Effects of ingesting 6% and 12% glucose/electrolyte beverages during prolonged intermittent cycling in the heat

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.

'I think I'm gonna hurl': A Narrative Review of the Causes of Nausea and Vomiting in Sport

Exercise-associated gastrointestinal (GI) distress can negatively impact athletic performance and interfere with exercise training. Although there are a few universal underlying causes of GI distress, each symptom often has its own unique triggers and, therefore, its own prevention and management strategies. One of the most troubling GI symptoms an athlete can experience during training and competition is nausea/vomiting. The prevalence of nausea varies with several factors, two of the most important being exercise intensity and duration. Relatively brief, high-intensity exercise (e.g., sprinting, tempo runs) and ultra-endurance exercise are both associated with more frequent and severe nausea. The potential causes of nausea in sport are numerous and can include catecholamine secretion, hypohydration, heat stress, hyponatremia, altitude exposure, excessive fluid/food consumption, hypertonic beverage intake, pre-exercise intake of fatty- or protein-rich foods (especially in close proximity to exercise), prolonged fasting, various supplements (caffeine, sodium bicarbonate, ketones), certain drugs (antibiotics, opioids), GI infections, and competition-related anxiety. Beyond directly addressing these aforementioned causes, antiemetic drugs (e.g., ondansetron) may also be useful for alleviating nausea in some competitive situations. Given the commonness of nausea in sport and its potential impact on exercise performance, athletes and sports medicine practitioners should be aware of the origins of nausea and strategies for dealing with this troublesome gut complaint.

Water Intake, Water Balance, and the Elusive Daily Water Requirement

Water is essential for metabolism, substrate transport across membranes, cellular homeostasis, temperature regulation, and circulatory function. Although nutritional and physiological research teams and professional organizations have described the daily total water intakes (TWI, L/24h) and Adequate Intakes (AI) of children, women, and men, there is no widespread consensus regarding the human water requirements of different demographic groups. These requirements remain undefined because of the dynamic complexity inherent in the human water regulatory network, which involves the central nervous system and several organ systems, as well as large inter-individual differences. The present review analyzes published evidence that is relevant to these issues and presents a novel approach to assessing the daily water requirements of individuals in all sex and life-stage groups, as an alternative to AI values based on survey data. This empirical method focuses on the intensity of a specific neuroendocrine response (e.g., plasma arginine vasopressin (AVP) concentration) employed by the brain to regulate total body water volume and concentration. We consider this autonomically-controlled neuroendocrine response to be an inherent hydration biomarker and one means by which the brain maintains good health and optimal function. We also propose that this individualized method defines the elusive state of euhydration (i.e., water balance) and distinguishes it from hypohydration. Using plasma AVP concentration to analyze multiple published data sets that included both men and women, we determined that a mild neuroendocrine defense of body water commences when TWI is ˂1.8 L/24h, that 19⁻71% of adults in various countries consume less than this TWI each day, and consuming less than the 24-h water AI may influence the risk of dysfunctional metabolism and chronic diseases.

Sago supplementation for exercise performed in a thermally stressful environment: Rationale, efficacy and opportunity

Sago (Metroxylin sagu), a carbohydrate (CHO) based dietary staple of Southeast Asia is easily digestible and quickly absorbed, and thus has potential to be prescribed as an affordable pre-and post-exercise food in this part of the world. Compared to other CHO staples, research into the physiological response to sago ingestion is sparse, and only a few recent studies have investigated its value before, during, and after exercise. The purpose of this review is to describe the published literature pertaining to sago, particularly as a supplement in the peri-exercise period, and suggest further avenues of research, principally in an environment/climate which would be experienced in Southeast Asia i.e. hot/humid.

Glucose-fructose enhances performance versus isocaloric, but not moderate, glucose

PURPOSE

The effects of glucose-and-fructose (GF) coingestion on cycling time trial (TT) performance and physiological responses to exercise were examined under postprandial conditions.

METHODS

Eight trained male cyclists (age, 25 ± 6 yr; height, 180 ± 4 cm; weight, 77 ± 9 kg; V˙O2max, 62 ± 6 mL·kg·min) completed the study. Subjects ingested either an artificially sweetened placebo (PL), a moderate-glucose beverage (MG, 1.03 g·min), a high-glucose beverage (HG, 1.55 g·min), or a GF beverage (1.55 g·min, 2:1 ratio) during approximately 3 h of exercise, including 2 h of constant-load cycling (55% Wmax, 195 ± 17 W), immediately followed by a computer-simulated 30-km TT. Physiological responses (V˙E, V˙O2, RER, HR, blood glucose level, blood lactate level, and RPE) and incidences of gastrointestinal distress were assessed during early (15-20 min), middle (55-60 min), and late exercise (115-120 min) and during the TT. Magnitude-based qualitative inferences were used to evaluate differences between treatments.

