The effects of consuming carbohydrate-electrolyte beverages on gastric emptying and fluid absorption during and following exercise

Fluid and carbohydrate replacement during intermittent exercise

Most studies relating to fluid replacement have addressed the problem of drinking during prolonged exercise. Fluid replacement is also very important for intermittent exercise, although it has not been extensively studied. More studies in this area would help coaches and athletes understand the importance of fluid balance and carbohydrate supplementation during intermittent exercise. Based on available data, it can be concluded that: (i) because of high exercise intensity, sweat loss and glycogen depletion during intermittent exercise are at least comparable with those during continuous exercise for a similar period of time. Therefore, the need to ingest a sport drink or replacement beverage during intermittent exercise may be greater than that during continuous exercise in order to maintain a high level of performance and to help prevent the possibility of thermal injury when such activity occurs in a warm environment; (ii) the volume of ingested fluid is critical for both rapid gastric emptying and complete rehydration; and (iii) osmolality (250 to 370 mOsm/kg), carbohydrate concentration (5 to 7%), and carbohydrate type (multiple transportable carbohydrates) should be considered when choosing an effective beverage for rehydration and carbohydrate supplementation during intermittent exercise.

Factors influencing the restoration of fluid and electrolyte balance after exercise in the heat

Maintenance of fluid balance is a major concern for all athletes competing in events held in hot climates. This paper reviews recent work relating to optimisation of fluid replacement after sweat loss induced by exercising in the heat. Data are taken from studies undertaken in our laboratory. Issues investigated were drink composition, volume consumed, effects of consuming food with a drink, effects of alcohol in rehydration effectiveness, voluntary intake of fluid, and considerations for women related to the menstrual cycle. The results are presented as a series of summaries of experiments, followed by a discussion of the implications. The focus of this review is urine output after ingestion of a drink; fluid excreted in urine counteracts rehydration. Also included are data on the restoration of plasma volume losses. Ingestion of large volumes of plain water will inhibit thirst and will also promote a diuretic response. If effective rehydration is to be maintained for some hours after fluid ingestion, drinks should contain moderately high levels of sodium (perhaps as much as 50-60 mmol/l) and possibly also some potassium to replace losses in the sweat. To surmount ongoing obligatory urine losses, the volume consumed should be greater than the volume of sweat lost. Palatability of drinks is important in stimulating intake and ensuring adequate volume replacement. Where opportunities allow, the electrolytes required may be ingested as solid food consumed with a drink. There are no special concerns for women related to changes in hormone levels associated with the menstrual cycle. Ingestion of carbohydrate-electrolyte drinks in the post-exercise period restores exercise capacity more effectively than plain water. The effects on performance of an uncorrected fluid deficit should persuade all athletes to attempt to remain fully hydrated at all times, and the aim should be to start each bout of exercise in a fluid replete state. This will only be achieved if a volume of fluid in excess of the sweat loss is ingested together with sufficient electrolytes.

Use of radionuclide imaging to determine gastric emptying of carbohydrate solutions during exercise

OBJECTIVE

To investigate the repeatability of continual assessment of the gastric emptying rates of carbohydrate solutions in exercising subjects using 99mtechnetium labelling.

METHODS

Gastric emptying of a 5% glucose solution and an iso-osmotic maltodextrin solution was measured using 3 MBq of 99mtechnetium labelled diethylene triamine penta-acetic acid (DTPA) and continuous gamma camera imaging in five male subjects. The subjects performed four 1 h trials at 70% VO2 peak on a cycle ergometer. After 15 min, 200 ml of a radiolabelled solution of glucose or maltodextrin were ingested in a blind crossover protocol. The two solutions were each ingested on separate occasions (trial 1 and trial 2) to establish repeatability.

RESULTS

Statistical analysis showed no differences between trial 1 and trial 2 for both solutions. There were no significant differences for the emptying rates between the two test solutions.

