Effect of hypohydration on gastric emptying and intestinal absorption during exercise

The Impact of Heat Acclimation on Gastrointestinal Function following Endurance Exercise in a Hot Environment

To determine the effects of heat acclimation on gastrointestinal (GI) damage and the gastric emptying (GE) rate following endurance exercise in a hot environment. Fifteen healthy men were divided into two groups: endurance training in hot (HOT, 35 °C, n = 8) or cool (COOL, 18 °C, n = 7) environment. All subjects completed 10 days of endurance training (eight sessions of 60 min continuous exercise at 50% of the maximal oxygen uptake (V·O2max). Subjects completed a heat stress exercise tests (HST, 60 min exercise at 60% V·O2max) to evaluate the plasma intestinal fatty acid-binding protein (I-FABP) level and the GE rate following endurance exercise in a hot environment (35 °C) before (pre-HST) and after (post-HST) the training period. We assessed the GE rate using the ¹³C-sodium acetate breath test. The core temperature during post-HST exercise decreased significantly in the HOT group compared to the pre-HST (p = 0.004) but not in the COOL group. Both the HOT and COOL groups showed exercise-induced plasma I-FABP elevations in the pre-HST (p = 0.002). Both groups had significantly attenuated exercise-induced I-FABP elevation in the post-HST. However, the reduction of exercise-induced I-FABP elevation was not different significantly between both groups. GE rate following HST did not change between pre- and post-HST in both groups, with no significant difference between two groups in the post-HST. Ten days of endurance training in a hot environment improved thermoregulation, whereas exercise-induced GI damage and delay of GE rate were not further attenuated compared with training in a cool environment.

Comparisons of isomaltulose, sucrose, and mixture of glucose and fructose ingestions on postexercise hydration state in young men

PURPOSE

Isomaltulose is a low glycemic and insulinaemic carbohydrate available as a constituent in sports drink. However, it remains unclear whether postexercise rehydration achieved by isomaltulose drink ingestion alone differs as compared to other carbohydrates.

METHODS

Thirteen young men performed intermittent exercise in the heat (35 °C and relative humidity 40%) to induce a state of hypohydration as defined by a 2% loss in body mass. Thereafter, participants were rehydrated by ingesting drinks equal to the volume of body mass loss with either a mixture of 3.25% glucose and 3.25% fructose, 6.5% sucrose (SUC), or 6.5% isomaltulose (ISO) within the first 30 min of a 3-h recovery. The change in plasma volume (ΔPV) from pre-exercise baseline, blood glucose, and plasma insulin concentration were assessed every 30-min.

RESULTS

ΔPV was lower in ISO as compared to SUC until 90 min of the recovery (all P ≤ 0.038) with no difference thereafter (all P ≥ 0.391). The ΔPV were paralleled by concomitant changes in blood glucose levels that were greater in ISO as compared to other drinks after 90 min of the recovery (all P ≤ 0.035). Plasma insulin secretion, which potentially enhances renal sodium reabsorption and fluid retention, did not differ between the trials (interaction, P = 0.653). ISO induced a greater net fluid volume retention as compared to SUC (P = 0.010).

CONCLUSION

We showed that rehydration with an isomaltulose drink following exercise-heat stress induces comparable recovery of PV and a greater net fluid retention as compared to other drinks, albeit this response is delayed. The delayed water transport along with glucose absorption may modulate this response. This trial was registered in 25th Sep 2019 at https://www.umin.ac.jp/ as UMIN000038099. (249/250).

National Athletic Trainers' Association Position Statement: Fluid Replacement for the Physically Active

OBJECTIVE

  To present evidence-based recommendations that promote optimized fluid-maintenance practices for physically active individuals.

BACKGROUND

  Both a lack of adequate fluid replacement (hypohydration) and excessive intake (hyperhydration) can compromise athletic performance and increase health risks. Athletes need access to water to prevent hypohydration during physical activity but must be aware of the risks of overdrinking and hyponatremia. Drinking behavior can be modified by education, accessibility, experience, and palatability. This statement updates practical recommendations regarding fluid-replacement strategies for physically active individuals.

RECOMMENDATIONS

  Educate physically active people regarding the benefits of fluid replacement to promote performance and safety and the potential risks of both hypohydration and hyperhydration on health and physical performance. Quantify sweat rates for physically active individuals during exercise in various environments. Work with individuals to develop fluid-replacement practices that promote sufficient but not excessive hydration before, during, and after physical activity.

Effect of Different Osmolalities, CHO Types, and [CHO] on Gastric Emptying in Humans

PURPOSE

This study investigated the effect of beverage osmolalities, carbohydrate (CHO) type, and CHO concentration on gastric emptying in euhydrated subjects at rest.