RESULTS

In comparison with that in PL (52.9 ± 3.7 min), TT performances were faster with GF (50.4 ± 2.2 min, "very likely" benefit), MG (51.1 ± 2.4 min, "likely" benefit), and HG (52.0 ± 3.7 min, "possible" benefit). GF resulted in a "likely" improvement versus HG (3.0%) and an "unclear" effect relative to MG (1.2%). MG was "possibly" beneficial versus HG (1.8%). Few incidences of GI distress were reported in any trials.

CONCLUSIONS

GF ingestion seems to enhance performance, relative to PL and HG. However, it is unclear whether GF improves performance versus moderate doses of glucose.

Energy drink usage among university students in a Caribbean country: Patterns of use and adverse effects

Objective: There has been little inquiry addressing whether or not concerns about adverse effects of energy drink usage are relevant in the Caribbean. This survey investigated energy drink usage and adverse consequences among tertiary level students in Trinidad and Tobago.

Methods: A cross-sectional survey of 1994 students from eight institutions was conducted using a de novo questionnaire based on findings from a focus group of students. Chi-squared analyses and logistic regression were used to assess relationships between energy drink usage, adverse effects and other factors affecting energy drink use, and to verify predictors of energy drink use.

Results: Prevalence of use was 86%; 38% were current users. Males were more likely to use, used more frequently and at an earlier age. Energy drinks were used most commonly to increase energy (50%), combat sleepiness (45%) and enhance academic performance (40%), and occurred during sports (23%) and mixed with alcohol (22.2%). The majority (79.6%) consumed one energy drink per sitting; 62.2% experienced adverse effects, most commonly restlessness (22%), jolt and crash (17.1%) and tachycardia (16.6%). Awareness of adverse effects was associated with no use (p = 0.004), but adverse effects were not a deterrent to continued use.

Conclusion: Energy drink usage is prevalent among students. The use is not excessive, but associated with high rates of adverse effects and occurs in potentially dangerous situations like during exercise and with alcohol. There is a need to educate students about the potential adverse effects of energy drinks.

Do glucose containing beverages play a role in thermoregulation, thermal sensation, and mood state?

Introduction

Dehydration limits the appropriate delivery of oxygen and substrates to the working muscle. Further, the brain’s ability to function may also be compromised whereby thermal sensation and mood state may be altered.

Purpose

The purpose of the present investigation was to compare the thermoregulatory, perceptual, and negative mood state profile in glucose (GLU) vs. non-glucose beverage (NON-GLU) condition.

Methods

Ten healthy men volunteered and were counterbalanced either a GLU or NON-GLU containing beverage on separate mornings. In each condition, they were exposed to 37°C, 50% relative humidity (RH) for baseline, exercise, rehydration, and recovery periods. The exercise period elicited the desired level of dehydration (mean of 2.6 ± 0.3% body weight losses). Upon completion of the protracted exercise, participants were administered either a GLU or NON-GLU containing electrolyte based sports drink ad libitum for 30 min, followed by a recovery period of 15 min in 37°C, 50% RH. Rectal (Tre) and mean skin temperatures (Tsk) were continuously monitored. Gagge (TS) and heated thermal sensation (HTS), profile of mood state (POMS) were measure at the end of each period.

Results

During recovery after rehydration, Tre was not significantly different between conditions (GLU vs. NON-GLU) (37.4 ± 0.8 vs. 37.0 ± 1.2°C); Tsk was also not affected by rehydration in both conditions (36.0 ± 0.5 vs. 36.0 ± 0.6°C) and, TS and HTS did not differ between conditions (0.9 ± 1.3 vs.1.3 ± 0.7) and (1.0 ± 0.8 vs.0.8 ± 0.3). Total mood disturbance (TMD) score for the POMS was utilized for overall negative mood state and demonstrated a main effect for time (p < 0.05). TMD during recovery was decreased compared to before hydration in both conditions.

Conclusion

The non-glucose containing beverage maintained plasma volume and was effective at maintaining body temperature homeostasis in a similar fashion compared to the glucose containing beverage. Furthermore, negative mood state was not different between the two conditions. The non-glucose beverages can serve a valuable role in the exercise environment depending upon the sport, the ambient temperature, the individual, duration of the exercise, the age and training states of the individual.

