Post-exercise rehydration in man: effects of electrolyte addition to ingested fluids

Evolution of the thirst mechanism in Homo: The need and limitations of thirst and hydration

There is a view that the perception of thirst and actual body fluid balance may affect cognitive and exercise performance. The evolutionary evidence suggests that our survival was dependent on our ability to sweat profusely when hunting during the heat of the day (persistence hunting), so if water deficits were not tolerated, consequently the thirst mechanism would limit our persistence hunting capability. This also means that hunting and searching for water was undertaken with some extent of water deficit, and in turn suggests that performance; physical and cognitive, was conducted with a degree of dehydration. Given the current views on the maintenance of body water for performance, there is a need to evaluate the evidence relating to tolerance limits for water deficits with respect to both physical and cognitive performance. This review considers the thirst mechanism and the conditions and selective pressures under which this might have evolved. Consideration will be given to how the thirst mechanism influences our physical and cognitive performance. The review suggests that Homo developed appropriate tolerances for water deficits and thirst perception, with a safety margin that prevented detrimental declines in physical and cognitive performance to the point of inhibiting corrective action. This would have offered a selective advantage, enabling the search for water and functioning adequately during periods of water scarcity.

Lowering blood pressure by exercise: investigating the effect of sweating

High blood pressure (hypertension), is a common medical condition, affecting millions of people and is associated with significant health risks. Exercise has been suggested to manage hypertension by inducing sweating and the corresponding loss of sodium and water from the body.Thus, a variety of epidemiological and clinical studies have been conducted to investigate the relationship between sweating and exercise-induced blood pressure reduction and its impacts on hypertension. The mechanisms underlying exercise-induced blood pressure reduction are complex and still not fully understood. However, several pathways have been suggested, including the loss of sodium and water through sweat, a decrease in peripheral resistance, and an improvement in endothelial function in the blood vessels. The decrease in sodium and water content in the body associated with sweating may result in a reduction in blood volume and thus a decrease in blood pressure. Moreover, the reduction in peripheral resistance is thought to be mediated by the activation of the nitric oxide synthase pathway and the release of vasodilators such as prostacyclin and bradykinin, which lead to vasodilation and, thus, a reduction in blood pressure. In conclusion, exercise-induced sweating and consequent sodium and water loss appear to be a reliable biological link to the blood pressure-reducing effects of exercise in hypertensive individuals. Additionally, the mechanisms underlying exercise-induced blood pressure reduction are complex and involve several biological pathways in the cardiovascular system. Therefore, understanding the role of sweat production in blood pressure management is important for developing effective exercise interventions to prevent and manage hypertension.

Trends and Missing Links in (De)Hydration Research: A Narrative Review

Despite decades of literature on (de)hydration in healthy individuals, many unanswered questions remain. To outline research and policy priorities, it is fundamental to recognize the literature trends on (de)hydration and identify current research gaps, which herein we aimed to pinpoint. From a representative sample of 180 (de)hydration studies with 4350 individuals, we found that research is mainly limited to small-scale laboratory-based sample sizes, with high variability in demographics (sex, age, and level of competition); to non-ecological (highly simulated and controlled) conditions; and with a focus on recreationally active male adults (e.g., Tier 1, non-athletes). The laboratory-simulated environments are limiting factors underpinning the need to better translate scientific research into field studies. Although, consistently, dehydration is defined as the loss of 2% of body weight, the hydration status is estimated using a very heterogeneous range of parameters. Water is the most researched hydration fluid, followed by alcoholic beverages with added carbohydrates (CHO). The current research still overlooks beverages supplemented with proteins, amino acids (AA), and glycerol. Future research should invest more effort in "real-world" studies with larger and more heterogeneous cohorts, exploring the entire available spectrum of fluids while addressing hydration outcomes more harmoniously.

Compositional Aspects of Beverages Designed to Promote Hydration Before, During, and After Exercise: Concepts Revisited

Hypohydration can impair aerobic performance and deteriorate cognitive function during exercise. To minimize hypohydration, athletes are recommended to commence exercise at least euhydrated, ingest fluids containing sodium during long-duration and/or high-intensity exercise to prevent body mass loss over 2% and maintain elevated plasma osmolality, and rapidly restore and retain fluid and electrolyte homeostasis before a second exercise session. To achieve these goals, the compositions of the fluids consumed are key; however, it remains unclear what can be considered an optimal formulation for a hydration beverage in different settings. While carbohydrate-electrolyte solutions such as sports drinks have been extensively explored as a source of carbohydrates to meet fuel demands during intense and long-duration exercise, these formulas might not be ideal in situations where fluid and electrolyte balance is impaired, such as practicing exercise in the heat. Alternately, hypotonic compositions consisting of moderate to high levels of electrolytes (i.e., ≥45 mmol/L), mainly sodium, combined with low amounts of carbohydrates (i.e., <6%) might be useful to accelerate intestinal water absorption, maintain plasma volume and osmolality during exercise, and improve fluid retention during recovery. Future studies should compare hypotonic formulas and sports drinks in different exercise settings, evaluating different levels of sodium and/or other electrolytes, blends of carbohydrates, and novel ingredients for addressing hydration and rehydration before, during, and after exercise.

Post-Exercise Rehydration in Athletes: Effects of Sodium and Carbohydrate in Commercial Hydration Beverages

The effects of varying sodium (Na) and carbohydrate (CHO) in oral rehydration solutions (ORS) and sports drinks (SD) for rehydration following exercise are unclear. We compared an ORS and SD for the percent of fluid retained (%FR) following exercise-induced dehydration and hypothesized a more complete rehydration for the ORS (45 mmol Na/L and 2.5% CHO) and that the %FR for the ORS and SD (18 mmol Na/L and 6% CHO) would exceed the water placebo (W). A placebo-controlled, randomized, double-blind clinical trial was conducted. To induce 2.6% body mass loss (BML, p > 0.05 between treatments), 26 athletes performed three 90 min interval training sessions without drinking fluids. Post-exercise, participants replaced 100% of BML and were observed for 3.5 h for the %FR. Mean ± SD for the %FR at 3.5 h was 58.1 ± 12.6% (W), 73.9 ± 10.9% (SD), and 76.9 ± 8.0% (ORS). The %FR for the ORS and SD were similar and greater than the W (p < 0.05 ANOVA and Tukey HSD). Two-way ANOVA revealed a significant interaction with the ORS having greater suppression of urine production in the first 60 min vs. W (SD did not differ from W). By 3.5 h, the ORS and SD promoted greater rehydration than did W, but the pattern of rehydration early in recovery favored the ORS.

