Clinical Scenario
Exercise in the heat can lead to performance decrements and increase the risk of heat illness.1,2 Heat acclimation refers to the systematic and gradual increase in exercise in a controlled, laboratory environment with hot environmental conditions and is an impactful strategy that can be used to optimize performance and safety when exercising in the heat.1 Higher sweat rate, plasma volume expansion, decreased heart rate, and lower internal body temperature are observed following heat acclimation, and these adaptations decrease the risk of heat illness and increase exercise performance in the heat.1–3 Heat acclimation induction protocols typically occur over the course of 5 to 14 days (<7 d, short term; 7–14 d, medium term; and >14 d, long term) and produce a variety of physiological gains in performance.1,3 Many heat acclimation studies have examined the hydration status of the subjects exercising in the heat4–8 and it is well known that hydration status impacts physiological and performance responses in the heat.9 Some of the physiological responses that are desired to elicit heat acclimation (ie, higher heart rate and internal body temperature) are exacerbated in a dehydrated state.10 Thus, euhydration (optimal hydration) and dehydration trials during heat acclimation induction have been conducted to determine if there are additional benefits to dehydrated exercise trials on plasma volume, heart rate, internal body temperature, skin temperature, and sweat rate. However, there is still much debate over hydration status and its effect on heat acclimation.
Focused Clinical Question
Does dehydration affect the adaptations of plasma volume, heart rate, internal body temperature, skin temperature, and sweat rate during the induction phase of heat acclimation?
Search Strategy
Terms Used to Guide Search
Our key terms included the following: hydration, heat, environment, acclimation, thermoregulation, and exercise.
- •Patient/Client Group: regularly trained active males
- •Intervention/Assessment: heat acclimation exercise sessions for 3+days
- •Comparison: euhydration trial and dehydration trial
- •Outcome: plasma volume, heart rate, internal body temperature, skin temperature, and sweat rate
Source of Evidence Searched
- •PubMed
- •Scopus
- •SPORTDiscus
Inclusion Criteria
- •Regularly trained active males
- •At least one of the following dependent variables were included as dependent variables: plasma volume, heart rate, internal body temperature, skin temperature, and sweat rate
- •There was an intervention of euhydration and dehydration
- •Heat acclimation exercise sessions for 3+ days
- •Full articles were published
Exclusion Criteria
- •Heat acclimatization trials
- •Studies not available in the English language
Results of Search
In the initial search, 5 out of 265 studies were recognized. Four of 5 studies met the inclusion criteria. One excluded study did not include the intervention of euhydration and dehydration. The study designs were 1 cross-sectional and 3 crossover studies. Each study was evaluated using the 2011 Oxford Centre for Evidence-Based Medicine (See Table 1).
Best Evidence
The studies in Table 2 were identified as the best evidence available for this critically appraised topic. All 4 studies had a level 2 evidence rating, because they were randomized control clinical trials, and the participant’s VO2max was ≥53 mL·kg−1·min−1 to identify them as “active” and “trained.” At least one of following variables, including plasma volume, heart rate, internal body temperature, skin temperature, and sweat rate, was the dependent variable in all 4 studies. There were no differences in plasma volume, internal body temperature, and skin temperature following heat acclimation in all 4 studies. This is an indication that hydration status might not affect adaptations to heat acclimation.
Characteristics of Included Studies
| Article | Pethick et al6 | Neal et al11 | Schleh et al5 | Garrett et al12 |
|---|---|---|---|---|
| Design | RCT (cross-sectional study) | RCT (balanced crossover design) | RCT (random crossover design) | RCT (random crossover design) |
| Participants | 24 subelite male athletes Age, 28 (6) y BM, 74.4 (8.3) kg VO2max, 61.2 (7.0) mL·kg−1·min−1 | 8 trained male athletes Age, 21 (3) y BM, 77.3 (4.89) kg VO2max, 56.9 (7.2) mL·kg−1·min−1 | 13 recreationally active males Age, 23 (1) y BM, 81.4 (2.2) kg VO2max, 53.3 (1.7) mL·kg−1·min−1 | 9 trained males Age, 27 (7) y BM, 74.6 (4.4) kg VO2peak, 60.0 (7.0) mL·kg−1·min−1 |
| Intervention investigated | EUH (−0.2% [0.7%] BML) and DEH (2.2% [0.5%] BML) groups performed 90-min exercise with isothermal method in 35°C and 57% RH for 5 d. Plasma volume and resting Tint were measured prior to and following heat acclimation. | EUH (0.6% [0.7%] BML) and DEH (2.7% [0.8%] BML) groups performed 90-min exercise with isothermal method in 39°C and 56% RH for 8 d. Adaptations in plasma volume, HR, Tint, Tskin, and sweat loss were measured by heat stress test prior to and following heat acclimation. HST was consisted of 60 min at 35% of peak power output in 39°C and 53% RH. | EUH (1.4% BML) and DEH (2.4% BML) groups performed for 3 d of 90-min exercise at 50% VO2max in 40°C and 30% RH. Plasma volume, HR, Tskin, and sweat rate were measured by heat stress test prior to and following heat acclimation. HST was consisted of 90 min at 50% VO2max in 40°C and 30% RH. | EUH (−0.3% [0.6%] BML) and DEH (1.9% [0.6%] BML) groups performed 5 d of 90 min with isothermal method in 35°C and 60% RH. Plasma volume, HR, Tint, and Tskin were measured by HST prior to and following heat acclimation. HST was consisted of 90 min at 40% of peak power output in 35°C and 60% RH. |