Systematic review: Carbohydrate supplementation on exercise performance or capacity of varying durations

This systematic review examines the efficacy of carbohydrate (CHO) supplementation on exercise performance of varying durations. Included studies utilized an all-out or endurance-based exercise protocol (no team-based performance studies) and featured randomized interventions and placebo (water-only) trial for comparison against exclusively CHO trials (no other ingredients). Of the 61 included published performance studies (n = 679 subjects), 82% showed statistically significant performance benefits (n = 50 studies), with 18% showing no change compared with placebo. There was a significant (p = 0.0036) correlative relationship between increasing total exercise time and the subsequent percent increase in performance with CHO intake versus placebo. While not mutually exclusive, the primary mechanism(s) for performance enhancement likely differs depending on the duration of the exercise. In short duration exercise situations (∼1 h), oral receptor exposure to CHO, via either mouthwash or oral consumption (with enough oral contact time), which then stimulates the pleasure and reward centers of the brain, provide a central nervous system-based mechanism for enhanced performance. Thus, the type and (or) amount of CHO and its ability to be absorbed and oxidized appear completely irrelevant to enhancing performance in short duration exercise situations. For longer duration exercise (>2 h), where muscle glycogen stores are stressed, the primary mechanism by which carbohydrate supplementation enhances performance is via high rates of CHO delivery (>90 g/h), resulting in high rates of CHO oxidation. Use of multiple transportable carbohydrates (glucose:fructose) are beneficial in prolonged exercise, although individual recommendations for athletes should be tailored according to each athlete's individual tolerance.

Composite versus single transportable carbohydrate solution enhances race and laboratory cycling performance

When ingested at high rates (1.8-2.4 g·min(-1)) in concentrated solutions, carbohydrates absorbed by multiple (e.g., fructose and glucose) vs. single intestinal transporters can increase exogenous carbohydrate oxidation and endurance performance, but their effect when ingested at lower, more realistic, rates during intermittent high-intensity endurance competition and trials is unknown. Trained cyclists participated in two independent randomized crossover investigations comprising mountain-bike races (average 141 min; n = 10) and laboratory trials (94-min high-intensity intervals followed by 10 maximal sprints; n = 16). Solutions ingested during exercise contained electrolytes and fructose + maltodextrin or glucose + maltodextrin in 1:2 ratio ingested, on average, at 1.2 g carbohydrate·kg(-1)·h(-1). Exertion, muscle fatigue, and gastrointestinal discomfort were recorded. Data were analysed using mixed models with gastrointestinal discomfort as a mechanism covariate; inferences were made against substantiveness thresholds (1.2% for performance) and standardized difference. The fructose-maltodextrin solution substantially reduced race time (-1.8%; 90% confidence interval = ±1.8%) and abdominal cramps (-8.1 on a 0-100 scale; ±6.6). After accounting for gastrointestinal discomfort, the effect of the fructose-maltodextrin solution on lap time was reduced (-1.1%; ±2.4%), suggesting that gastrointestinal discomfort explained part of the effect of fructose-maltodextrin on performance. In the laboratory, mean sprint power was enhanced (1.4%; ±0.8%) with fructose-maltodextrin, but the effect on peak power was unclear (0.7%; ±1.5%). Adjusting out gastrointestinal discomfort augmented the fructose-maltodextrin effect on mean (2.6%; ±1.9%) and peak (2.5%; ±3.0%) power. Ingestion of multiple transportable vs. single transportable carbohydrates enhanced mountain-bike race and high-intensity laboratory cycling performance, with inconsistent but not irreconcilable effects of gut discomfort as a possible mediating mechanism.

Carbohydrate administration and exercise performance: what are the potential mechanisms involved?

