would have provided a more robust analysis of potential MC phase-mediated responses. Moreover, despite the similar tier classifications of athletic caliber, there was significant variability in the TT completion times among study participants. Previous research indicates that recreational athletes exhibit greater variability in performance tests compared with elite athletes, particularly under heat stress conditions where some individuals are more affected than others (Hopkins et al., 2001). Therefore, additional research with elite female cyclists is required to determine whether performance outcomes with hyperhydration are more uniform and of a similar magnitude. Finally, the absence of an exit questionnaire prevents ruling out potential placebo or nocebo effects of supplementation on performance and is, therefore, an important consideration for future hyperhydration research.

Conclusions

This is the first study to investigate the effects of sodium hyperhydration on exercise performance in female cyclists and with MC phase comparisons. Sodium hyperhydration improved cycling performance in the heat by ∼5% compared with placebo and was more effective during MC Phase 4. Sodium hyperhydration induced minimal GIS and allowed a higher workload without changes in thermal and cardiovascular strain, likely due to enhanced cardiovascular stability via increased fluid retention. Sodium hyperhydration is a beneficial acute heat mitigation strategy for female endurance athletes exercising and/or competing for a prolonged duration in the heat with insufficient fluid access and may provide additional hydration and performance benefits during MC Phase 4.

Acknowledgments

The authors would like to thank Mr. William Jardine for conducting the blinding of the study. Author Contributions: Conceptualization and methodology: Convit, Périard, Warmington, Carr, Snipe. Data collection: Convit, Beaugeois, Abraham. Data curation, validation, and formal analysis: Convit, Orellana. Writing original draft preparation: Convit, Snipe. Writing review and editing: Convit, Orellana, Périard, Warmington, Carr, Beaugeois, Abraham, Snipe. All authors have read and agreed to the published version of the manuscript. Funding Sources: This research was supported by Deakin University School of Exercise and Nutrition Sciences. Availability of Data: Data generated or analyzed during this study are available from the corresponding author upon reasonable request.

References

  • Baker, F.C., Siboza, F., & Fuller, A. (2020). Temperature regulation in women: Effects of the menstrual cycle. Temperature, 7(3), 226262.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Beis, L.Y., Polyviou, T., Malkova, D., & Pitsiladis, Y.P. (2011). The effects of creatine and glycerol hyperhydration on running economy in well trained endurance runners. Journal of the International Society of Sports Nutrition, 8, Article 24.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Borg, G. (1998). Borg’s perceived exertion and pain scales. Human Kinetics.

  • Casa, D.J. (1999). Exercise in the heat. I. Fundamentals of thermal physiology, performance implications, and dehydration. Journal of Athletic Training, 34(3), 246252.

    • Search Google Scholar
    • Export Citation
  • Che Jusoh, M., Morton, R., Stannard, S., & Mündel, T. (2015). A reliable preloaded cycling time trial for use in conditions of significant thermal stress. Scandinavian Journal of Medicine & Science in Sports, 25, 296301.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Coles, M.G., & Luetkemeier, M.J. (2005). Sodium-facilitated hypervolemia, endurance performance, and thermoregulation. International Journal of Sports Medicine, 26(3), 182187.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Decroix, L., De Pauw, K., Foster, C., & Meeusen, R. (2016). Guidelines to classify female subject groups in sport-science research. International Journal of Sports Physiology and Performance, 11(2), 204213.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Elliott-Sale, K.J., Minahan, C.L., de Jonge, X.A.K.J., Ackerman, K.E., Sipilä, S., Constantini, N.W., Lebrun, C.M., & Hackney, A.C. (2021). Methodological considerations for studies in sport and exercise science with women as participants: A working guide for standards of practice for research on women [Original Paper]. Sports Medicine, 51(5), Article 843.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Gagnon, D., & Kenny, G.P. (2012). Sex differences in thermoeffector responses during exercise at fixed requirements for heat loss. Journal of Applied Physiology, 113(5), 746757.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Gaskell, S.K., Snipe, R.M., & Costa, R.J. (2019). Test–retest reliability of a modified visual analog scale assessment tool for determining incidence and severity of gastrointestinal symptoms in response to exercise stress. International Journal of Sport Nutrition and Exercise Metabolism, 29(4), 411419.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Gelman, A. (2007). Data analysis using regression and multilevel/hierarchical models. Cambridge University Press.