Post-exercise rehydration: Comparing the efficacy of three commercial oral rehydration solutions

Introduction

This study compared the efficacy of three commercial oral rehydration solutions (ORS) for restoring fluid and electrolyte balance, after exercise-induced dehydration.

Method

Healthy, active participants (N = 20; ♀ = 3; age ∼27 y, V˙O₂peak ∼52 ml/kg/min) completed three randomised, counterbalanced trials whereby intermittent exercise in the heat (∼36°C, ∼50% humidity) induced ∼2.5% dehydration. Subsequently, participants rehydrated (125% fluid loss in four equal aliquots at 0, 1, 2, 3 h) with a glucose-based (G-ORS), sugar-free (Z-ORS) or amino acid-based sugar-free (AA-ORS) ORS of varying electrolyte composition. Urine output was measured hourly and capillary blood samples collected pre-exercise, 0, 2 and 5 h post-exercise. Sodium, potassium, and chloride concentrations in urine, sweat, and blood were determined.

Results

Net fluid balance peaked at 4 h and was greater in AA-ORS (141 ± 155 ml) and G-ORS (101 ± 195 ml) than Z-ORS (-47 ± 208 ml; P ≤ 0.010). Only AA-ORS achieved positive sodium and chloride balance post-exercise, which were greater for AA-ORS than G-ORS and Z-ORS (P ≤ 0.006), as well as for G-ORS than Z-ORS (P ≤ 0.007) from 1 to 5 h.

Conclusion

when provided in a volume equivalent to 125% of exercise-induced fluid loss, AA-ORS produced comparable/superior fluid balance and superior sodium/chloride balance responses to popular glucose-based and sugar-free ORS.

Tracing Acid-Base Variables in Exercising Horses: Effects of Pre-Loading Oral Electrolytes

Oral electrolyte supplementation may influence acid-base state during exercise due to the intestinal absorption of administered water and electrolytes used to mitigating sweat losses. This study examined the effect of pre-exercise electrolyte supplementation (3 and 8 L) on plasma acid-base variables at rest, during moderate intensity exercise and during recovery. It was hypothesized that electrolyte supplementation will result in improved acid-base state compared to the alkalosis typical of prolonged exercise. In randomized crossover fashion, four horses were administered 3 L or 8 L of a hypotonic electrolyte solution (PNW) intended to replace sweat losses, or water alone (CON), 1 h before treadmill exercise to fatigue (at 35% of peak VO2) or for 45 min at 50% peak VO2. Blood was sampled at 10-min intervals before, during and after exercise, and analyzed for dependent and independent acid-base variables. Effects of 3 L of supplementation at low exercise intensities were minimal. In the 8 L trials, plasma [H+] decreased (p < 0.05) during exercise and early recovery in CON but not PNW. Plasma TCO2 decreased (p < 0.05) by 30 min after PNW reaching a nadir of 28.0 ± 1.5 mmol/L during the early exercise period (p = 0.018). Plasma pCO2 and strong ion difference [SID] were the primary contributors to changes in [H+] and [TCO2], respectively. Pre-exercise PNW of 8 L intended to fully replenish sweat loses maintained [H+], decreased [TCO2] and mitigated the mild alkalosis during moderate intensity exercise.

Oral Electrolyte and Water Supplementation in Horses

Horses that sweat for prolonged periods lose considerable amounts of water and electrolytes. Maintenance of hydration and prevention of dehydration requires that water and electrolytes are replaced. Dehydration is common in equine disciplines and can be avoided, thus promoting equine wellness, improved performance and enhanced horse and rider safety. Significant dehydration occurs through exercise or transport lasting one hour or more. Oral electrolyte supplementation is an effective strategy to replace water and electrolytes lost through sweating. The stomach and small intestine serve as a reservoir for uptake of water and electrolytes consumed 1 to 2 h prior to exercise and transport. The small intestine is the primary site of very rapid absorption of ions and water. Water and ions absorbed in the small intestine are taken up by muscles, and also transported via the blood to the skin where they serve to replace or augment the losses of water and ions in the body. Effective electrolyte supplements are designed to replace the proportions of ions lost through sweating; failure to do so can result in electrolyte imbalance. Adequate water must be consumed with electrolytes so as to maintain solution osmolality less than that of body fluids in order to promote gastric emptying and intestinal absorption. The electrolyte supplement should taste good, and horses should be trained to drink the solution voluntarily prior to and during transport, and prior to and after exercise.

The Hydrating Effects of Hypertonic, Isotonic and Hypotonic Sports Drinks and Waters on Central Hydration During Continuous Exercise: A Systematic Meta-Analysis and Perspective

Background

Body-fluid loss during prolonged continuous exercise can impair cardiovascular function, harming performance. Delta percent plasma volume (dPV) represents the change in central and circulatory body-water volume and therefore hydration during exercise; however, the effect of carbohydrate–electrolyte drinks and water on the dPV response is unclear.

Objective

To determine by meta-analysis the effects of ingested hypertonic (> 300 mOsmol kg⁻¹), isotonic (275–300 mOsmol kg⁻¹) and hypotonic (< 275 mOsmol kg⁻¹) drinks containing carbohydrate and electrolyte ([Na⁺] < 50 mmol L⁻¹), and non-carbohydrate drinks/water (< 40 mOsmol kg⁻¹) on dPV during continuous exercise.

Methods

A systematic review produced 28 qualifying studies and 68 drink treatment effects. Random-effects meta-analyses with repeated measures provided estimates of effects and probability of superiority (p₊) during 0–180 min of exercise, adjusted for drink osmolality, ingestion rate, metabolic rate and a weakly informative Bayesian prior.

Results

Mean drink effects on dPV were: hypertonic − 7.4% [90% compatibility limits (CL) − 8.5, − 6.3], isotonic − 8.7% (90% CL − 10.1, − 7.4), hypotonic − 6.3% (90% CL − 7.4, − 5.3) and water − 7.5% (90% CL − 8.5, − 6.4). Posterior contrast estimates relative to the smallest important effect (dPV = 0.75%) were: hypertonic-isotonic 1.2% (90% CL − 0.1, 2.6; p₊ = 0.74), hypotonic-isotonic 2.3% (90% CL 1.1, 3.5; p₊ = 0.984), water-isotonic 1.3% (90% CL 0.0, 2.5; p₊ = 0.76), hypotonic-hypertonic 1.1% (90% CL 0.1, 2.1; p₊ = 0.71), hypertonic-water 0.1% (90% CL − 0.8, 1.0; p₊ = 0.12) and hypotonic-water 1.1% (90% CL 0.1, 2.0; p₊ = 0.72). Thus, hypotonic drinks were very likely superior to isotonic and likely superior to hypertonic and water. Metabolic rate, ingestion rate, carbohydrate characteristics and electrolyte concentration were generally substantial modifiers of dPV.