| Outcome measures | Plasma volume Resting Tint | Plasma volume HR Tint Tskin Sweat loss | Plasma volume HR Tskin Sweat rate | Plasma volume HR Tint Tskin |
| Main findings | Plasma volume was not different in EUH (+4.8% [10.2%]) and DEH (+1.7% [10.1%]) following heat acclimation (P > .05). Resting Tint was not different in EUH (37.3°C [0.3°C]) and DEH (37.4°C [0.3°C]) following heat acclimation (P > .05). | Plasma volume, HR, Tint, Tskin, and sweat loss were not different in EUH and DEH following heat acclimation. (Data in article represented by figures.) | Sweat rate increased greater following heat acclimation with DEH (1.9 [0.1] L·h−1) compared with EUH (1.8 [0.1] L·h−1) (P = .02). There were no differences in plasma volume (EUH, 4.4% [2.5%]; DEH, 7.1% [1.8%]), HR (EUH, 167.4 [3.7] bpm; DEH, 164.1 [4.4] bpm), and Tskin (EUH, 37.6 [0.2]; DEH, 37.4 [0.1]) between EUH and DEH (P > .05). | Plasma volume (EUH, 4% [3%]; DEH, 8% [3%], P = .06), Tint (EUH, −0.2°C; DEH, −0.4°C, P = .52), and Tskin (EUH, −0.3°C; DEH, −0.4°C, P = .15) were not different in EUH and DEH following heat acclimation. However, HR with DEH demonstrated a greater extent of change (−19 bpm) compared with EUH (−10 bpm) following heat acclimation (P = .05). |
| Level of evidence | 2 | 2 | 2 | 2 |
| Validity scorea | 6 | 7 | 7 | 6 |
| Conclusion | Dehydration did not affect adaptations in plasma volume and resting Tint following heat acclimation. | Dehydration did not affect adaptations in plasma volume, HR, Tint, Tskin, and sweat loss following heat acclimation. | Dehydration did not affect plasma volume, HR, and Tskin following heat acclimation. However, sweat rate increased greater following heat acclimation with dehydration. | Dehydration did not affect adaptations in plasma volume, Tint, Tskin, and sweat loss following heat acclimation. However, HR decreased greater following heat acclimation with dehydration. |
Abbreviations: BM, body mass; BML, body mass loss; bpm, beats per minute; DEH, dehydration; EUH, euhydration; HR, heart rate; HST, heat stress test; RCT, randomized controlled trial; RH, relative humidity; Tint, internal body temperature; Tskin, skin temperature; VO2max, maximal oxygen consumption; VO2peak, peak oxygen consumption.
aValidity score was measured by PEDro scale.
Clinical Bottom Line
There is minimal literature assessing the effects of hydration status on heat acclimation induction. In the studies that met the inclusion criteria, there were no differences in plasma volume, internal body temperature, and skin temperature between euhydration and dehydration trials following heat acclimation.5,6,11,12
Strength of Recommendation
Based on the Oxford Centre for Evidence-Based Medicine Scale, Level 2 evidence exists.
Implications for Practice, Education, and Further Research
The results of the 4 studies included in this critically appraised topic demonstrate that hydration status does not change physiological adaptations in plasma volume, internal body temperature, and skin temperature. While heat acclimation protocols can vary, the studies that met the inclusion criteria had exercise trials in the heat from 3 to 8 days. The study to show differences in sweat rate had the shortest heat acclimation protocol. Trials were held over a 3-day period, which included exercising in the heat for 90 minutes 3 times a day.5 Plasma volume and heart rate remained unchanged. Sweat rate was the only variable that had changes between dehydrated (1.5 [0.1] to 1.9 [0.1] L·h−1) and euhydrated trials (1.6 [0.1] to 1.8 [0.1] L·h−1, P = .02). In addition, only 1 study demonstrated greater changes in heart rate with dehydration trials following heat acclimation compared with preheat acclimation heart rate. However, the absolute heart rate values were not different between euhydration and dehydration trials following heat acclimation. As all 4 studies did not show changes in plasma volume, internal body temperature, and skin temperature, and 2 out of 3 studies did not indicate the differences in heart rate, between euhydrated and dehydrated trials, which indicates that intentional dehydration does not influence effectiveness of heat acclimation. When designing a heat acclimation protocol, researchers need to keep in mind the target population and ultimate training goal to optimize heat acclimation. Each study included healthy, aerobically fit participants. As hydration status has not been demonstrated to attenuate or enhance the ability to acclimate to the heat, the maintenance of euhydration is considered optimal to prevent athletes from the risk of heat illness and decreases in performance during heat acclimation induction.
References
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Garrett AT, Goosens NG, Rehrer NJ, et al. Short-term heat acclimation is effective and may be enhanced rather than impaired by dehydration. Am J Hum Biol. 2014;26(3):311–320. PubMed ID: 24469986 doi:10.1002/ajhb.22509