CONCLUSIONS

Posterior imaging using a computer linked gamma camera following the ingestion of 99mtechnetium labelled DTPA mixed with carbohydrate solutions provides a repeatable method of assessing gastric emptying characteristics in exercising subjects. This technique showed no significant differences between the emptying rates of a single dose of iso-osmotic glucose or maltodextrin solution.

Preparing for and competing in the heat: the human perspective

This review provides an overview of the challenges that face man and horses when exercising in the heat. Some of the strategies that are used and are being developed for human athletes exercising in the heat are reviewed. There are many similarities between human and equine physiological responses to exercise in the heat; and equine exercise science may gain some useful insights from the training, fluid replacement and heat acclimatisation strategies used by human athletes. There are, however, some important differences that impact on the ability of horses to thermoregulate and to regulate fluid and electrolyte balance. The major differences are the low surface area to body mass ratio in horses compared to man; and the high metabolic capacity of equine skeletal muscle. These 2 factors may limit the ability of horses to dissipate heat when exercise is performed under hot conditions. Some of the more important equine differences are highlighted within the context of the "human perspective'.

Osmolarity does not affect the gastric emptying rate of oral rehydration solutions

BACKGROUND

The objective of this study was to determine the effect of either carbohydrate content or osmolarity on gastric emptying rate in normal healthy subjects.

METHODS

In total 12 test drinks were ingested as a single 8 mL/kg per body weight bolus on an empty stomach. Six of these drinks had a different carbohydrate content, increasing stepwise from 45 to 90 g/L, but all with the same osmolarity (330 mOsm/kg). The other six drinks all contained 60 g carbohydrate/L but differed stepwise in osmolarity because of the use of maltodextrins with a difference in chain length (243 to 374 mOsm/kg).

RESULTS

The results show a significant negative relation between carbohydrate content and gastric emptying in the six drinks with a uniform osmolarity but progressively increasing carbohydrate content. The six drinks, which had the same carbohydrate-energy content but different osmolarities, emptied all at the same rate from the stomach. The delivery of carbohydrate-energy per minute from the stomach to the small intestine was the same for all drinks.

CONCLUSIONS

From these data we conclude that the rate of gastric emptying of carbohydrate-containing solutions is triggered by the carbohydrate-energy drink content or by the delivery rate of carbohydrate-energy to the gut. Osmolarity in the range studied here had no effect.

The effect of osmolality and carbohydrate content on the rate of gastric emptying of liquids in man

1. The effect of osmolality and carbohydrate content on the rate of gastric emptying was assessed by using the double sampling gastric aspiration technique to measure the rate of gastric emptying of isoenergetic and isosmotic solutions of glucose and glucose polymer. Six healthy male subjects were each studied on four separate occasions using a test drink volume of 600 ml. 2. The half-emptying time (t1/2, mean +/- S.E.M.) for a dilute (40 g l-1) solution of glucose (LG, 230 mosmol kg-1) was 17 +/- 1 min. This was greater than that (14 +/- 1 min) for a glucose polymer solution with the same energy content (LP, 42 mosmol kg-1). A concentrated (188 g l-1) glucose polymer solution (HP, 237 mosmol kg-1) emptied faster (t1/2 = 64 +/- 8 min) than the corresponding isoenergetic glucose solution (HG, 1300 mosmol kg-1, t1/2 = 130 +/- 18 min). 3. The dilute (40 g l-1) glucose solution emptied faster than the concentrated (188 g l-1) glucose polymer solution with the same osmolality (LG, 230 mosmol kg-1; HP, 237 mosmol kg-1). 4. The two dilute solutions (40 g l-1) delivered a similar amount of carbohydrate to the small intestine, whereas the concentrated (188 g l-1) glucose polymer solution delivered a greater amount of carbohydrate at 20, 40 and 50 min than the isoenergetic glucose solution. 5. These results indicate that both osmolality and carbohydrate content influence gastric emptying of liquids in man, but the carbohydrate content appears to have greater influence than osmolality.(ABSTRACT TRUNCATED AT 250 WORDS)