Conclusion

Hypotonic carbohydrate–electrolyte drinks ingested continuously during exercise provide the greatest benefit to hydration.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s40279-021-01558-y.

The potential nutrition-, physical- and health-related benefits of cow's milk for primary-school-aged children

Cow's milk is a naturally nutrient-dense foodstuff. A significant source of many essential nutrients, its inclusion as a component of a healthy balanced diet has been long recommended. Beyond milk's nutritional value, an increasing body of evidence illustrates cow's milk may confer numerous benefits related to health. Evidence from adult populations suggests that cow's milk may have a role in overall dietary quality, appetite control, hydration and cognitive function. Although evidence is limited compared with the adult literature, these benefits may be echoed in recent paediatric studies. This article, therefore, reviews the scientific literature to provide an evidence-based evaluation of the associated health benefits of cow's milk consumption in primary-school-aged children (4-11 years). We focus on seven key areas related to nutrition and health comprising nutritional status, hydration, dental and bone health, physical stature, cognitive function, and appetite control. The evidence consistently demonstrates cow's milk (plain and flavoured) improves nutritional status in primary-school-aged children. With some confidence, cow's milk also appears beneficial for hydration, dental and bone health and beneficial to neutral concerning physical stature and appetite. Due to conflicting studies, reaching a conclusion has proven difficult concerning cow's milk and cognitive function; therefore, a level of caution should be exercised when interpreting these results. All areas, however, would benefit from further robust investigation, especially in free-living school settings, to verify conclusions. Nonetheless, when the nutritional-, physical- and health-related impact of cow's milk avoidance is considered, the evidence highlights the importance of increasing cow's milk consumption.

Non-Alcoholic Beverages, Old and Novel, and Their Potential Effects on Human Health, with a Focus on Hydration and Cardiometabolic Health

The Beverage Guidance System has established dietary recommendations for daily intake of commonly consumed beverages including water, tea, coffee, milk, non-calorically sweetened beverages, and calorically sweetened beverages. As obesity in America continues to be a growing problem, this guidance becomes of increasing importance due to many beverages' potential links to Type 2 Diabetes Mellitus (T2DM), Cardiovascular disease (CVD), and numerous other harmful health effects. However, the growing popularity of "better for you" beverages is causing a shift in the market, with consumers pushing for healthier beverage alternatives. Beverages simultaneously present advantages while posing concerns that need to be evaluated and considered. In this review, health effects of nonalcoholic beverages are discussed including various aspects of consumption and current trends of the beverage market such as the novel Soft Seltzer category as an alternative to Hard Seltzer and various mashups. A variety of advisory boards and agencies responsible for dietary guidelines in various countries suggest drinking water as the preferred practice for hydration.

Self-selected fluid volume and flavor strength does not alter fluid intake, body mass loss, or physiological strain during moderate-intensity exercise in the heat

INTRODUCTION

The purpose of this study was to determine the effects of ad libitum flavor and fluid intake on changes in body mass (BM) and physiological strain during moderate intensity exercise in the heat.

METHODS

Ten subjects (24±3yrs, 7M/3F) performed 60 min of treadmill walking at 1.3 m/s and 7% grade in an environmental chamber set to 33 °C and 10% relative humidity while carrying a 22.7 kg pack on two different occasions. Subjects consumed either plain water or water plus flavor (Infuze), ad libitum, at each visit. Pre and post exercise, fluid consumption (change in fluid reservoir weight) and BM (nude) were measured. During exercise, heart rate (HR), systolic blood pressure (SBP), rate of perceived exertion (RPE), oxygen consumption (VO₂), respiratory exchange ratio (RER), core temperature (T_C), and physiological strain index (PSI) were recorded every 15 min during exercise.

RESULTS

No significant differences were observed for fluid consumption between fluid conditions (512 ± 97.2 mL water vs. 414.3 ± 62.5 mL Infuze). Despite a significant decrease from baseline, there were no significant differences in overall change of BM (Δ -1.18 vs. -0.64 Kg) or percent body weight loss for water and Infuze conditions, respectively (1.58 ± 0.6 and 0.79 ± 0.2%). Furthermore, there were no significant differences in HR (144 ± 6 vs. 143 ± 8 bpm), SBP (157 ± 5 vs. 155 ± 5 mmHg), RPE, VO₂ (27.4 ± 0.9 vs. 28.1 ± 1.2 ml/Kg/min), RER, T_C (38.1 ± 0.1 vs. 37.0 ± 0.1 °C), and peak PSI (5.4 ± 0.4 vs. 5.7 ± 0.8) between conditions.

CONCLUSIONS

Offering individuals the choice to actively manipulate flavor strength did not significantly influence ad libitum fluid consumption, fluid loss, or physiological strain during 60 min of moderate intensity exercise in the heat.

Impact of Nutrient Intake on Hydration Biomarkers Following Exercise and Rehydration Using a Clustering-Based Approach

We investigated the impact of nutrient intake on hydration biomarkers in cyclists before and after a 161 km ride, including one hour after a 650 mL water bolus consumed post-ride. To control for multicollinearity, we chose a clustering-based, machine learning statistical approach. Five hydration biomarkers (urine color, urine specific gravity, plasma osmolality, plasma copeptin, and body mass change) were configured as raw- and percent change. Linear regressions were used to test for associations between hydration markers and eight predictor terms derived from 19 nutrients merged into a reduced-dimensionality dataset through serial k-means clustering. Most predictor groups showed significant association with at least one hydration biomarker: (1) Glycemic Load + Carbohydrates + Sodium, (2) Protein + Fat + Zinc, (3) Magnesium + Calcium, (4) Pinitol, (5) Caffeine, (6) Fiber + Betaine, and (7) Water; potassium + three polyols, and mannitol + sorbitol showed no significant associations with any hydration biomarker. All five hydration biomarkers were associated with at least one nutrient predictor in at least one configuration. We conclude that in a real-life scenario, some nutrients may serve as mediators of body water, and urine-specific hydration biomarkers may be more responsive to nutrient intake than measures derived from plasma or body mass.