It is well established that carbohydrate (CHO) administration increases performance during prolonged exercise in humans and animals. The mechanism(s), which could mediate the improvement in exercise performance associated with CHO administration, however, remain(s) unclear. This review focuses on possible underlying mechanisms that could explain the increase in exercise performance observed with the administration of CHO during prolonged muscle contractions in humans and animals. The beneficial effect of CHO ingestion on performance during prolonged exercise could be due to several factors including (i) an attenuation in central fatigue; (ii) a better maintenance of CHO oxidation rates; (iii) muscle glycogen sparing; (iv) changes in muscle metabolite levels; (v) reduced exercise-induced strain; and (vi) a better maintenance of excitation-contraction coupling. In general, the literature indicates that CHO ingestion during exercise does not reduce the utilization of muscle glycogen. In addition, data from a meta-analysis suggest that a dose-dependent relationship was not shown between CHO ingestion during exercise and an increase in performance. This could support the idea that providing enough CHO to maintain CHO oxidation during exercise may not always be associated with an increase in performance. Emerging evidence from the literature shows that increasing neural drive and attenuating central fatigue may play an important role in increasing performance during exercise with CHO supplementation. In addition, CHO administration during exercise appears to provide protection from disrupted cell homeostasis/integrity, which could translate into better muscle function and an increase in performance. Finally, it appears that during prolonged exercise when the ability of metabolism to match energy demand is exceeded, adjustments seem to be made in the activity of the Na+/K+ pump. Therefore, muscle fatigue could be acting as a protective mechanism during prolonged contractions. This could be alleviated when CHO is administered resulting in the better maintenance of the electrical properties of the muscle fibre membrane. The mechanism(s) by which CHO administration increases performance during prolonged exercise is(are) complex, likely involving multiple factors acting at numerous cellular sites. In addition, due to the large variation in types of exercise, durations, intensities, feeding schedules and CHO types it is difficult to assess if the mechanism(s) that could explain the increase in performance with CHO administration during exercise is(are) similar in different situations. Experiments concerning the identification of potential mechanism(s) by which performance is increased with CHO administration during exercise will add to our understanding of the mechanism(s) of muscle/central fatigue. This knowledge could have significant implications for improving exercise performance.

Energy beverages: content and safety

Exercise is making a resurgence in many countries, given its benefits for fitness as well as prevention of obesity. This trend has spawned many supplements that purport to aid performance, muscle growth, and recovery. Initially, sports drinks were developed to provide electrolyte and carbohydrate replacement. Subsequently, energy beverages (EBs) containing stimulants and additives have appeared in most gyms and grocery stores and are being used increasingly by "weekend warriors" and those seeking an edge in an endurance event. Long-term exposure to the various components of EBs may result in significant alterations in the cardiovascular system, and the safety of EBs has not been fully established. For this review, we searched the MEDLINE and EMBASE databases from 1976 through May 2010, using the following keywords: energy beverage, energy drink, power drink, exercise, caffeine, red bull, bitter orange, glucose, ginseng, guarana, and taurine. Evidence regarding the effects of EBs is summarized, and practical recommendations are made to help in answering the patient who asks, "Is it safe for me to drink an energy beverage when I exercise?"

Accumulation of 2H2O in plasma and eccrine sweat during exercise-heat stress

The purpose of this research was to characterize the movement of ingested water through body fluids, during exercise-heat stress. Deuterium oxide ((2)H(2)O) accumulation in plasma and eccrine sweat was measured at two sites (back and forehead). The exercise of 14 males was controlled via cycle ergometry in a warm environment (60 min; 28.7 degrees C, 51%rh). Subjects consumed (2)H(2)O (0.15 mg kg(-1), 99.9% purity) mixed in flavored, non-caloric, colored water before exercise, then consumed 3.0 ml kg(-1) containing no (2)H(2)O every 15 min during exercise. We hypothesized that water transit from mouth to skin would occur before 15 min. (2)H(2)O appeared rapidly in both plasma and sweat (P < 0.05), within 10 min of water consumption. The ratio (2)H(2)O/H(2)O (D:H) was 47.3-55.0 times greater in plasma than in back sweat at minutes 10, 20, and 30 (DeltaD:H relative to baseline). At elapsed minute 20, the mean rate of deuterium accumulation (DeltaD:H min(-1)) in plasma was 14.9 and 23.7 times greater than in forehead and back sweat samples, respectively. Mean (+/-SE) whole-body sweat rate was 1.04 +/- 0.05 L h(-1) and subjects with the greatest whole-body sweat rate exhibited the greatest peak deuterium enrichment in sweat (r(2) = 0.87, exponential function); the peak (2)H(2)O enrichment in sweat was not proportional (P > 0.05) to body mass, volume of the deuterium dose, or total volume of fluid consumed. These findings clarify the time course of fluid movement from mouth to eccrine sweat glands, and demonstrate considerable differences of (2)H(2)O enrichment in plasma versus sweat.

Core temperature and metabolic responses after carbohydrate intake during exercise at 30 degrees C

CONTEXT

Carbohydrate ingestion has recently been associated with elevated core temperature during exercise in the heat when testing for ergogenic effects. Whether the association holds when metabolic rate is controlled is unclear. Such an effect would have undesirable consequences for the safety of the athlete.