Exercise in the heat: strategies to minimize the adverse effects on performance

Exercise in the heat is usually associated with reduced performance; both dehydration and hyperthermia adversely affect mental and physical performance. For athletes from temperate climates, the negative effects of heat had humidity can be attenuated by a period of acclimatization. This requires up to 10-14 days. Endurance-trained individuals already show some of the adaptations that accompany acclimatization, but further adaptation occurs with training in the heat. Prior dehydration has a negative effect even on exercise of short duration where sweat losses are small. The athlete must begin exercise fully hydrated and regular ingestion of fluids is beneficial where the exercise duration exceeds 40 min. Dilute carbohydrate-electrolyte (sodium) drinks are best for fluid replacement and also supply some substrate for the exercising muscles. Post-exercise rehydration requires electrolyte as well as volume replacement. In extreme conditions, neither acclimatization nor fluid replacement will allow hard exercise to be performed without some risk of heat illness.

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

This study examined the effects on water balance of adding electrolytes to fluids ingested after exercise-induced dehydration. Eight healthy male volunteers were dehydrated by approximately 2% of body mass by intermittent cycle exercise. Over a 30-min period after exercise, subjects ingested one of the four test drinks of a volume equivalent to their body mass loss. Drink A was a 90 mmol.l-1 glucose solution; drink B contained 60 mmol.l-1 sodium chloride; drink C contained 25 mmol.l-1 potassium chloride; drink D contained 90 mmol.l-1 glucose, 60 mmol.l-1 sodium chloride and 25 mmol.l-1 potassium chloride. Treatment order was randomised. Blood and urine samples were obtained at intervals throughout the study; subjects remained fasted throughout. Plasma volume increased to the same extent after the rehydration period on all treatments. Serum electrolyte (Na+, K+ and Cl-) concentrations fell initially after rehydration before returning to their pre-exercise levels. Cumulative urine output was greater after ingestion of drink A than after ingestion of any of the other drinks. On the morning following the trial, subjects were in greater net negative fluid balance [mean (SEM); P < 0.02] on trial A [745 (130) ml] than on trials B [405 (51) ml], C [467 (87) ml] or D [407 (34) ml]. There were no differences at any time between the three electrolyte-containing solutions in urine output or net fluid balance. One hour after the end of the rehydration period, urine osmolality had fallen, with a significant treatment effect (P = 0.016); urine osmolality was lowest after ingestion of drink A.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of postexercise glucose ingestion upon serum potassium levels and ECG function

Thirteen trained runners were studied to determine whether postexercise glucose ingestion contributes to electrocardiogram (ECG) alterations by enhancing decreases in serum potassium (K+) concentrations. For the two randomly ordered trials, subjects ingested a 100 g (25% w/v glucose polymer) drink, either alone or with the addition of 3 g of potassium chloride (KCl), within 15 min following a 90-min run. ECG parameters, serum K+, and glucose concentrations were measured preexercise (Time 0), 2-3 min post-exercise (Time 1), and 25 (Time 2) and 60 (Time 3) min postexercise. The data suggest that postexercise glucose ingestion may cause ECG changes that are not directly related to the return of K+ to muscle, and that these changes, although characteristic of hypokalemia, may be related to serum glucose excursions rather than to absolute levels of serum K+. The addition of KCl may have prevented these changes by delaying gastric emptying of glucose.

Position of the American Dietetic Association and the Canadian Dietetic Association: nutrition for physical fitness and athletic performance for adults

The importance of diet and healthful food choices in optimizing health status, fitness levels, and athletic performance has been recognized by both participants and professionals. There continues to be a need for the interpretation of new research findings in this fast-growing discipline and for the dissemination of nutrition information and training techniques for a broad spectrum of individuals involved in various forms of physical activity. The registered dietitian who has specialized in exercise physiology and sports nutrition has the knowledge and counseling skills to act as the provider of this nutrition information. Additional information may be obtained in the Sports Nutrition Manual, 2nd edition, published by The American Dietetic Association and the Sports and Cardiovascular Nutrition dietetic practice group as well as in Sport Nutrition for the Athletes of Canada, published by the Sport Nutrition Advisory Committee of the Sports Medicine and Science Council of Canada.