Effects of whey protein in carbohydrate-electrolyte drinks on post-exercise rehydration

The purpose of this study was to examine the effects of different amounts of whey protein in carbohydrate-electrolyte (CE) drinks on post-exercise rehydration. Ten males completed 5 trials in a randomised cross-over design. A 4-h recovery was applied after a 60-min run at 65% VO_2peak in each trial. During recovery, the participants ingested a high-carbohydrate CE drink (CE-H), a low-carbohydrate CE drink (CE-L), a high-whey-protein (33 g·L^-1) CE drink (CW-H), a medium-whey-protein (22 g·L^-1) CE drink (CW-M) or a low-whey-protein (15 g·L^-1) CE drink (CW-L) in a volume equivalent to 150% of their body mass (BM) loss. The drinks were provided in six equal boluses and consumed by the participants within 150 min in each trial. After exercise, a BM loss of 2.15% ± 0.05% was achieved. Urine production was less in the CW-M and CW-H trials during recovery, which induced a greater fluid retention in the CW-M (51.0% ± 5.7%) and CW-H (55.4% ± 3.8%) trials than in any other trial (p < .05). The plasma albumin content was higher in the CW-H trial than in the CE-H and CE-L trials at 2 h (p < .05) and 3 h (p < .01) during recovery. The aldosterone concentration was lower in the CE-H trial than in the CW-M and CW-H trials after recovery (p < .05). It is concluded that the rehydration was improved when whey protein was co-ingested with CE drinks during a 4-h recovery after a 60-min run. However, this additive effect was only observed when whey protein concentration was at least 22 g·L^-1 in the current study.

Normative data on regional sweat-sodium concentrations of professional male team-sport athletes

BACKGROUND

The purpose of this paper was to report normative data on regional sweat sweat-sodium concentrations of various professional male team-sport athletes, and to compare sweat-sodium concentrations among sports. Data to this effect would inform our understanding of athlete sodium requirements, thus allowing for the individualisation of sodium replacement strategies. Accordingly, data from 696 athletes (Soccer, n = 270; Rugby, n = 181; Baseball, n = 133; American Football, n = 60; Basketball, n = 52) were compiled for a retrospective analysis. Regional sweat-sodium concentrations were collected using the pilocarpine iontophoresis method, and compared to self-reported measures collected via questionnaire.

RESULTS

Sweat-sodium concentrations were significantly higher (p < 0.05) in American football (50.4 ± 15.3 mmol·L-1), baseball (54.0 ± 14.0 mmol·L-1), and basketball (48.3 ± 14.0 mmol·L-1) than either soccer (43.2 ± 12.0 mmol·L-1) or rugby (44.0 ± 12.1 mmol·L-1), but with no differences among the N.American or British sports. There were strong positive correlations between sweat-sodium concentrations and self-reported sodium losses in American football (rs = 0.962, p < 0.001), basketball (rs = 0.953, p < 0.001), rugby (rs = 0.813, p < 0.001), and soccer (rs = 0.748, p < 0.001).

CONCLUSIONS

The normative data provided on sweat-sodium concentrations might assist sports science/medicine practitioners in generating bespoke hydration and electrolyte-replacement strategies to meet the sodium demands of professional team-sport athletes. Moreover, these novel data suggest that self-reported measures of sodium loss might serve as an effective surrogate in the absence of direct measures; i.e., those which are more expensive or non-readily available.

Impact of Isotonic Beverage on the Hydration Status of Healthy Chinese Adults in Air-Conditioned Environment

People living in tropical climates spend much of their time in confined air-conditioned spaces, performing normal daily activities. This study investigated the effect of distilled water (W) or isotonic beverage (IB) on the hydration status in subjects living under these conditions. In a randomized crossover design, forty-nine healthy male subjects either consumed beverage or IB over a period of 8 h (8 h) in a controlled air-conditioned environment. Blood, urine, and saliva samples were collected at baseline and after 8 h. Hydration status was assessed by body mass, urine output, blood and plasma volume, fluid retention, osmolality, electrolyte concentration and salivary flow rate. In the IB group, urine output (1862 ± 86 mL vs. 2104 ± 98 mL) was significantly lower and more fluids were retained (17% ± 3% vs. 7% ± 3%) as compared to W (p < 0.05) after 8 h. IB also resulted in body mass gain (0.14 ± 0.06 kg), while W led to body mass loss (-0.04 ± 0.05 kg) (p = 0.01). A significantly smaller drop in blood volume and lower free water clearance was observed in IB (-1.18% ± 0.43%; 0.55 ± 0.26 mL/min) compared to W (-2.11% ± 0.41%; 1.35 ± 0.24 mL/min) (p < 0.05). IB increased salivary flow rate (0.54 ± 0.05 g/min 0.62 ± 0.04 g/min). In indoor environments, performing routine activities and even without excessive sweating, isotonic beverages may be more effective at retaining fluids and maintaining hydration status by up to 10% compared to distilled water.

Optimizing the restoration and maintenance of fluid balance after exercise-induced dehydration

Hypohydration, or a body water deficit, is a common occurrence in athletes and recreational exercisers following the completion of an exercise session. For those who will undertake a further exercise session that day, it is important to replace water losses to avoid beginning the next exercise session hypohydrated and the potential detrimental effects on performance that this may lead to. The aim of this review is to provide an overview of the research related to factors that may affect postexercise rehydration. Research in this area has focused on the volume of fluid to be ingested, the rate of fluid ingestion, and fluid composition. Volume replacement during recovery should exceed that lost during exercise to allow for ongoing water loss; however, ingestion of large volumes of plain water results in a prompt diuresis, effectively preventing longer-term maintenance of water balance. Addition of sodium to a rehydration solution is beneficial for maintenance of fluid balance due to its effect on extracellular fluid osmolality and volume. The addition of macronutrients such as carbohydrate and protein can promote maintenance of hydration by influencing absorption and distribution of ingested water, which in turn effects extracellular fluid osmolality and volume. Alcohol is commonly consumed in the postexercise period and may influence postexercise rehydration, as will the coingestion of food. Future research in this area should focus on providing information related to optimal rates of fluid ingestion, advisable solutions to ingest during different duration recovery periods, and confirmation of mechanistic explanations for the observations outlined.

A metered intake of milk following exercise and thermal dehydration restores whole-body net fluid balance better than a carbohydrate–electrolyte solution or water in healthy young men

Appropriate rehydration and nutrient intake in recovery is a key component of exercise performance. This study investigated whether the recovery of body net fluid balance (NFB) following exercise and thermal dehydration to −2 % of body mass (BM) was enhanced by a metered rate of ingestion of milk (M) compared with a carbohydrate–electrolyte solution (CE) or water (W). In randomised order, seven active men (aged 26·2 (sd 6·1) years) undertook exercise and thermal dehydration to −2 % of BM on three occasions. A metered replacement volume of M, CE or W equivalent to 150 % of the BM loss was then consumed within 2–3 h. NFB was subsequently measured for 5 h from commencement of rehydration. A higher overall NFB in M than CE (P=0·001) and W (P=0·006) was observed, with no difference between CE and W (P=0·69). After 5 h, NFB in M remained positive (+117 (sd 122) ml) compared with basal, and it was greater than W (−539 (sd 390) ml, P=0·011) but not CE (−381 (sd 460) ml, P=0·077, d=1·6). Plasma osmolality (Posm) and K remained elevated above basal in M compared with CE and W. The change in Posm was associated with circulating pre-provasopressin (rs 0·348, P<0·001), a biomarker of arginine vasopressin, but could not account fully for the augmented NFB in M compared with CE and W. These data suggest that a metered approach to fluid ingestion acts in synergy with the nutrient composition of M in the restoration of NFB following exercise and thermal dehydration.