  • Gigou, P.Y., Dion, T., Asselin, A., Berrigan, F., & Goulet, E.D. (2012). Pre-exercise hyperhydration-induced bodyweight gain does not alter prolonged treadmill running time-trial performance in warm ambient conditions. Nutrients, 4(8), 949966.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Goulet, E.D., De La Flore, A., Savoie, F.A., & Gosselin, J. (2018). Salt + glycerol-induced hyperhydration enhances fluid retention more than salt -or glycerol-induced hyperhydration. International Journal of Sport Nutrition, 28(3), 246252.

    • Search Google Scholar
    • Export Citation
  • Greenleaf, J.E. (1992). Problem: Thirst, drinking behavior, and involuntary dehydration [Article]. Medicine & Science in Sports & Exercise, 24(6), 645656.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Greenleaf, J.E., Looft-Wilson, R., Wisherd, J.L., McKenzie, M.A., Jensen, C.D., & Whittam, J.H. (1997). Pre-exercise hypervolemia and cycle ergometer endurance in men. Biology of Sport, 14(2), 103114.

    • Search Google Scholar
    • Export Citation
  • Hashimoto, H., Ishijima, T., Suzuki, K., & Higuchi, M. (2016). The effect of the menstrual cycle and water consumption on physiological responses during prolonged exercise at moderate intensity in hot conditions. Journal of Sports Medicine Physical Fitness, 56(9), 951960.

    • Search Google Scholar
    • Export Citation
  • Havenith, G., Coenen, J.M., Kistemaker, L., & Kenney, W.L. (1998). Relevance of individual characteristics for human heat stress response is dependent on exercise intensity and climate type. European Journal of Applied Physiology and Occupational Physiology, 77(3), 231241.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Havenith, G., Luttikholt, V.G., & Vrijkotte, T. (1995). The relative influence of body characteristics on humid heat stress response. European Journal of Applied Physiology and Occupational Physiology, 70(3), 270279.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Hopkins, W.G., Schabort, E.J., & Hawley, J.A. (2001). Reliability of power in physical performance tests. Sports Medicine, 31(3), 211234.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Janse de Jonge, X. (2003). Effects of the menstrual cycle on exercise performance. Sports Medicine, 33(11), 833851.

  • Janse de Jonge, X., Thompson, M.W., Chuter, V.H., Silk, L.N., & Thom, J.M. (2012). Exercise performance over the menstrual cycle in temperate and hot, humid conditions. Medicine & Science in Sports & Exercise, 44(11), 21902198.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Jardine, W.T., Aisbett, B., Kelly, M.K., Burke, L.M., Ross, M.L., Condo, D., Périard, J.D., & Carr, A.J. (2023). The effect of pre-exercise hyperhydration on exercise performance, physiological outcomes and gastrointestinal symptoms: A systematic review. Sports Medicine, 53, 21112134.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kenefick, R.W. (2018). Drinking strategies: Planned drinking versus drinking to thirst. Sports Medicine, 48(Suppl. 1), 3137.

  • Kolka, M.A., & Stephenson, L.A. (1997). Effect of luteal phase elevation in core temperature on forearm blood flow during exercise. Journal of Applied Physiology, 82(4), 10791083.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • McKay, A.K., Stellingwerff, T., Smith, E.S., Martin, D.T., Mujika, I., Goosey-Tolfrey, V.L., Sheppard, J., & Burke, L.M. (2021). Defining training and performance caliber: A participant classification framework. International Journal of Sports Physiology and Performance, 17(2), 317331.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Mesen, T.B., & Young, S.L. (2015). Progesterone and the luteal phase: A requisite to reproduction. Obstetrics and Gynecology Clinics, 42(1), 135151.