Is the gut an athletic organ? Digestion, absorption and exercise

Digestion is a process which takes place in resting conditions. Exercise is characterised by a shift in blood flow away from the gastrointestinal (GI) tract towards the active muscle and the lungs. Changes in nervous activity, in circulating hormones, peptides and metabolic end products lead to changes in GI motility, blood flow, absorption and secretion. In exhausting endurance events, 30 to 50% of participants may suffer from 1 or more GI symptoms, which have often been interpreted as being a result of maldigestion, malabsorption, changes in small intestinal transit, and improper food and fluid intake. Results of field and laboratory studies show that pre-exercise ingestion of foods rich in dietary fibre, fat and protein, as well as strongly hypertonic drinks, may cause upper GI symptoms such as stomach ache, vomiting and reflux or heartburn. There is no evidence that the ingestion of nonhypertonic drinks during exercise induces GI distress and diarrhoea. In contrast, dehydration because of insufficient fluid replacement has been shown to increase the frequency of GI symptoms. Lower GI symptoms, such as intestinal cramps, diarrhoea--sometimes bloody--and urge to defecate seem to be more related to changes in gut motility and tone, as well as a secretion. These symptoms are to a large extent induced by the degree of decrease in GI blood flow and the secretion of secretory substances such as vasoactive intestinal peptide, secretin and peptide-histidine-methionine. Intensive exercise causes considerable reflux, delays small intestinal transit, reduces absorption and tends to increase colonic transit. The latter may reduce whole gut transit time. The gut is not an athletic organ in the sense that it adapts to increased exercise-induced physiological stress. However, adequate training leads to a less dramatic decrease of GI blood flow at submaximal exercise intensities and is important in the prevention of GI symptoms.

Blood and urine responses to ingesting fluids of various salt and glucose concentrations

Several hours before returning to Earth, Space Shuttle astronauts consume fluid and salt tablets equivalent to a liter of 0.9% saline as a countermeasure to postflight orthostatic intolerance. This countermeasure is not completely successful. Therefore, in search of a countermeasure that would protect against orthostatic intolerance better and for a longer duration, the authors compared the blood and urine responses of five men (21-41 yr) after they drank 1 L of 0.9% saline to their responses after drinking five other solutions: distilled water, 1% glucose, 0.74% saline with 1% glucose, 0.9% saline with 1% glucose, and 1.07% saline. Each subject ingested a different solution on 6 different days and remained seated for the ensuing 4 hours. Heart rate, blood pressures, and urine variables were measured before ingestion of the fluids and every 30 minutes thereafter; blood samples were drawn before, immediately after, and every 60 minutes after ingestion. Change in plasma volume, which was estimated from hemoglobin and hematocrit, was considered the most critical variable. Data for all solutions were compared by analysis of variance. Since plasma volume was increased most after ingestion of 1.07% saline, all variables (at 2 hours, at 3 hours and at 4 hours) were compared between 1.07% saline and 0.9% saline, the current countermeasure. Plasma volume was increased more after 1.07% saline than after 0.9% saline, and this difference was most significant at 4 hours after ingestion (P = .056). Diuresis occurred promptly after ingestion of the two saline-free solutions, water and 1% glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of ingesting polylactate or glucose polymer drinks during prolonged exercise

Five trained, fasted male cyclists rode a cycle ergometer three times at 50% of VO2max for 180 min. Using a balanced order, double-blind procedure, subjects were given either a solution containing polylactate (PL: 80% polylactate, 20% sodium lactate, in 7% solution with water), glucose polymer (GP: multidextrin in 7% solution with water), or control (C: water sweetened with aspartame) 5 min before exercise and at 20-min intervals during exercise. Venous blood samples were taken at rest and at 20-min intervals during exercise. In general, PL and GP rendered similar results except that pH and bicarbonate (HCO3-) were higher in PL. There were no differences between treatments in perceived exertion, sodium, potassium, chloride, lactate, heart rate, oxygen consumption, rectal temperature, or selected skin temperatures. These data show that polylactate may help maintain blood glucose and enhance blood buffering capacity during prolonged exercise and could be a useful component in an athletic fluid replacement beverage.