Effects of protein addition to carbohydrate-electrolyte solutions on postexercise rehydration

Background/Objective

This study aimed to examine the effects of the addition of whey or casein protein, the two major proteins in milk, to carbohydrate-electrolyte (CE) solutions on postexercise rehydration.

Methods

Ten young men aged 20.7 ± 1.4 years with an average VO_2max of 60.7 mL/kg/min ran for 60 minutes at 65% VO_2max on three occasions followed by 4 hours' recovery. During recovery, the participants consumed either CE solution with 66 g/L carbohydrate (CHO), or CE plus whey protein solution (CW trial, 44 g/L CHO, 22 g/L whey), or CE plus casein protein solution (CC trial, 44 g/L CHO, 22 g/L casein); the solutions were matched for energy and electrolyte content.

Results

The participants lost 2.36 ± 0.32% of their pre-exercise body weight after the exercise. Total urine output after recovery was greater in the CE and CC trials than CW trial (CE vs. CW vs. CC: 1184 ± 378 mL vs. 1005 ± 214 mL vs. 1256 ± 413 mL; p < 0.05). Fluid retention after ingestion of CW solution was greater than CE and CC solutions (CE vs. CW vs. CC: 46.9 ± 16.5% vs. 54.9 ± 9.2% vs. 45.8 ± 17.3%; p < 0.05). Lower urine specific gravity and urine osmolality were observed by the end of recovery in the CE trial compared with CW trial (p < 0.05). No difference was found in the changes in plasma volume in all trials.

Conclusion

These results suggest that during the 4 hours' recovery after a 60-minute run, the CW solution was more effective for rehydration compared with the CE or CC solution.

Hydration Status over 24-H Is Not Affected by Ingested Beverage Composition

OBJECTIVE

To investigate the 24-h hydration status of healthy, free-living, adult males when given various combinations of different beverage types.

METHODS

Thirty-four healthy adult males participated in a randomized, repeated-measures design in which they consumed: water only (treatment A), water+cola (treatment B), water+diet cola (treatment C), or water+cola+diet cola+orange juice (treatment D) over a sedentary 24-h period across four weeks of testing. Volumes of fluid were split evenly between beverages within each treatment, and when accounting for food moisture content and metabolic water production, total fluid intake from all sources was equal to 35 ± 1 ml/kg body mass. Urine was collected over the 24-h intervention period and analyzed for osmolality (Uosm), volume (Uvol) and specific gravity (USG). Serum osmolality (Sosm) and total body water (TBW) via bioelectrical impedance were measured after the 24-h intervention.

RESULTS

24-h hydration status was not different between treatments A, B, C, and D when assessed via Uosm (590 ± 179; 616 ± 242; 559 ± 196; 633 ± 222 mOsm/kg, respectively) and Uvol (1549 ± 594; 1443 ± 576; 1690 ± 668; 1440 ± 566 ml) (all p > 0.05). A -difference in 24-h USG was observed between treatments A vs. D (1.016 ± 0.005 vs. 1.018 ± 0.007; p = 0.049). There were no differences between treatments at the end of the 24-h with regard to Sosm (291 ± 4; 293 ± 5; 292 ± 5; 293 ± 5 mOsm/kg, respectively) and TBW (43.9 ± 5.9; 43.8 ± 6.0; 43.7 ± 6.1; 43.8 ± 6.0 kg) (all p > 0.05).

CONCLUSIONS

Regardless of the beverage combination consumed, there were no differences in providing adequate hydration over a 24-h period in free-living, healthy adult males. This confirms that beverages of varying composition are equally effective in hydrating the body.

Recovery interventions and strategies for improved tennis performance

Improving the recovery capabilities of the tennis athlete is receiving more emphasis in the research communities, and also by practitioners (coaches, physical trainers, tennis performance specialists, physical therapists, etc). The purpose of this article was to review areas of recovery to limit the severity of fatigue and/or speed recovery from fatigue. This review will cover four broad recovery techniques commonly used in tennis with the belief that the interventions may improve athlete recovery and therefore improve adaptation and future performance. The four areas covered are: (1) temperature-based interventions, (2) compressive clothing, (3) electronic interventions and (4) nutritional interventions.

Optimal composition of fluid-replacement beverages

The objective of this article is to provide a review of the fundamental aspects of body fluid balance and the physiological consequences of water imbalances, as well as discuss considerations for the optimal composition of a fluid replacement beverage across a broad range of applications. Early pioneering research involving fluid replacement in persons suffering from diarrheal disease and in military, occupational, and athlete populations incurring exercise- and/or heat-induced sweat losses has provided much of the insight regarding basic principles on beverage palatability, voluntary fluid intake, fluid absorption, and fluid retention. We review this work and also discuss more recent advances in the understanding of fluid replacement as it applies to various populations (military, athletes, occupational, men, women, children, and older adults) and situations (pathophysiological factors, spaceflight, bed rest, long plane flights, heat stress, altitude/cold exposure, and recreational exercise). We discuss how beverage carbohydrate and electrolytes impact fluid replacement. We also discuss nutrients and compounds that are often included in fluid-replacement beverages to augment physiological functions unrelated to hydration, such as the provision of energy. The optimal composition of a fluid-replacement beverage depends upon the source of the fluid loss, whether from sweat, urine, respiration, or diarrhea/vomiting. It is also apparent that the optimal fluid-replacement beverage is one that is customized according to specific physiological needs, environmental conditions, desired benefits, and individual characteristics and taste preferences.

