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The purpose of this study was to compare the gastric emptying rates (GER) of water, a 6% carbohydrate (CHO) beverage, and a 20% CHO beverage and to contrast those rates against the rate at which deuterium oxide in the drinks accumulated in plasma (DAR) following beverage ingestion. Ten subjects (8 males, 2 females) cycled at 60% ${\text{VO}}_2\max$ for 70 min; at 13 min, the subjects ingested 400 ml of one of the beverages. The GER and DAR of water and 6% CHO were similar, while GER and DAR were both significantly slowed by ingestion of 20% CHO. Although there was a significant correlation (r = .63, p < .05) between GER and DAR, only 40% of the variation in DAR could be accounted for by variation in GER. These data support the contention that DAR is partially determined by GER, with differences in the rate of fluid absorption across the intestine and other factors accounting for the remaining variation in DAR.

The authors measured 24-h fluid-turnover (FTO) rate during 6 d of preseason training in U.S. college football players. Players, training (T, n = 9, full gear and contact drills) and reference (R, n = 4, conditioning without gear or contact), ingested a deuterium oxide (D₂O) dose and provided urine samples every 24 h for analysis of D₂O. During one ~2.3–h practice (wet-bulb globe temperature 24.6 °C), body-mass change, urine production, and voluntary fluid intake were measured to calculate gross sweat loss (GSL). Average FTO was 10.3 ± 2.2 L/d for T and 7.0 ± 1.0 L/d for R. GSL was 3.4 ± 1.5 L for T and 1.7 ± 1.3 for R (P > 0.05). By Day 6, body mass decreased significantly in T (–2.4 ± 1.3 kg, P < 0.05) but not in R (0.38 ± 0.95 kg). With preseason training under moderate environmental stress, football players had high FTO and sweat rates, which might have contributed to a loss of body mass during preseason football training.

Isotonic sports drinks are often consumed to offset the effects of dehydration and improve endurance performance, but hypotonic drinks may be more advantageous. The purpose of the study was to compare absorption and effects on performance of a commercially available hypotonic sports drink (Mizone Rapid: 3.9% carbohydrate [CHO], 218 mOsmol/kg) with those of an isotonic drink (PowerAde: 7.6% CHO, 281 mOsmol/kg), a hypertonic drink (Gatorade: 6% CHO, 327 mOsmol/kg), and a noncaloric placebo (8 mOsmol/kg). In a crossover, 11 cyclists consumed each drink on separate days at 250 ml/15 min during a 2-hr preload ride at 55% peak power followed by an incremental test to exhaustion. Small to moderate increases in deuterium oxide enrichment in the preload were observed with Mizone Rapid relative to PowerAde, Gatorade, and placebo (differences of 88, 45, and 42 parts per million, respectively; 90% confidence limits ±28). Serum osmolality was moderately lower with Mizone Rapid than with PowerAde and Gatorade (–1.9, –2.4; mOsmol/L; ±1.2 mOsmol/L) but not clearly different vs. placebo. Plasma volume reduction was small to moderate with Mizone Rapid, PowerAde, and Gatorade relative to placebo (–1.9%, –2.5%, –2.9%; ± 2.5%). Gut comfort was highest with Mizone Rapid but clearly different (8.4% ± 4.8%) only vs PowerAde. Peak power was highest with Mizone Rapid (380 W) vs. placebo and other drinks (1.2–3.0%; 99% confidence limits ±4.7%), but differences were inconclusive with reference to the smallest important effect (~1.2%). The outcomes are consistent with fastest fluid absorption with the hypotonic sports drink. Further research should determine whether the effect has a meaningful impact on performance.

Caffeine is regarded as a diuretic despite evidence that hydration is not impaired with habitual ingestion. The purpose of this study was to determine whether a caffeinated sports drink impairs fluid delivery and hydration during exercise in warm, humid conditions (28.5 °C, 60% relative humidity). Sixteen cyclists completed 3 trials: placebo (P), carbohydrate-electrolyte (CE), and caffeinated (195 mg/L) sports drink (CAF+CE). Subjects cycled for 120 min at 60–75%VO_2max followed by 15 min of maximal-effort cycling. Heart rate and rectal temperature were similar until the final 15 min, when these responses and exercise intensity were higher with CAF+CE than with CE and P. Sweat rate, urine output, plasma-volume losses, serum electrolytes, and blood deuterium-oxide accumulation were not different. Serum osmolality was higher with CAF+CE vs. P but not CE. The authors conclude that CAF+CE appears as rapidly in blood as CE and maintains hydration and sustains cardiovascular and thermoregulatory function as well as CE during exercise in a warm, humid environment.

Our purpose was to determine if sports drinks with 6 and 8% CHO differentially affect physiological responses or run performance in the heat. Ten men ran 32 km while ingesting: placebo (P), 6% carbohydrate-electrolyte (CE6), and 8% carbohydrate-electrolyte (CE8). At 15 km, a 250 mL drink labeled with deuterium oxide (D₂O) was ingested. Blood glucose and respiratory exchange ratio were significantly higher (P < 0.05) for CE6 and CE8 compared to P. Rectal temperature (T_re) at 32 km was higher for CE8 (40.1 ± 0.2 °C) compared to P (39.5 ± 0.2 °C) but similar to CE6 (39.8 ± 0.2 °C). D₂O accumulation was not different among drink trials. Run performance was 8% faster for CE8 (1062 ± 31 s) compared to P (1154 ± 56 s) and similar to CE6 (1078 ± 33 s). Confirming the ACSM Position Stand, 8% CE are acceptable during exercise in the heat and attenuate the decline in performance.

Total body water (TBW) and water turnover rates (WTR) of 8 competitive swimmers (SW) and 6 age-matched non-training individuals (CON) were determined using deuterium oxide dilution and elimination. During the 7-day study, individuals in the SW group trained 9 times, swimming on average 42.4 km, while the CON group did no regular exercise. Water temperature in the swimming pool was between 26 and 29 °C during training sessions. Body mass at the beginning and end of the study period remained essentially the same in the SW (67.8 ± 6.3 kg) and CON (61.1 ± 8.5 kg) groups. Mean ± SD TBW of the SW (38.7 ± 5.6 L) was similar to that of the CON (37.5 ± 8:0 L). Mean WTR was faster in the SW (54 ± 18 ml · kg · day⁻¹) than the CON (28 ± 21 ml · kg · day⁻¹). Mean daily urine output was similar in the SW (14 ± 5 ml · kg · day⁻¹) and CON (14 ± 3 ml · kg · day⁻¹). Calculated non-renal daily water loss was faster in the SW (41 ± 21 ml · kg · day⁻¹) than the CON (13 ± 20 ml · kg · day⁻¹). This study demonstrates that WTR are faster in young swimmers who exercise strenuously in cool water than in non-training individuals and that the difference was due to the approximately 3-times greater non-renal water losses that the exercising group incurred. This suggests that exercise-induced increases in sweat rates are a major factor in water loss in swimmers training in cool water.