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Dual energy x-ray absorptiometry (DXA) is a popular tool to determine body composition (BC) in athletes, and is used for analysis of fat-free soft tissue mass (FFST) or fat mass (FM) gain/loss in response to exercise or nutritional interventions. The aim of the current study was to assess the effect of exercise-heat stress induced hypohydration (HYP, >2% of body mass (BM) loss) vs. maintenance of euhydration (EUH) on DXA estimates of BC, sum of skinfolds (SF), and impedance (IMP) measurements in athletes. Competitive athletes (23 males and 15 females) recorded morning nude BM for 7 days before the first main trial. Measurements on the first trial day were conducted in a EUH condition, and again after exercise-heat stress induced HYP. On the second trial day, fluid and electrolyte losses were replaced during exercise using a sports drink. A reduction in total BM (1.6 ± 0.4 kg; 2.3 ± 0.4% HYP) and total FFST (1.3 ± 0.4 kg), mainly from trunk (1.1 ± 0.5 kg), was observed using DXA when participants were HYP, reflecting the sweat loss. Estimated fat percent increased (0.3 ± 0.3%), however, total FM did not change (0.1 ± 0.2 kg). SF and IMP declined with HYP (losses of 1.5 ± 2.9% and 1.6 ± 3% respectively) suggesting FM loss. When EUH was maintained there were no significant changes in BM, DXA estimates, or SF values pre to post exercise, but IMP still declined. We conclude that use of DXA for FFST assessment in athletes must ensure a EUH state, particularly when considering changes associated with nutritional or exercise interventions.

This study investigated the effect of drinking rate on fluid retention of milk and water following exercise-induced dehydration. In Part A, 12 male participants lost 1.9% ± 0.3% body mass through cycle exercise on four occasions. Following exercise, plain water or low-fat milk equal to the volume of sweat lost during exercise was provided. Beverages were ingested over 30 or 90 min, resulting in four beverage treatments: water 30 min, water 90 min, milk 30 min, and milk 90 min. In Part B, 12 participants (nine males and three females) lost 2.0% ± 0.3% body mass through cycle exercise on four occasions. Following exercise, plain water equal to the volume of sweat lost during exercise was provided. Water was ingested over 15 min (DR15), 45 min (DR45), or 90 min (DR90), with either DR15 or DR45 repeated. In both trials, nude body mass, urine volume, urine specific gravity and osmolality, plasma osmolality, and subjective ratings of gastrointestinal symptoms were obtained preexercise and every hour for 3 hr after the onset of drinking. In Part A, no effect of drinking rate was observed on the proportion of fluid retained, but milk retention was greater (p < .01) than water (water 30 min: 57% ± 16%, water 90 min: 60% ± 20%, milk 30 min: 83% ± 6%, and milk 90 min: 85% ± 7%). In Part B, fluid retention was greater in DR90 (57% ± 13%) than DR15 (50% ± 11%, p < .05), but this was within test–retest variation determined from the repeated trials (coefficient of variation: 17%). Within the range of drinking rates investigated the nutrient composition of a beverage has a more pronounced impact on fluid retention than the ingestion rate.

This study systematically examined the influence of carbohydrate (sucrose), sodium, and caffeine on the fluid retention potential of beverages under euhydrated conditions, using the beverage hydration index method. Three cohorts, each of 12 young, healthy, active men, ingested 1 L of beverages containing four different concentrations of a single component (sucrose, sodium, or caffeine) in a double-blind, crossover manner. Urine output was collected for the subsequent 4 hr. Cumulative urine output was lower and net fluid balance was higher after 10 and 20% sucrose beverages than 0 and 5% sucrose beverages (p < .05), and after 27 and 52 mmol/L sodium beverages than 7 and 15 mmol/L sodium beverages (p < .05). No difference in urine output or net fluid balance was apparent following ingestion of caffeine at concentrations of 0–400 mg/L (p = .83). Consequently, the calculated beverage hydration index was greater in beverages with higher sucrose or sodium content, but caffeine had no effect. No difference was observed in arginine vasopressin or aldosterone between any trials. These data highlight that the key drivers promoting differences in the fluid retention potential of beverages when euhydrated are energy density, likely through slowed fluid delivery to the circulation (carbohydrate content effect), or electrolyte content through improved fluid retention (sodium content effect). These data demonstrate that beverage carbohydrate and sodium content influence fluid delivery and retention in the 4 hr after ingestion, but caffeine up to 400 mg/L does not. Athletes and others can use this information to guide their daily hydration practices.