Postexercise rehydration: potassium-rich drinks versus water and a sports drink

Fluid retention, thirst quenching, tolerance, and palatability of different drinks were assessed. On 4 different days, 12 healthy, physically active volunteers (24.4 ± 3.2 years old, 74.75 ± 11.36 kg body mass (mean ± S.D)), were dehydrated to 2.10% ± 0.24% body mass by exercising in an environmental chamber (32.0 ± 0.4 °C dry bulb, 53.8 ± 5.2% relative humidity). Each day they drank 1 of 4 beverages in random order: fresh coconut water (FCW), bottled water (W), sports drink (SD), or potassium-rich drink (NEW); volume was 120% of weight loss. Urine was collected and perceptions self-reported for 3 h. Urine output was higher (p < 0.05) for W (894 ± 178 mL) than SD (605 ± 297 mL) and NEW (599 ± 254 mL). FCW (686 ± 250 mL) was not different from any other drink (p > 0.05). Fluid retention was higher for SD than W (68.2% ± 13.0% vs. 51.3% ± 12.6%, p = 0.013), but not for FCW and NEW (62.5% ± 15.4% and 65.9% ± 15.4%, p > 0.05). All beverages were palatable and well tolerated; none maintained a positive net fluid balance after 3 h, but deficit was greater in W versus SD (p = 0.001). FCW scored higher for sweetness (p = 0.03). Thirst increased immediately after exercise but returned to baseline after drinking a small volume (p < 0.0005). In conclusion, additional potassium in FCW and NEW did not result in additional rehydration benefits over those already found in a conventional sports drink with sodium.

An amino acid-electrolyte beverage may increase cellular rehydration relative to carbohydrate-electrolyte and flavored water beverages

BACKGROUND

In cases of dehydration exceeding a 2% loss of body weight, athletic performance can be significantly compromised. Carbohydrate and/or electrolyte containing beverages have been effective for rehydration and recovery of performance, yet amino acid containing beverages remain unexamined. Therefore, the purpose of this study is to compare the rehydration capabilities of an electrolyte-carbohydrate (EC), electrolyte-branched chain amino acid (EA), and flavored water (FW) beverages.

METHODS

Twenty men (n = 10; 26.7 ± 4.8 years; 174.3 ± 6.4 cm; 74.2 ± 10.9 kg) and women (n = 10; 27.1 ± 4.7 years; 175.3 ± 7.9 cm; 71.0 ± 6.5 kg) participated in this crossover study. For each trial, subjects were dehydrated, provided one of three random beverages, and monitored for the following three hours. Measurements were collected prior to and immediately after dehydration and 4 hours after dehydration (3 hours after rehydration) (AE = -2.5 ± 0.55%; CE = -2.2 ± 0.43%; FW = -2.5 ± 0.62%). Measurements collected at each time point were urine volume, urine specific gravity, drink volume, and fluid retention.

RESULTS

No significant differences (p > 0.05) existed between beverages for urine volume, drink volume, or fluid retention for any time-point. Treatment x time interactions existed for urine specific gravity (USG) (p < 0.05). Post hoc analysis revealed differences occurred between the FW and EA beverages (p = 0.003) and between the EC and EA beverages (p = 0.007) at 4 hours after rehydration. Wherein, EA USG returned to baseline at 4 hours post-dehydration (mean difference from pre to 4 hours post-dehydration = -0.0002; p > 0.05) while both EC (-0.0067) and FW (-0.0051) continued to produce dilute urine and failed to return to baseline at the same time-point (p < 0.05).

CONCLUSION

Because no differences existed for fluid retention, urine or drink volume at any time point, yet USG returned to baseline during the EA trial, an EA supplement may enhance cellular rehydration rate compared to an EC or FW beverage in healthy men and women after acute dehydration of around 2% body mass loss.

Effect of varying the concentrations of carbohydrate and milk protein in rehydration solutions ingested after exercise in the heat

The present study investigated the relationship between the milk protein content of a rehydration solution and fluid balance after exercise-induced dehydration. On three occasions, eight healthy males were dehydrated to an identical degree of body mass loss (BML, approximately 1·8 %) by intermittent cycling in the heat, rehydrating with 150 % of their BML over 1 h with either a 60 g/l carbohydrate solution (C), a 40 g/l carbohydrate, 20 g/l milk protein solution (CP20) or a 20 g/l carbohydrate, 40 g/l milk protein solution (CP40). Urine samples were collected pre-exercise, post-exercise, post-rehydration and for a further 4 h. Subjects produced less urine after ingesting the CP20 or CP40 drink compared with the C drink (P< 0·01), and at the end of the study, more of the CP20 (59 (sd 12) %) and CP40 (64 (sd 6) %) drinks had been retained compared with the C drink (46 (sd 9) %) (P< 0·01). At the end of the study, whole-body net fluid balance was more negative for trial C ( − 470 (sd 154) ml) compared with both trials CP20 ( − 181 (sd 280) ml) and CP40 ( − 107 (sd 126) ml) (P< 0·01). At 2 and 3 h after drink ingestion, urine osmolality was greater for trials CP20 and CP40 compared with trial C (P< 0·05). The present study further demonstrates that after exercise-induced dehydration, a carbohydrate–milk protein solution is better retained than a carbohydrate solution. The results also suggest that high concentrations of milk protein are not more beneficial in terms of fluid retention than low concentrations of milk protein following exercise-induced dehydration.

Whey protein addition to a carbohydrate-electrolyte rehydration solution ingested after exercise in the heat

CONTEXT

Many active people finish exercise hypohydrated, so effective rehydration after exercise is an important consideration.

OBJECTIVE

To determine the effects of a rehydration solution containing whey protein isolate on fluid balance after exercise-induced dehydration.

DESIGN

Randomized controlled clinical trial.

SETTING

University research laboratory.

PATIENTS OR OTHER PARTICIPANTS

Twelve healthy men (age = 21 ± 1 years, height = 1.82 ± 0.08 m, mass = 82.71 ± 10.31 kg) participated.

INTERVENTION(S)

Participants reduced body mass by 1.86% ± 0.07% after intermittent exercise in the heat and rehydrated with a volume of drink in liters equivalent to 1.5 times their body mass loss in kilograms of a solution of either 65 g/L carbohydrate (trial C) or 50 g/L carbohydrate and 15 g/L whey protein isolate (trial CPl. Solutions were matched for energy density and electrolyte content. Urine samples were collected before and after exercise and for 4 hours after rehydration.

MAIN OUTCOME MEASURE(S)

We measured urine volume, drink retention, net fluid balance, urine osmolality, and subjective responses. Drink retention was calculated as the difference between the volume of drink ingested and urine produced. Net fluid balance was calculated from fluid gained through drink ingestion and fluid lost through sweat and urine production.

RESULTS

Total cumulative urine output after rehydration was not different between trial C (1173 ± 481 mL) and trial CP (1180 ± 330 mL) (F(1) = 0.002, P = .96), and drink retention during the study also was not different between trial C (50% ± 18%) and trial CP (49% ± 13%) (t(11) = -0.159, P =.88). At the end of the study, net fluid balance was negative compared with baseline for trial C (-432 ± 436 mL) (t(11) = 3.433, P = .03) and trial CP (-432 ± 302 mL) (t(11) = 4.958, P = .003).