    • Search Google Scholar
    • Export Citation
  • Morris, D.M., Huot, J.R., Jetton, A.M., Collier, S.R., & Utter, A.C. (2015). Acute sodium ingestion before exercise increases voluntary water consumption resulting in pre-exercise hyperhydration and improvement in exercise performance in the heat. International Journal of Sport Nutrition and Exercise Metabolism, 25(5), Article 456.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Périard, J.D., Eijsvogels, T.M., & Daanen, H.A. (2021). Exercise under heat stress: Thermoregulation, hydration, performance implications and mitigation strategies. Physiological Reviews, 101, 18731979.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Pickering, C., & Kiely, J. (2019). What should we do about habitual caffeine use in athletes? Sports Medicine, 49(6), 833842.

  • Pivarnik, J.M., Marichal, C.J., Spillman, T., & Morrow, J., Jr. (1992). Menstrual cycle phase affects temperature regulation during endurance exercise. Journal of Applied Physiology, 72(2), 543548.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Ramanathan, N. (1964). A new weighting system for mean surface temperature of the human body. Journal of Applied Physiology, 19(3), 531533.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Schliep, K.C., Mumford, S.L., Hammoud, A.O., Stanford, J.B., Kissell, K.A., Sjaarda, L.A., Perkins, N.J., Ahrens, K.A., Wactawski-Wende, J., & Mendola, P. (2014). Luteal phase deficiency in regularly menstruating women: Prevalence and overlap in identification based on clinical and biochemical diagnostic criteria. The Journal of Clinical Endocrinology & Metabolism, 99(6), E1007E1014.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Schneider, D.A., Phillips, S.E., & Stoffolano, S. (1993). The simplified V-slope method of detecting the gas exchange threshold. Medicine & Science in Sports & Exercise, 25(10), 11801184.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Sims, S.T., Rehrer, N.J., Bell, M.L., & Cotter, J.D. (2007). Preexercise sodium loading aids fluid balance and endurance for women exercising in the heat. Journal of Applied Physiology, 103, 534541.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Sims, S.T., van Vliet, L., Cotter, J., & Rehrer, N. (2007). Sodium loading aids fluid balance and reduces physiological strain of trained men exercising in the heat [Article]. Medicine & Science in Sports & Exercise, 39(1), 123130.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Stachenfeld, N.S. (2008). Sex hormone effects on body fluid regulation. Exercise and Sport Sciences Reviews, 36(3), Article 152.

  • Tanner, R., & Gore, C. (2012). Physiological tests for elite athletes. Human Kinetics.

  • Wu, Q., Burley, G., Li, L.C., Lin, S., & Shi, Y.C. (2023). The role of dietary salt in metabolism and energy balance: Insights beyond cardiovascular disease. Diabetes, Obesity and Metabolism, 25(5), 11471161.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yanovich, R., Ketko, I., & Charkoudian, N. (2020). Sex differences in human thermoregulation: Relevance for 2020 and beyond. Physiology, 35(3), 177184.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Young, A.J., Sawka, M.N., Epstein, Y., Decristofano, B., & Pandolf, K.B. (1987). Cooling different body surfaces during upper and lower body exercise. Journal of Applied Physiology, 63(3), 12181223.

    • Crossref
    • Search Google Scholar
    • Export Citation

Nontechnical Summary

Sodium hyperhydration involves adding sodium to fluids consumed before exercise to increase total body water levels. This increased body water can delay dehydration and help mitigate heat-related stress during prolonged physical activity in the heat. Our study examined the effects of sodium hyperhydration compared with fluid intake with no sodium on body temperature, heart rate, and exercise performance in female endurance athletes and compared these outcomes during different phases of the menstrual cycle (Phase 1: low estrogen and progesterone and Phase 4: moderate estrogen and high progesterone) where fluctuating hormone levels can influence fluid retention and body temperature.