Fluid replacement and exercise stress. A brief review of studies on fluid replacement and some guidelines for the athlete

Fluid ingestion during exercise has the twin aims of providing a source of carbohydrate fuel to supplement the body's limited stores and of supplying water and electrolytes to replace the losses incurred by sweating. Increasing the carbohydrate content of drinks will increase the amount of fuel which can be supplied, but will tend to decrease the rate at which water can be made available; where provision of water is the first priority, the carbohydrate content of drinks will be low, thus restricting the rate at which substrate is provided. The composition of drinks to be taken will thus be influenced by the relative importance of the need to supply fuel and water, this in turn depends on the intensity and duration of the exercise task, on the ambient temperature and humidity, and on the physiological and biochemical characteristics of the individual athlete. Carbohydrate ingested during exercise appears to be readily available as a fuel for the working muscles, at least when the exercise intensity does not exceed 70 to 75% of maximum oxygen uptake. Carbohydrate-containing solutions appear to be more effective in improving performance than plain water. Water and electrolytes are lost form the body in sweat: although the composition of sweat is rather variable, it is invariably hypotonic with respect to plasma. Sweat rate is determined primarily by the metabolic rate and the environmental temperature and humidity. The sweat rate may exceed the maximum rate of gastric emptying of ingested fluids, and some degree of dehydration is commonly observed. Excessive replacement of sweat losses with plain water or fluids with a low sodium content may result in hyponatraemia. Sodium replacement is essential for postexercise rehydration. The optimum frequency, volume and composition of drinks will vary widely depending on the intensity and duration of the exercise, the environmental conditions and the physiology of the individual. The athlete must determine by trial and error the most suitable regimen.

Fluid and electrolyte loss and replacement in exercise

Prolonged exercise leads to a progressive water and electrolyte loss from the body as sweat is secreted to promote heat loss. The rate of sweating depends on many factors and is increased in proportion to the work rate and the environmental temperature and humidity. Sweat rate is highly variable between individuals, and can exceed 21 h-1 for prolonged periods. Since it is established that dehydration will impair exercise capacity and can pose a risk to health, the intake of fluid during exercise to offset sweat loss is important. Fluid intake is also aimed at providing a source of substrate, usually in the form of carbohydrate. The availability of ingested fluids may be limited by gastric emptying or by intestinal absorption. Gastric emptying of liquids is slowed by the addition of carbohydrate in proportion to the carbohydrate concentration and osmolality of the solution. With increasing glucose concentration, the rate of fluid delivery to the small intestine is decreased, but the rate of glucose delivery is increased. Water absorption in the small intestine is a passive process and is stimulated by the active absorption of glucose and sodium. The optimum fluid for rehydration during exercise depends on many factors, particularly the intensity and duration of the exercise, the environmental conditions, and the individual physiology of the athlete. There is no advantage to fluid intake during exercise of less than 30 min duration. The composition of fluids to be used will depend on the relative needs to replace water and to provide substrate. Where rehydration is a priority the solution should contain some glucose and sodium and should not exceed isotonicity: this will require the glucose concentration to be low (20-309 g l-1) or the substitution of glucose polymers, and the sodium content to be high (perhaps as much as 60 mmol l-1). Where substrate provision is more important, a more concentrated solution, incorporating large amounts of glucose polymers in concentrations of 150-200 g l-1, is to be preferred. To minimize the limitation imposed by the rate of gastric emptying, the volume of fluid in the stomach should be kept as high as is comfortable by frequent ingestion of small amounts of fluid. Addition of sodium, and perhaps also of potassium, may be important for rehydration after exercise.