CONCLUSIONS

When matched for energy density and electrolyte content, a solution of carbohydrate and whey protein isolate neither increased nor decreased rehydration compared with a solution of carbohydrate.

Effects of rehydration fluid temperature and composition on body weight retention upon voluntary drinking following exercise-induced dehydration

The purpose of this study was to determine the effects of beverage temperature and composition on weight retention and fluid balance upon voluntary drinking following exercise induced-dehydration. Eight men who were not acclimated to heat participated in four randomly ordered testing sessions. In each session, the subjects ran on a treadmill in a chamber maintained at 37℃ without being supplied fluids until 2% body weight reduction was reached. After termination of exercise, they recovered for 90 min under ambient air conditions and received one of the following four test beverages: 10℃ water (10W), 10℃ sports drink (10S), 26℃ water (26W), and 26℃ sports drink (26S). They consumed the beverages ad libitum. The volume of beverage consumed and body weight were measured at 30, 60, and 90 min post-recovery. Blood samples were taken before and immediately after exercise as well as at the end of recovery in order to measure plasma parameters and electrolyte concentrations. We found that mean body weight decreased by 1.8-2.0% following exercise. No differences in mean arterial pressure, plasma volume, plasma osmolality, and blood electrolytes were observed among the conditions. Total beverage volumes consumed were 1,164 ± 388, 1,505 ± 614, 948 ± 297, and 1,239 ± 401 ml for 10W, 10S, 26W, and 26S respectively (P > 0.05). Weight retention at the end of recovery from dehydration was highest in 10S (1.3 ± 0.7 kg) compared to 10W (0.4 ± 0.5 kg), 26W (0.4 ± 0.4 kg), and (0.6 ± 0.4 kg) (P < 0.005). Based on these results, carbohydrate/electrolyte-containing beverages at cool temperature were the most favorable for consumption and weight retention compared to plain water and moderate temperature beverages.

Effect of milk protein addition to a carbohydrate–electrolyte rehydration solution ingested after exercise in the heat

The present study examined the effects of milk protein on rehydration after exercise in the heat, via the comparison of energy- and electrolyte content-matched carbohydrate and carbohydrate–milk protein solutions. Eight male subjects lost 1·9 (sd 0·2) % of their body mass by intermittent exercise in the heat and rehydrated with 150 % of their body mass loss with either a 65 g/l carbohydrate solution (trial C) or a 40 g/l carbohydrate, 25 g/l milk protein solution (trial CP). Urine samples were collected before and after exercise and for 4 h after rehydration. Total cumulative urine output after rehydration was greater for trial C (1212 (sd 310) ml) than for trial CP (931 (sd 254) ml) (P < 0·05), and total fluid retention over the study was greater after ingestion of drink CP (55 (sd 12) %) than that after ingestion of drink C (43 (sd 15) %) (P < 0·05). At the end of the study period, whole body net fluid balance (P < 0·05) was less negative for trial CP ( − 0·26 (sd 0·27) litres) than for trial C ( − 0·52 (sd 0·30) litres), and although net negative for both the trials, it was only significantly negative after ingestion of drink C (P < 0·05). The results of the present study suggest that when matched for energy density and fat content, as well as for Na and K concentration, and when ingested after exercise-induced dehydration, a carbohydrate–milk protein solution is better retained than a carbohydrate solution. These results suggest that gram-for-gram, milk protein is more effective at augmenting fluid retention than carbohydrate.

Nutritional aspects of post exercise skeletal muscle glycogen synthesis in horses: a comparative review

Carbohydrate (CHO) stored in the form of skeletal muscle glycogen is the main energy source for glycolytic and oxidative ATP production during vigorous exercise in mammals. In man, horse and dog both short-term high intensity and prolonged submaximal exercise deplete muscle glycogen. In horses, however, muscle glycogen synthesis is 2-3-fold slower than in man and rat, even when a diet high in soluble CHO is fed. There appear to be significant differences in CHO and glycogen metabolism between horses and other mammals, and it is becoming increasingly clear that many conclusions drawn from human exercise physiology do not apply to horses. This review aims to provide a comprehensive, comparative summary of the research on muscle glycogen synthesis in horse, man and rodent. Species differences in CHO uptake and utilisation are examined and the issues with feeding high soluble CHO diets to horses are discussed. Alternative feeding strategies, including protein and long and short chain fatty acid supplementation and the importance of rehydration, are explored.

Effect of preexercise electrolyte ingestion on fluid balance in men and women

PURPOSE

This article aimed to study the effect of preexercise ingestion of an electrolyte-containing beverage and meal on fluid balance during exercise in men and women.

METHODS

Twenty healthy, college-aged people (10 males, 10 females; mean +/- SD = 51.2 +/- 9.8 mL x kg x min(-1)) exercised at 58 +/- 4% V O 2 peak for 90 min, 45 min after ingesting 355 mL of chicken noodle soup (SOUP; 167 mmol x L(-1) Na +), carbohydrate-electrolyte beverage (CE; 16 mmol x L(-1) Na+), or water (WATER). After 90 min of exercise, participants completed a physical performance task (PPT) consisting of the calculated work that would be completed in 30 min at 60% V O 2 peak (n = 19). Water was allowed ad libitum throughout all trials.

RESULTS

Fluid balance was improved in SOUP compared with WATER (-251 +/- 418 vs -657 +/- 593 g, respectively; P = 0.002) because of greater water intake and retention throughout the trial. Water intake was also greater in CE compared with WATER mostly because of an increase during the PPT. Plasma osmolality increased after ingestion of SOUP and remained elevated throughout exercise compared with both CE and WATER. Men and women had similar fluid balance results, with women having lower relative water intake and evaporative water losses compared with men. Physical performance was similar in all trials.

CONCLUSIONS

SOUP ingested before exercise improves fluid balance because of increased ad libitum water intake and reduced proportional urinary water loss. The increase in water intake and, subsequently, the improved fluid balance may be because of a greater plasma osmolality before and throughout exercise.