Our findings indicate that sodium hyperhydration increased fluid retention compared with fluid without sodium. Despite the lack of significant changes in body temperature or heart rate, sodium hyperhydration led to a ∼5% improvement in self-paced exercise performance in female athletes exercising in the heat, with greater improvements observed during Phase 4 of the menstrual cycle.

These results indicate that sodium hyperhydration may be particularly beneficial for female endurance athletes exercising in the heat when fluid access is limited and during Phase 4 of the menstrual cycle where fluid retention is naturally higher.

Sodium hyperhydration involves adding sodium to fluids consumed before exercise to increase total body water levels. This increased body water can delay dehydration and help mitigate heat-related stress during prolonged physical activity in the heat. Our study examined the effects of sodium hyperhydration compared with fluid intake with no sodium on body temperature, heart rate, and exercise performance in female endurance athletes and compared these outcomes during different phases of the menstrual cycle (Phase 1: low estrogen and progesterone and Phase 4: moderate estrogen and high progesterone) where fluctuating hormone levels can influence fluid retention and body temperature.

Our findings indicate that sodium hyperhydration increased fluid retention compared with fluid without sodium. Despite the lack of significant changes in body temperature or heart rate, sodium hyperhydration led to a ∼5% improvement in self-paced exercise performance in female athletes exercising in the heat, with greater improvements observed during Phase 4 of the menstrual cycle.

These results indicate that sodium hyperhydration may be particularly beneficial for female endurance athletes exercising in the heat when fluid access is limited and during Phase 4 of the menstrual cycle where fluid retention is naturally higher.

  • Collapse
  • Expand
  • Figure 1

    —Schematic of the experimental sessions. FFM = fat-free mass; RH = relative humidity; TT = time trial; SS = steady state; HR = heart rate; BM = body mass.

  • Figure 2

    —Physiological responses during the SS cycling (75 min) and the 200-kJ cycling TT in the heat with sodium and placebo hyperhydration across Phases 1 and 4 of the MC (n = 12). (A) Tre, (B) Tsk, (C) HR, (D) RPE, and (E) TS. Linear mixed model included “intervention,” “phase,” and “time” (categorical) and all possible interactions as fixed effects. MC = menstrual cycle; TS = thermal sensation; RPE = rating of perceived exertion; TT = time trial; SS = steady state; HR = heart rate; Tsk = mean skin temperature; Tre = rectal temperature.

  • Figure 3

    —200-kJ cycling TT completion time (minutes) in the heat with sodium and placebo hyperhydration across Phases 1 and 4 of the MC (n = 12). *p < .05 between sodium and placebo within the MC phase. #p < .05 between sodium and placebo estimates independent of MC phase. Linear mixed model included “intervention,” “phase,” and their interaction as fixed effects with participant as a random effect. TT = time trial; MC = menstrual cycle.

  • Baker, F.C., Siboza, F., & Fuller, A. (2020). Temperature regulation in women: Effects of the menstrual cycle. Temperature, 7(3), 226262.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Beis, L.Y., Polyviou, T., Malkova, D., & Pitsiladis, Y.P. (2011). The effects of creatine and glycerol hyperhydration on running economy in well trained endurance runners. Journal of the International Society of Sports Nutrition, 8, Article 24.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Borg, G. (1998). Borg’s perceived exertion and pain scales. Human Kinetics.

  • Casa, D.J. (1999). Exercise in the heat. I. Fundamentals of thermal physiology, performance implications, and dehydration. Journal of Athletic Training, 34(3), 246252.