OBJECTIVE

To examine whether ingesting fluids containing carbohydrate contributed to an accelerated rise in core temperature and greater overall body heat production during 1 hour of exercise at 30 degrees C when the effort was maintained at steady state.

DESIGN

Crossover design (repeated measures) in randomized order of treatments of drinking fluids with carbohydrate and electrolytes (CHO) or flavored-water placebo with electrolytes (PLA). The beverages were identical except for the carbohydrate content: CHO = 93.7 +/- 11.2 g, PLA = 0 g.

SETTING

Research laboratory.

PATIENTS OR OTHER PARTICIPANTS

Nine physically fit, endurance-trained adult males.

INTERVENTION(S)

Using rectal temperature sensors, we measured core temperature during 30 minutes of rest and 60 minutes of exercise at 65% of maximal oxygen uptake (Vo(2) max) in the heat (30.6 degrees C, 51.8% relative humidity). Participants drank equal volumes (1.6 L) of 2 beverages in aliquots 30 minutes before and every 15 minutes during exercise. Volumes were fixed to approximate sweat rates and minimize dehydration.

MAIN OUTCOME MEASURE(S)

Rectal temperature and metabolic response (Vo(2), heart rate).

RESULTS

Peak temperature, rate of temperature increase, and metabolic responses did not differ between beverage treatments. Initial hydration status, sweat rate, and fluid replacement were also not different between trials, as planned.

CONCLUSIONS

Ingestion of carbohydrate in fluid volumes that minimized dehydration during 1 hour of steady-state exercise at 30 degrees C did not elicit an increase in metabolic rate or core temperature.

Superior endurance performance with ingestion of multiple transportable carbohydrates

INTRODUCTION

The aim of the present study was to investigate the effect of ingesting a glucose plus fructose drink compared with a glucose-only drink (both delivering carbohydrate at a rate of 1.8 g.min(-1)) and a water placebo on endurance performance.

METHODS

Eight male trained cyclists were recruited (age 32 +/- 7 yr, weight 84.4 +/- 6.9 kg, .VO(2max) 64.7 +/- 3.9 mL.kg(-1).min(-1), Wmax 364 +/- 31 W). Subjects ingested either a water placebo (P), a glucose (G)-only beverage (1.8 g.min(-1)), or a glucose and fructose (GF) beverage in a 2:1 ratio (1.8 g.min(-1)) during 120 min of cycling exercise at 55% Wmax followed by a time trial in which subjects had to complete a set amount of work as quickly as possible (approximately 1 h). Every 15 min, expired gases were analyzed and blood samples were collected.

RESULTS

Ingestion of GF resulted in an 8% quicker time to completion during the time trial (4022 s) compared with G (3641 s) and a 19% improvement compared with W (3367 s). Total carbohydrate (CHO) oxidation was not different between GF (2.54 +/- 0.25 g.min(-1)) and G (2.50 g.min(-1)), suggesting that GF led to a sparing of endogenous CHO stores, because GF has been shown to have a greater exogenous CHO oxidation than G.

CONCLUSION

Ingestion of GF led to an 8% improvement in cycling time-trial performance compared with ingestion of G.

The effect of sweetness on the efficacy of carbohydrate supplementation during exercise in the heat

The aim of the present study was to investigate potential mechanisms responsible for the improvement in prolonged exercise capacity in hot environments with exogenous carbohydrate. Eight endurance-trained men (VO(2)max 60.5 +/- 2.4 ml.kg(-1).min(-1), mean +/- SE) cycled to exhaustion on three occasions at 60% VO(2)max at an ambient temperature of 35 degrees C. They ingested either a sweet 6.4% carbohydrate solution (SC), a nonsweet 6.4% carbohydrate solution (NSC), or water (W). Exercise capacity was significantly increased with SC and NSC compared to W, the improvements corresponding to 15.8% and 11.8%, respectively. No difference in exercise capacity was seen between SC and NSC solutions. Plasma glucose concentrations were higher during the SC and NSC trials compared to W, significantly so at 10 min and at fatigue. Rates of carbohydrate oxidation were higher in the SC and NSC trials, although the rates never declined below 2.1 +/- 0.2 g.min(-1) in the W trial. There was no difference in the rate of rise of rectal temperature between trials, but there was a trend for subjects to fatigue at higher temperatures during the two carbohydrate trials. In conclusion, exogenous carbohydrate, independent of sweetness, improves exercise capacity in the heat compared to water alone.