Carbohydrate exerts a mild influence on fluid retention following exercise-induced dehydration

Rapid and complete rehydration, or restoration of fluid spaces, is important when acute illness or excessive sweating has compromised hydration status. Many studies have investigated the effects of graded concentrations of sodium and other electrolytes in rehydration solutions; however, no study to date has determined the effect of carbohydrate on fluid retention when electrolyte concentrations are held constant. The purpose of this study was to determine the effect of graded levels of carbohydrate on fluid retention following exercise-induced dehydration. Fifteen heat-acclimatized men exercised in the heat for 90 min with no fluid to induce 2–3% dehydration. After a 30-min equilibration period, they received, over the course of 60 min, one of five test beverages equal to 100% of the acute change in body mass. The experimental beverages consisted of a flavored placebo with no electrolytes (P), placebo with electrolytes (P + E), 3%, 6%, and 12% carbohydrate solutions with electrolytes. All beverages contained the same type and concentration of electrolytes (18 meq/l Na+, 3 meq/l K+, 11 meq/l Cl−). Subjects voided their bladders at 60, 90, 120, 180, and 240 min, and urine specific gravity and urine volume were measured. Blood samples were taken before exercise and 30, 90, 180, and 240 min following exercise and were analyzed for glucose, sodium, hemoglobin, hematocrit, renin, aldosterone, and osmolality. Body mass was measured before and after exercise and a final body mass was taken at 240 min. There were no differences in percent dehydration, sweat loss, or fluid intake between trials. Fluid retention was significantly greater for all carbohydrate beverages compared with P (66.3 ± 14.4%). P + E (71.8 ± 9.9%) was not different from water, 3% (75.4 ± 7.8%) or 6% (75.4 ± 16.4%) but was significantly less than 12% (82.4 ± 9.2%) retention of the ingested fluid. No difference was found between the carbohydrate beverages. Carbohydrate at the levels measured exerts a mild influence on fluid retention in postexercise recovery.

Postexercise rehydration in man: the effects of osmolality and carbohydrate content of ingested drinks

OBJECTIVE

This study investigated the effect of the osmolality and carbohydrate content of drinks on their rehydration effectiveness after exercise-induced dehydration.

METHODS

Six healthy male volunteers were dehydrated by 1.9+/-0.1% of body mass by intermittent cycle ergometer exercise in the heat before ingesting one of three solutions with different carbohydrate contents and osmolalities over a period of 1h. Thirty minutes after the cessation of exercise, subjects drank a volume that amounted to 150% (130-150, median [range]) of their body mass loss. Drinks contained 25 mmol/L Na(+) and 0%, 2%, or 10% glucose with osmolalities of (mean+/-SD) 79+/-4, 193+/-5, and 667+/-12 mosm/kg, respectively. Blood and urine samples were collected before exercise, after exercise, and 0, 1, 2, 3, 4, and 6h after the end of the rehydration period.

RESULTS

Significantly more of the ingested fluid was retained in the 10% trial (46+/-9%) than in the 0% trial (27+/-13%), with 40+/-14% retained in the 2% trial. Subjects remained euhydrated for 1h longer in the 10% glucose trial than in the 2% glucose trial. In the 2% glucose trial, plasma volume was elevated immediately after and 1h after rehydration.

CONCLUSION

This study suggests that, following the rehydration protocol used, hypertonic glucose-sodium drinks may be more effective at restoring and maintaining hydration status after sweat loss than more dilute solutions when the sodium concentration is comparable.

Fluid and electrolyte supplementation after prolonged moderate-intensity exercise enhances muscle glycogen resynthesis in Standardbred horses

We hypothesized that postexercise rehydration using a hypotonic electrolyte solution will increase the rate of recovery of whole body hydration, and that this is associated with increased muscle glycogen and electrolyte recovery in horses. Gluteus medius biopsies and jugular venous blood were sampled from six exercise-conditioned Standardbreds on two separate occasions, at rest and for 24 h following a competitive exercise test (CET) designed to simulate the speed and endurance test of a 3-day event. After the CETs, horses were given water ad libitum, and either a hypotonic commercial electrolyte solution (electrolyte) via nasogastric tube, followed by a typical hay/grain meal, or a hay/grain meal alone (control). The CET resulted in decreased total body water and muscle glycogen concentration of 8.4 ± 0.3 liters and 22.6%, respectively, in the control treatment, and 8.2 ± 0.4 liters and 21.9% in the electrolyte treatment. Electrolyte resulted in an enhanced rate of muscle glycogen resynthesis and faster restoration of hydration (as evidenced by faster recovery of plasma protein concentration, maintenance of plasma osmolality, and greater muscle intracellular fluid volume) during the recovery period compared with control. There were no differences in muscle Na, K, Cl, or Mg contents between the two treatments. It is concluded that oral administration of a hypotonic electrolyte solution after prolonged moderate-intensity exercise enhanced the rate of muscle glycogen resynthesis during the recovery period compared with control. It is speculated that postexercise dehydration may be one key contributor to the slow muscle glycogen replenishment in horses.

Milk as an effective post-exercise rehydration drink

The effectiveness of low-fat milk, alone and with an additional 20 mmol/l NaCl, at restoring fluid balance after exercise-induced hypohydration was compared to a sports drink and water. After losing 1.8 (sd 0.1) % of their body mass during intermittent exercise in a warm environment, eleven subjects consumed a drink volume equivalent to 150 % of their sweat loss. Urine samples were collected before and for 5 h after exercise to assess fluid balance. Urine excretion over the recovery period did not change during the milk trials whereas there was a marked increase in output between 1 and 2 h after drinking water and the sports drink. Cumulative urine output was less after the milk drinks were consumed (611 (sd 207) and 550 (sd 141) ml for milk and milk with added sodium, respectively, compared to 1184 (sd 321) and 1205 (sd 142) ml for the water and sports drink; P < 0.001). Subjects remained in net positive fluid balance or euhydrated throughout the recovery period after drinking the milk drinks but returned to net negative fluid balance 1 h after drinking the other drinks. The results of the present study suggest that milk can be an effective post-exercise rehydration drink and can be considered for use after exercise by everyone except those individuals who have lactose intolerance.

Fluid and electrolyte needs for preparation and recovery from training and competition

For a person undertaking regular exercise, any fluid deficit that is incurred during one exercise session can potentially compromise the next exercise session if adequate fluid replacement does not occur. Fluid replacement after exercise can, therefore, frequently be thought of as hydration before the next exercise bout. The importance of ensuring euhydration before exercise and the potential benefits of temporary hyperhydration with sodium salts or glycerol solutions are also important issues. Post-exercise restoration of fluid balance after sweat-induced dehydration avoids the detrimental effects of a body water deficit on physiological function and subsequent exercise performance. For effective restoration of fluid balance, the consumption of a volume of fluid in excess of the sweat loss and replacement of electrolyte, particularly sodium, losses are essential. Intravenous fluid replacement after exercise has been investigated to a lesser extent and its role for fluid replacement in the dehydrated but otherwise well athlete remains equivocal.