    • Search Google Scholar
    • Export Citation
  • Che Jusoh, M., Morton, R., Stannard, S., & Mündel, T. (2015). A reliable preloaded cycling time trial for use in conditions of significant thermal stress. Scandinavian Journal of Medicine & Science in Sports, 25, 296301.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Coles, M.G., & Luetkemeier, M.J. (2005). Sodium-facilitated hypervolemia, endurance performance, and thermoregulation. International Journal of Sports Medicine, 26(3), 182187.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Decroix, L., De Pauw, K., Foster, C., & Meeusen, R. (2016). Guidelines to classify female subject groups in sport-science research. International Journal of Sports Physiology and Performance, 11(2), 204213.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Elliott-Sale, K.J., Minahan, C.L., de Jonge, X.A.K.J., Ackerman, K.E., Sipilä, S., Constantini, N.W., Lebrun, C.M., & Hackney, A.C. (2021). Methodological considerations for studies in sport and exercise science with women as participants: A working guide for standards of practice for research on women [Original Paper]. Sports Medicine, 51(5), Article 843.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Gagnon, D., & Kenny, G.P. (2012). Sex differences in thermoeffector responses during exercise at fixed requirements for heat loss. Journal of Applied Physiology, 113(5), 746757.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Gaskell, S.K., Snipe, R.M., & Costa, R.J. (2019). Test–retest reliability of a modified visual analog scale assessment tool for determining incidence and severity of gastrointestinal symptoms in response to exercise stress. International Journal of Sport Nutrition and Exercise Metabolism, 29(4), 411419.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Gelman, A. (2007). Data analysis using regression and multilevel/hierarchical models. Cambridge University Press.

  • Gigou, P.Y., Dion, T., Asselin, A., Berrigan, F., & Goulet, E.D. (2012). Pre-exercise hyperhydration-induced bodyweight gain does not alter prolonged treadmill running time-trial performance in warm ambient conditions. Nutrients, 4(8), 949966.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Goulet, E.D., De La Flore, A., Savoie, F.A., & Gosselin, J. (2018). Salt + glycerol-induced hyperhydration enhances fluid retention more than salt -or glycerol-induced hyperhydration. International Journal of Sport Nutrition, 28(3), 246252.

    • Search Google Scholar
    • Export Citation
  • Greenleaf, J.E. (1992). Problem: Thirst, drinking behavior, and involuntary dehydration [Article]. Medicine & Science in Sports & Exercise, 24(6), 645656.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Greenleaf, J.E., Looft-Wilson, R., Wisherd, J.L., McKenzie, M.A., Jensen, C.D., & Whittam, J.H. (1997). Pre-exercise hypervolemia and cycle ergometer endurance in men. Biology of Sport, 14(2), 103114.

    • Search Google Scholar
    • Export Citation
  • Hashimoto, H., Ishijima, T., Suzuki, K., & Higuchi, M. (2016). The effect of the menstrual cycle and water consumption on physiological responses during prolonged exercise at moderate intensity in hot conditions. Journal of Sports Medicine Physical Fitness, 56(9), 951960.

    • Search Google Scholar
    • Export Citation
  • Havenith, G., Coenen, J.M., Kistemaker, L., & Kenney, W.L. (1998). Relevance of individual characteristics for human heat stress response is dependent on exercise intensity and climate type. European Journal of Applied Physiology and Occupational Physiology, 77(3), 231241.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Havenith, G., Luttikholt, V.G., & Vrijkotte, T. (1995). The relative influence of body characteristics on humid heat stress response. European Journal of Applied Physiology and Occupational Physiology, 70(3), 270279.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Hopkins, W.G., Schabort, E.J., & Hawley, J.A. (2001). Reliability of power in physical performance tests. Sports Medicine, 31(3), 211234.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Janse de Jonge, X. (2003). Effects of the menstrual cycle on exercise performance. Sports Medicine, 33(11), 833851.

  • Janse de Jonge, X., Thompson, M.W., Chuter, V.H., Silk, L.N., & Thom, J.M. (2012). Exercise performance over the menstrual cycle in temperate and hot, humid conditions. Medicine & Science in Sports & Exercise, 44(11), 21902198.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Jardine, W.T., Aisbett, B., Kelly, M.K., Burke, L.M., Ross, M.L., Condo, D., Périard, J.D., & Carr, A.J. (2023). The effect of pre-exercise hyperhydration on exercise performance, physiological outcomes and gastrointestinal symptoms: A systematic review. Sports Medicine, 53, 21112134.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kenefick, R.W. (2018). Drinking strategies: Planned drinking versus drinking to thirst. Sports Medicine, 48(Suppl. 1), 3137.