Fluids and hydration in prolonged endurance performance

Numerous studies have confirmed that performance can be impaired when athletes are dehydrated. Endurance athletes should drink beverages containing carbohydrate and electrolyte during and after training or competition. Carbohydrates (sugars) favor consumption and Na(+) favors retention of water. Drinking during competition is desirable compared with fluid ingestion after or before training or competition only. Athletes seldom replace fluids fully due to sweat loss. Proper hydration during training or competition will enhance performance, avoid ensuing thermal stress, maintain plasma volume, delay fatigue, and prevent injuries associated with dehydration and sweat loss. In contrast, hyperhydration or overdrinking before, during, and after endurance events may cause Na(+) depletion and may lead to hyponatremia. It is imperative that endurance athletes replace sweat loss via fluid intake containing about 4% to 8% of carbohydrate solution and electrolytes during training or competition. It is recommended that athletes drink about 500 mL of fluid solution 1 to 2 h before an event and continue to consume cool or cold drinks in regular intervals to replace fluid loss due to sweat. For intense prolonged exercise lasting longer than 1 h, athletes should consume between 30 and 60 g/h and drink between 600 and 1200 mL/h of a solution containing carbohydrate and Na(+) (0.5 to 0.7 g/L of fluid). Maintaining proper hydration before, during, and after training and competition will help reduce fluid loss, maintain performance, lower submaximal exercise heart rate, maintain plasma volume, and reduce heat stress, heat exhaustion, and possibly heat stroke.

Using a non-invasive stable isotope tracer to measure the absorption of water in humans

The development of solutions that prevent dehydration or promote adequate re-hydration play a vital role in preventing fatigue during exercise, however, the methods commonly used to assess the hydration ability of such solutions are invasive and often assess the components of absorption separately. This paper describes using a non-invasive deuterium tracer technique that assesses gastric emptying and intestinal absorption simultaneously to evaluate the uptake of water during rest and exercise. The kinetics of absorption are further examined by mathematical modelling of the data generated. For the rest group, 0.05 g/kg of body weight of deuterium, contained in gelatine capsules, was ingested with ordinary tap water and saliva samples were collected every 5 min for one hour while the subject remained seated. The deuterium was administered as above for the exercise group but sample collection was during one hour of exercise on a treadmill at 55% of the subject's maximum heart rate. The enrichment data for each subject were mathematically modelled and the parameters obtained were compared across groups using an independent samples t-test. Compared with the rest condition, the exercise group showed delayed absorption of water as indicated by significant differences for the modelling parameters t2, t1/2, maximum absorption rate and solution absorption amount at t1. Labelling with a deuterium tracer is a good measure of the relative rate ingested fluids are absorbed by the body. Mathematical modelling of the data generates rates of maximum absorption and allows calculation of the percentage of the solution that is absorbed at any given time during the testing period.

Carbohydrate supplementation improves moderate and high-intensity exercise in the heat

The aim of the present study was to clarify the effect of carbohydrate (CHO) supplementation on moderate and high-intensity endurance exercise in the heat. Eight endurance-trained men [maximal oxygen uptake ( VO(2max)) 59.5+/-1.6 ml kg(-1) bw(-1), mean+/-SE] cycled to exhaustion twice at 60% VO(2max) and twice at 73% VO(2max) at an ambient temperature of 35 degrees C. Subjects ingested either a 6.4% maltodextrin solution (CHO) or an artificially flavoured and coloured placebo (PLA). Time to fatigue was significantly greater with CHO in both the 60% and 73% VO(2max) trials (14.5% and 13.5% improvement, respectively). Heart rate and oxygen uptake ( VO(2)) did not differ at any point between PLA and CHO. Hypoglycaemia was not seen in any condition but plasma glucose concentrations tended to be higher at both intensities when CHO was fed. CHO oxidation rates were similar at 60% VO(2max) between CHO and PLA. There were no differences between PLA and CHO in the rate of rise of rectal temperatures ( T(rec)) at either intensity but there was a trend for subjects to fatigue at a high temperature when taking CHO. Ratings of perceived exertion (RPE) tended to be lower throughout both CHO trials; this was significant at 80 min and at fatigue at 60% VO(2max). It is concluded that supplementation with CHO improves exercise performance in the heat at both moderate and high endurance intensities. In the absence of a clear metabolic explanation, a central effect involving an increased tolerance of rising deep body temperature merits further investigation.