Influence of a carbohydrate-electrolyte beverage on performance and blood homeostasis during recovery from football

This study compared the efficacy of a 7% glucose polymer beverage containing electrolytes (GP) versus a nonnutrient, nonelectrolyte placebo (P) in maintaining blood homeostasis during recovery from football and determined whether consumption of the GP beverage improved anaerobic performance immediately after football competition when compared with the placebo. Forty-four high school football players participated in a 50-play scrimmage designed to simulate game conditions. At each of six periods before and during the scrimmage, players consumed 170 ml of the GP or P beverage. Eight maximal-effort 40-yd sprints (40-sec rest intervals) were performed before and after the scrimmage to assess the decrement in anaerobic performance from the scrimmage. Venous blood samples were drawn before and after the scrimmage and analyzed. The pre- to postscrimmage differences in mean and peak sprint velocities did not differ between treatments, nor did body weight and plasma. In contrast, the percent decrease in plasma volume was significantly greater in the P group. Postscrimmage increases in glucose and insulin were greater in the GP group. These data suggest that CHO-electrolyte drinks do not prevent a decline in anaerobic performance when compared to water, but a CHO-electrolyte drink is more effective in maintaining PV than water during recovery from anaerobic exercise.

Heat illness. Fluid and electrolyte issues for pediatric and adolescent athletes

The primary mechanism for maintaining normal body temperature during physical exercise in the heat is the evaporation of sweat. With profuse sweating, water loss far exceeds electrolyte loss. Rigorous exercise in the heat places the athlete at risk for thermoregulatory dysfunction from dehydration. Because children are inherently less efficient thermoregulators than adults, they are at even greater risk for heat illness. The three primary syndromes of heat illness are heat cramps, heat exhaustion, and heat stroke. Treatment of heat illness is based on reduction of body temperature and rehydration. Heat stroke is a true medical emergency with a high mortality rate; immediate reduction of body temperature is critical to the survival of these patients. Prevention of heat illness is based on reducing known risk factors. Physical activity should be modified in the face of high ambient temperature and humidity. The athlete should begin exercise well hydrated; frequent consumption of cold water during exercise decreases likelihood of significant dehydration. After exercise, the athlete should continue drinking to replace fluid losses. Clothing should be lightweight; the more skin exposed, the greater the available evaporative surface. A preseason conditioning program, when combined with an 8- to 14-day period of acclimatization, further reduces the risk of heat injury. Although athletes engaged in endurance sports may benefit from drinking carbohydrate/electrolyte-containing solutions, for the majority of young athletes, cold water remains the preferred choice for fluid replacement during exercise. The relatively greater body surface area of young athletes also places them at risk for hypothermia. Special attention should be given when these athletes are competing under cold environmental conditions.

The effect of exercise on the gastrointestinal tract

Surveys of athletes, primarily runners, have shown that digestive disorders are common, associated both with training and racing. Women, in particular, seem to suffer most commonly. Nearly half have loose stools and nausea and vomiting occur frequently after hard runs. Diarrhoea, incontinence and rectal bleeding occur with surprising frequency. Runners may use medications prophylactically to minimise some of these symptoms. Upper digestive symptoms seem to occur more commonly in multisport events such as triathlons or enduro. The published literature is difficult to analyse and the basic intestinal physiology not well studied. Most gastroenterologists are accustomed to evaluating the fasting patient at rest and exercise physiologists are seldom experienced with digestive techniques. Digestive symptoms occurring with exercise referable to the oesophagus include chest pain, gastro-oesophageal reflux symptoms, or symptoms related to alterations in motility. While little is known of the oesophageal physiology during exercise, it is believed that only minimal changes occur in most subjects. Gastro-oesophageal reflux occurs more frequently with exercise than at rest and may produce symptoms of chest pain suggestive of ischaemic disease. Acid exposure may be reduced by pretreatment with histamine H2-receptor antagonists. Oesophageal symptoms, though common, are rarely disabling to the athlete, and the clinical importance lies in confusion with ischaemic disease. Cases of acute gastric stasis following running have been reported and gastric physiology during exercise, particularly bicycling, has been more actively investigated. Gastric emptying during exercise is subject to a number of factors including calorie count, meal osmolality, meal temperature and exercise conditions. However, it is generally accepted that light exercise accelerates liquid emptying, vigorous exercise delays solid emptying and has little effect upon liquid emptying until near exhaustion. Gastric acid secretion probably changes little with exercise although some have postulated that ulcer patients may increase secretion with exercise. Some exercise-associated digestive symptoms, such as diarrhoea and abdominal pain, have been attributed to changes in intestine function. Small bowel transit is delayed by exercise when measured by breath hydrogen oral caecal transit times and motility may be reduced as well. Intestinal absorption during exercise has not been well evaluated but probably changes little in ordinary circumstances. Passive absorption of water, electrolytes and xylose are not affected by submaximal effort. Colonic transit and function is even more difficult to evaluate and published results have been conflicting. However, it is likely that many of the lower digestive complaints of runners such as diarrhoea and lower abdominal cramps are due to direct effects of exercise upon the colon.(ABSTRACT TRUNCATED AT 250 WORDS)