  • Kolka, M.A., & Stephenson, L.A. (1997). Effect of luteal phase elevation in core temperature on forearm blood flow during exercise. Journal of Applied Physiology, 82(4), 10791083.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • McKay, A.K., Stellingwerff, T., Smith, E.S., Martin, D.T., Mujika, I., Goosey-Tolfrey, V.L., Sheppard, J., & Burke, L.M. (2021). Defining training and performance caliber: A participant classification framework. International Journal of Sports Physiology and Performance, 17(2), 317331.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Mesen, T.B., & Young, S.L. (2015). Progesterone and the luteal phase: A requisite to reproduction. Obstetrics and Gynecology Clinics, 42(1), 135151.

    • Search Google Scholar
    • Export Citation
  • Morris, D.M., Huot, J.R., Jetton, A.M., Collier, S.R., & Utter, A.C. (2015). Acute sodium ingestion before exercise increases voluntary water consumption resulting in pre-exercise hyperhydration and improvement in exercise performance in the heat. International Journal of Sport Nutrition and Exercise Metabolism, 25(5), Article 456.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Périard, J.D., Eijsvogels, T.M., & Daanen, H.A. (2021). Exercise under heat stress: Thermoregulation, hydration, performance implications and mitigation strategies. Physiological Reviews, 101, 18731979.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Pickering, C., & Kiely, J. (2019). What should we do about habitual caffeine use in athletes? Sports Medicine, 49(6), 833842.

  • Pivarnik, J.M., Marichal, C.J., Spillman, T., & Morrow, J., Jr. (1992). Menstrual cycle phase affects temperature regulation during endurance exercise. Journal of Applied Physiology, 72(2), 543548.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Ramanathan, N. (1964). A new weighting system for mean surface temperature of the human body. Journal of Applied Physiology, 19(3), 531533.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Schliep, K.C., Mumford, S.L., Hammoud, A.O., Stanford, J.B., Kissell, K.A., Sjaarda, L.A., Perkins, N.J., Ahrens, K.A., Wactawski-Wende, J., & Mendola, P. (2014). Luteal phase deficiency in regularly menstruating women: Prevalence and overlap in identification based on clinical and biochemical diagnostic criteria. The Journal of Clinical Endocrinology & Metabolism, 99(6), E1007E1014.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Schneider, D.A., Phillips, S.E., & Stoffolano, S. (1993). The simplified V-slope method of detecting the gas exchange threshold. Medicine & Science in Sports & Exercise, 25(10), 11801184.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Sims, S.T., Rehrer, N.J., Bell, M.L., & Cotter, J.D. (2007). Preexercise sodium loading aids fluid balance and endurance for women exercising in the heat. Journal of Applied Physiology, 103, 534541.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Sims, S.T., van Vliet, L., Cotter, J., & Rehrer, N. (2007). Sodium loading aids fluid balance and reduces physiological strain of trained men exercising in the heat [Article]. Medicine & Science in Sports & Exercise, 39(1), 123130.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Stachenfeld, N.S. (2008). Sex hormone effects on body fluid regulation. Exercise and Sport Sciences Reviews, 36(3), Article 152.

  • Tanner, R., & Gore, C. (2012). Physiological tests for elite athletes. Human Kinetics.

  • Wu, Q., Burley, G., Li, L.C., Lin, S., & Shi, Y.C. (2023). The role of dietary salt in metabolism and energy balance: Insights beyond cardiovascular disease. Diabetes, Obesity and Metabolism, 25(5), 11471161.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yanovich, R., Ketko, I., & Charkoudian, N. (2020). Sex differences in human thermoregulation: Relevance for 2020 and beyond. Physiology, 35(3), 177184.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Young, A.J., Sawka, M.N., Epstein, Y., Decristofano, B., & Pandolf, K.B. (1987). Cooling different body surfaces during upper and lower body exercise. Journal of Applied Physiology, 63(3), 12181223.

    • Crossref
    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 1570 1570 0
Full Text Views 3386 3386 310
PDF Downloads 1418 1418 114