Special issues and concerns for the high school- and college-aged athletes

Increasing numbers of high school- and college-aged students are participating in sports. As sport participation and intensity increases, the frequency of associated heat related illnesses and acute and chronic overuse injuries will continue to become more prevalent. The sports medicine physician plays an essential role not only in the practice and treatment of injuries but also in educating athletes about safe and healthy exercise habits that will provide life long benefits.

Food and fluid intake during exercise

The intake of fluid and CHO offers benefits to the performance of a number of sports events and exercise activities. The effects of dehydration on performance are now well known, with the penalties ranging from subtle, but often important, decrements in performance at low levels of fluid deficit to the severe health risks associated with substantial fluid losses during exercise in the heat. Although evidence of the beneficial effects of CHO intake during exercise have existed for over 70 years, sports scientists are still to discover all the situations in which benefits occur and to explain the mechanisms involved. Optimal strategies for CHO and fluid intake during exercise are yet to be fine-tuned, and ultimately will be determined by practical issues such as the opportunity to eat or drink during an event, and gastrointestinal comfort.

The effectiveness of commercially available sports drinks

The purpose of this review is to evaluate the effectiveness of commercially available sports drinks by answering the questions: (i) will consuming a sports drink be beneficial to performance? and (ii) do different sports drinks vary in their effectiveness? To answer these questions we have considered the composition of commercially available sports drinks, examined the rationale for using them, and critically reviewed the vast number of studies that have investigated the effectiveness of sports drinks on performance. The focus is on the drinks that contain low carbohydrate concentrations (<10%) and are marketed for general consumption before and during exercise rather than those with carbohydrate concentrations >10%, which are intended for carbohydrate loading. Our conclusions are 3-fold. First, because of variations in drink composition and research design, much of the sports drinks research from the past cannot be applied directly to the effectiveness of currently available sports drinks. Secondly, in studies where a practical protocol has been used along with a currently available sports beverage, there is evidence to suggest that consuming a sports drinks will improve performance compared with consuming a placebo beverage. Finally, there is little evidence that any one sports drink is superior to any of the other beverages on the market.

Water and electrolyte requirements for exercise

Exercise performance can be compromised by a body water deficit, particularly when exercise is performed in hot climates. It is recommended that individuals begin exercise when adequately hydrated. This can be facilitated by drinking 400 mL to 600 mL of fluid 2 hours before beginning exercise and drinking sufficient fluid during exercise to prevent dehydration from exceeding 2% body weight. A practical recommendation is to drink small amounts of fluid (150-300 mL) every 15 to 20 minutes of exercise, varying the volume depending on sweating rate. Core temperature, heart rate, and perceived effort remain lowest when fluid replacement comes closest to matching the rate of sweat loss. During exercise lasting less than 90 minutes, water alone is sufficient for fluid replacement. During prolonged exercise lasting longer than 90 minutes, commercially available carbohydrate electrolyte beverages should be considered to provide an exogenous carbohydrate source to sustain carbohydrate oxidation and endurance performance. Electrolyte supplementation is generally not necessary because dietary intake is adequate to offset electrolytes lost in sweat and urine; however, during initial days of hot-weather training or when meals are not calorically adequate, supplemental salt intake may be indicated to sustain sodium balance.

Failure of low dose carbohydrate feeding to attenuate glucoregulatory hormone responses and improve endurance performance

The effects of ingesting a low dose of CHO on plasma glucose, glucoregulatory hormone responses, and performance during prolonged cycling were investigated. Nine male subjects cycled for 165 min at approximately 67% peak VO2 followed by a two-stage performance ride to exhaustion on two occasions in the laboratory. Every 20 min during exercise, subjects consumed either a flavored water placebo (P) or a dilute carbohydrate beverage (C). Blood samples were collected immediately before, every 20 min throughout, and immediately after exercise. Plasma was analyzed for glucose, lactate, free fatty acids (FFA), and various glucoregulatory hormones. VO2, RER, heart rate, perceived exertion, and exercise performance were also measured. Lactate, FFA, epinephrine, norepinephrine, ACTH, cortisol, and glucagon increased with exercise whereas glucose and insulin decreased (p < or = .05). Except for a small difference in glucose at 158 min of exercise and at exhaustion, no significant differences were found between drinks for any of the variables studied (P > or = .05). Ingestion of 13 g carbohydrate per hour is not sufficient to maintain plasma glucose, attenuate the glucoregulatory hormone response, and improve performance during prolonged moderate intensity cycling.