Applied physiology of a triathlon

The triathlon is an endurance contest in which contestants must compete in 3 consecutive events, usually swimming, cycling and running. Success in a triathlon depends upon the ability of the triathlete to perform each of the sequential events at optimal pace without creating fatigue that will hinder performance in the next event. The successful triathlete must, therefore, have highly developed oxygen transport and utilisation systems as well as the ability to efficiently produce a high energy output for prolonged periods without creating metabolic acidosis. Accordingly, mean VO2max values for groups of triathletes during treadmill running have been reported to range from 52.4 to 72 ml/kg/min in men; 58.7 to 65.9 ml/kg/min in women. VO2max values during cycle ergometry were 3 to 6% less than treadmill running values; tethered swimming maximums 13 to 18% less. Predictable and well-known adaptations occur in the cardiovascular systems of triathletes. Structural adaptations of the heart that have been documented in triathletes include increased left ventricular cavity size or wall thickness, or both. Morphological characteristics of the triathlete's heart appear to be unrelated to success in triathlon races. Following the acute stress of triathlon competition, alterations in both systolic and diastolic function have been observed. Heart muscle fatigue is the most likely reason for these changes, since there is a rapid return to normal with rest. Like the cardiovascular system, the musculoskeletal system responds to triathlon training. Peripheral adaptations occur that lead to increased muscle respiratory capacity and to modifications in substrate utilisation. The musculoskeletal system is the site of most injuries to triathletes, and non-traumatic overuse injuries account for 80 to 85% of the musculoskeletal injuries. Maintenance of fluid and electrolyte balance is of primary importance for the triathlete both in day-to-day training and during races. Water may be an adequate replacement fluid for short distance triathlons, but some combination of carbohydrate, electrolyte and fluid replacement is necessary for longer races. Although the physiological bases for success in a triathlon are not well understood at present, the ability to maintain minimal alterations in the homeostasis of cardiovascular, haemodynamic, thermal, metabolic, and musculoskeletal functions are of obvious importance.

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)

Sports nutrition. Approaching the nineties

A sophisticated appreciation of the role of nutrition in athletic performance has been made possible by increasing knowledge of the physiology of exercise. The nutritional issues of training are of primary importance, since this occupies most of the athlete's effort. The nutritional support of an intense daily training programme includes an appropriately high energy intake, predominantly in the form of carbohydrate in order to continually replenish muscle glycogen stores. Recent review of the protein needs of athletes indicates that requirements may be substantially above those of sedentary subjects, to account for the oxidation of amino acids during exercise as well as the retention of nitrogen during periods of muscle building. However, these increased requirements are likely to be met by the generous protein intakes anticipated in a high energy diet. The same would seem to hold true for micronutrient considerations, although there is no evidence that vitamin requirements are considerably increased by exercise. Nevertheless, a high energy diet chosen from a sufficiently varied range of foods should allow micronutrient intakes well in excess of population recommended dietary intake levels. Current interest is focused on the mineral status of athletes, particularly that of iron and calcium. In the case of iron, there is a possibility that the increased level of loss by some endurance athletes will not be met by their usual dietary patterns. Screening for early signs of iron deficiency, and appropriate supplementation and dietary counselling seem warranted in high risk groups. Competition poses the challenge of identifying possible factors limiting performance, and taking steps to delay or reduce these. Of paramount importance is body temperature regulation through the maintenance of hydration levels. This issue has long been recognised, but recent studies of gastric emptying and the benefits of carbohydrate supplementation during exercise have caused an update of the advice to athletes regarding fluid intake during exercise. It now seems possible to simultaneously achieve fluid and carbohydrate requirements for endurance exercise within a wide range of choice of beverages containing up to 10% carbohydrate. Concern about the adequacy of carbohydrate fuel stores in endurance exercise situations is also well known. The recognition that training achieves various physiological adaptations to enhance the lifespan of fuel stores has taken away some of the attention previously focussed on carbohydrate-loading techniques.(ABSTRACT TRUNCATED AT 400 WORDS)

Gastric emptying during walking and running: effects of varied exercise intensity

Gastric emptying is increased during running (50%-70% maximal aerobic uptake, VO2max) as compared to rest. Whether this increase varies as a function of mode (i.e. walking vs running) and intensity of treadmill exercise is unknown. To examine the gastric emptying characteristics of water during treadmill exercise performed over a wide range of intensities relative to resting conditions, 10 men ingested 400 ml of water prior to each of six 15 min exercise bouts or 15 min of seated rest. Three bouts of walking exercise (1.57 m.s-1) were performed at increasing grades eliciting approximately 28%, 41% or 56% of VO2max. On a separate day, three bouts of running (2.68 ms-1) exercise were performed at grades eliciting approximately 57%, 65% or 75% of VO2max. Gastric emptying was increased during treadmill exercise at all intensities excluding 75% VO2max as compared to rest. Gastric emptying was similar for all intensities during walking and at 57% and 65% VO2max during running. However, running at 74% VO2max decreased the volume of original drink emptied as compared to all lower exercise intensities. Stomach secretions were markedly less during running as compared to walking and rest. These data demonstrate that gastric emptying is similarly increased during both moderate intensity (approximately 28%-65% VO2max) walking or running exercise as compared to resting conditions. However, gastric emptying decreases during high intensity exercise. Increases in gastric emptying during moderate intensity treadmill exercise may be related to increases in intragastric pressure brought about by contractile activity of the abdominal muscles.

Gastric emptying during exercise: effects of heat stress and hypohydration

To determine the effects of acute heat stress, heat acclimation and hypohydration on the gastric emptying rate of water (W) during treadmill exercise, ten physically fit men ingested 400 ml of W before each of three 15 min bouts of exercise (treadmill, approximately 50% VO2max) on five separate occasions. Stomach contents were aspirated after each exercise bout. Before heat acclimation (ACC), experiments were performed in a neutral (18 degrees C), hot (49 degrees C) and warm (35 degrees C) environment. Subjects were euhydrated for all experiments before ACC. After ACC, the subjects completed two more experiments in the warm (35 degrees C) environment; one while euhydrated and a final one while hypohydrated (-5% of body weight). The volume of ingested water emptied into the intestines at the completion of each exercise bout was inversely correlated (P less than 0.01) with the rectal temperature (r = -0.76). The following new observations were made: 1) exercise in a hot (49 degrees C) environment impairs gastric emptying rate as compared with a neutral (18 degrees C) environment, 2) exercise in a warm (35 degrees C) environment does not significantly reduce gastric emptying before or after heat acclimation, but 3) exercise in a warm environment (35 degrees C) when hypohydrated reduces gastric emptying rate and stomach secretions. Reductions in gastric emptying appear to be related to the severity of the thermal strain induced by an exercise/heat stress.