Timing and method of increased carbohydrate intake to cope with heavy training, competition and recovery

Based upon the fact that fatigue during intense prolonged exercise is commonly due to depletion of muscle and liver glycogen which limits both training and competitive performance, this paper has proposed extraordinary dietary practices which generally advocate high carbohydrate intake at all times before, during and after exercise. The simple goal is to have as much carbohydrate in the body as possible during the latter stages of prolonged intense exercise when the ability for intense exercise usually becomes limiting to performance. This theory is put into practice by recommending that carbohydrate intake after exhaustive exercise should average 50 g per 2 h of mostly moderate and high glycaemic carbohydrate foods. The aim should be to ingest a total of about 600 g in 24 h. Carbohydrate intake should not be avoided during the 4 h period before exercise and in fact it is best to eat at least 200 g during this time. When possible, carbohydrate should be ingested during exercise, generally in the form of solutions containing glucose/sucrose/maltodextrins, at a rate of 30-60 g h-1. Emphasis has been placed upon eating the optimal amount and best type of carbohydrate at the proper times because these practices demand a large amount of food. When diet is not carefully planned according to these guidelines, endurance athletes tend to consume too little carbohydrate because they become satiated with high fat in their diet and they go through periods in the day when recovery of glycogen stores is suboptimal and thus precious time is wasted.

Physiological effects of dehydration and rehydration with water and acidic or neutral carbohydrate electrolyte solutions

Five healthy young men exercised on an ergocycle for six 25-min periods separated by 5-min rest intervals in a warm dry environment (36 degrees C). After 1 h of exercise without fluid intake, the subjects continued to be dehydrated or were rehydrated either with water (W) or with isosmotic electrolyte carbohydrate solutions, either acidic (AISO) or close to neutrality (NISO). The average amount of the fluid ingested progressively every 10 min (120 ml) at 20 degrees C was calculated so as to compensate for 80% of the whole body water loss due to exercise in the heat. Dehydration associated with hyperosmotic hypovolaemia elicited large increases in heart rate (HR), and in rectal temperature (Tre), while no decrease was found in either whole body or local sweat rates. Rehydration with water significantly reduced the observed disturbances, except for plasma osmolality and Na+ concentration which were significantly lower than normal. With both AISO and NISO there was no plasma volume reduction and osmolality increase. Although a plasma volume expansion was induced by NISO ingestion, the cardiac cost was not improved, as reflected by the absence of a decrease in HR. With NISO, sweating was not enhanced and Tre tended to remain higher. It is concluded that efficient rehydration requires the avoidance of plasma volume expansion at the expense of interstitial and intracellular rehydration. During rehydration by oral ingestion of fluid, the pH of the drink may be an important factor; its effect remains unclear, however.

Carbohydrate feeding and exercise: effect of beverage carbohydrate content

The purpose of this study was to determine the effect of ingesting fluids of varying carbohydrate content upon sensory response, physiologic function, and exercise performance during 1.25 h of intermittent cycling in a warm environment (Tdb = 33.4 degrees C). Twelve subjects (7 male, 5 female) completed four separate exercise sessions; each session consisted of three 20 min bouts of cycling at 65% VO2max, with each bout followed by 5 min rest. A timed cycling task (1200 pedal revolutions) completed each exercise session. Immediately prior to the first 20 min cycling bout and during each rest period, subjects consumed 2.5 ml.kg BW-1 of water placebo (WP), or solutions of 6%, 8%, or 10% sucrose with electrolytes (20 mmol.l-1 Na+, 3.2 mmol.l-1 K+). Beverages were administered in double blind, counterbalanced order. Mean (+/- SE) times for the 1200 cycling task differed significantly: WP = 13.62 +/- 0.33 min, *6% = 13.03 +/- 0.24 min, 8% = 13.30 +/- 0.25 min, 10% = 13.57 +/- 0.22 min (* = different from WP and 10%, P less than 0.05). Compared to WP, ingestion of the CHO beverages resulted in higher plasma glucose and insulin concentrations, and higher RER values during the final 20 min of exercise (P less than 0.05). Markers of physiologic function and sensory perception changed similarly throughout exercise; no differences were observed among subjects in response to beverage treatments for changes in plasma concentrations of lactate, sodium, potassium, for changes in plasma volume, plasma osmolality, rectal temperature, heart rate, oxygen uptake, rating of perceived exertion, or for indices of gastrointestinal distress, perceived thirst, and overall beverage acceptance.(ABSTRACT TRUNCATED AT 250 WORDS)