METHODS

The gastric emptying of water (W), four glucose beverages (2%, 4%, 6%, and 8% glucose: 2G, 4G, 6G, and 8G), and four sucrose beverages (2%, 4%, 6%, and 8% sucrose: 2S, 4S, 6S, and 8S) were determined in eight healthy subjects using the modified George double-sampling technique. Subjects ingested a beverage (7 mL·kg body weight) containing 25 ppm phenol red as quickly as possible (≤1.0 min), and subsequent gastric and blood samples were collected every 10 min for 40 min. A linear regression and a repeated-measures ANOVA were used for statistical analysis.

RESULTS

The gastric secretion volume was not significantly different among beverages across time. Gastric residual beverage volume (GRBV) at each sampling time point was not different among 2S, 4S, 6S, 8S, and water (P > 0.05). The 8G resulted in a significantly greater GRBV compared with other beverages at 20, 30, and 40 min (P < 0.05). GRBV from 6G was significantly higher than 2G at 30 min, but no other statistical differences were found among W, 2G, 4G, and 6G. The 8S had a greater GRBV compared with W at 40 min (P < 0.05). Mean gastric osmolality positively correlated to mean GRBV (r = 0.93). Gastric emptying rate was negatively correlated to the calories emptied (r = 0.84) with a greater effect from glucose beverages compared with sucrose beverages.

CONCLUSIONS

These data suggest that glucose exerts a stronger inhibitory stimulus compared with sucrose on gastric emptying and that a physiological threshold exists for the combined influence of glucose concentration and beverage osmolality to trigger the feedback inhibition of gastric emptying.

Systematic review: exercise-induced gastrointestinal syndrome-implications for health and intestinal disease

BACKGROUND

"Exercise-induced gastrointestinal syndrome" refers to disturbances of gastrointestinal integrity and function that are common features of strenuous exercise.

AIM

To systematically review the literature to establish the impact of acute exercise on markers of gastrointestinal integrity and function in healthy populations and those with chronic gastrointestinal conditions.

METHODS

Search literature using five databases (PubMed, EBSCO, Web of Science, SPORTSdiscus, and Ovid Medline) to review publications that focused on the impact of acute exercise on markers of gastrointestinal injury, permeability, endotoxaemia, motility and malabsorption in healthy populations and populations with gastrointestinal diseases/disorders.

RESULTS

As exercise intensity and duration increases, there is considerable evidence for increases in indices of intestinal injury, permeability and endotoxaemia, together with impairment of gastric emptying, slowing of small intestinal transit and malabsorption. The addition of heat stress and running mode appears to exacerbate these markers of gastrointestinal disturbance. Exercise stress of ≥2 hours at 60% VO_2max appears to be the threshold whereby significant gastrointestinal perturbations manifest, irrespective of fitness status. Gastrointestinal symptoms, referable to upper- and lower-gastrointestinal tract, are common and a limiting factor in prolonged strenuous exercise. While there is evidence for health benefits of moderate exercise in patients with inflammatory bowel disease or functional gastrointestinal disorders, the safety of more strenuous exercise has not been established.

CONCLUSIONS

Strenuous exercise has a major reversible impact on gastrointestinal integrity and function of healthy populations. The safety and health implications of prolonged strenuous exercise in patients with chronic gastrointestinal diseases/disorders, while hypothetically worrying, has not been elucidated and requires further investigation.

Ad-libitum drinking and performance during a 40-km cycling time trial in the heat

The aim of this study was to investigate if drinking ad-libitum can counteract potential negative effects of a hypohydrated start caused by fluid restriction during a 40-km time trial (TT) in the heat. Twelve trained males performed one 40-km cycling TT euhydrated (EU: no water during the TT) and two 40-km cycling TTs hypohydrated. During one hypohydrated trial no fluid was ingested (HYPO), during the other trial ad-libitum water ingestion was allowed (FLUID). Ambient temperature was 35.2 ± 0.2 °C, relative humidity 51 ± 3% and airflow 7 m·s(-1). Body mass (BM) was determined at the start of the test, and before and after the TT. During the TT, power output, heart rate (HR), gastrointestinal temperature, mean skin temperature, rating of perceived exertion (RPE), thermal sensation, thermal comfort and thirst sensation were measured. Prior to the start of the TT, BM was 1.2% lower in HYPO and FLUID compared to EU. During the TT, BM loss in FLUID was lower compared to EU and HYPO (1.0 ± 0.8%, 2.7 ± 0.2% and 2.6 ± 0.3%, respectively). Hydration status had no effect on power output (EU: 223 ± 32 W, HYPO: 217 ± 39 W, FLUID: 224 ± 35 W), HR, gastrointestinal temperature, mean skin temperature, RPE, thermal sensation and thermal comfort. Thirst sensation was higher in HYPO than in EU and FLUID. It was concluded that hypohydration did not adversely affect performance during a 40-km cycling TT in the heat. Therefore, whether or not participants consumed fluid during exercise did not influence their TT performance.

Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations

Gastrointestinal problems are common, especially in endurance athletes, and often impair performance or subsequent recovery. Generally, studies suggest that 30-50% of athletes experience such complaints. Most gastrointestinal symptoms during exercise are mild and of no risk to health, but hemorrhagic gastritis, hematochezia, and ischemic bowel can present serious medical challenges. Three main causes of gastrointestinal symptoms have been identified, and these are either physiological, mechanical, or nutritional in nature. During intense exercise, and especially when hypohydrated, mesenteric blood flow is reduced; this is believed to be one of the main contributors to the development of gastrointestinal symptoms. Reduced splanchnic perfusion could result in compromised gut permeability in athletes. However, although evidence exists that this might occur, this has not yet been definitively linked to the prevalence of gastrointestinal symptoms. Nutritional training and appropriate nutrition choices can reduce the risk of gastrointestinal discomfort during exercise by ensuring rapid gastric emptying and the absorption of water and nutrients, and by maintaining adequate perfusion of the splanchnic vasculature. A number of nutritional manipulations have been proposed to minimize gastrointestinal symptoms, including the use of multiple transportable carbohydrates, and potentially the use of nutrients that stimulate the production of nitric oxide in the intestine and thereby improve splanchnic perfusion. However, at this stage, evidence for beneficial effects of such interventions is lacking, and more research needs to be conducted to obtain a better understanding of the etiology of the problems and to improve the recommendations to athletes.

Sports nutrition needs: before, during, and after exercise

This article discusses how athletes should properly fuel their bodies before, during, and after exercise to maximize athletic performance. Emphasis is placed on hydration status and glycogen stores being maintained above deficits that negatively affect sport performance. Timing of nutrient intake is as important as composition.

Physiology and pathophysiology of splanchnic hypoperfusion and intestinal injury during exercise: strategies for evaluation and prevention

Physical exercise places high demands on the adaptive capacity of the human body. Strenuous physical performance increases the blood supply to active muscles, cardiopulmonary system, and skin to meet the altered demands for oxygen and nutrients. The redistribution of blood flow, necessary for such an increased blood supply to the periphery, significantly reduces blood flow to the gut, leading to hypoperfusion and gastrointestinal (GI) compromise. A compromised GI system can have a negative impact on exercise performance and subsequent postexercise recovery due to abdominal distress and impairments in the uptake of fluid, electrolytes, and nutrients. In addition, strenuous physical exercise leads to loss of epithelial integrity, which may give rise to increased intestinal permeability with bacterial translocation and inflammation. Ultimately, these effects can deteriorate postexercise recovery and disrupt exercise training routine. This review provides an overview on the recent advances in our understanding of GI physiology and pathophysiology in relation to strenuous exercise. Various approaches to determine the impact of exercise on the individual athlete's GI tract are discussed. In addition, we elaborate on several promising components that could be exploited for preventive interventions.

Review article: the pathophysiology and management of gastrointestinal symptoms during physical exercise, and the role of splanchnic blood flow

BACKGROUND

The prevalence of exercise-induced gastrointestinal (GI) symptoms has been reported up to 70%. The pathophysiology largely remains unknown.

AIM

To review the physiological and pathophysiological changes of the GI-tract during physical exercise and the management of the most common gastrointestinal symptoms.

METHODS

Search of the literature published in the English and Dutch languages using the Pubmed database to review the literature that focused on the relation between splanchnic blood flow (SBF), development of ischaemia, postischaemic endotoxinemia and motility.

RESULTS

During physical exercise, the increased activity of the sympathetic nervous system (SNS) redistributes blood flow from the splanchnic organs to the working muscles. With prolonged duration and/or intensity, the SBF may be decreased by 80% or more. Most studies point in the direction of increased SNS-activity as central driving force for reduction in SBF. A severely reduced SBF may frequently cause GI ischaemia. GI-ischaemia combined with reduced vagal activity probably triggers changes in GI-motility and GI absorption derangements. GI-symptoms during physical exercise may be prevented by lowering the exercise intensity, preventing dehydration and avoiding the ingestion of hypertonic fluids.

CONCLUSIONS

Literature on the pathophysiology of exercise-induced GI-symptoms is scarce. Increased sympathetic nervous system activity and decreased splanchnic blood flow during physical exercise seems to be the key factor in the pathogenesis of exercise-induced GI-symptoms, and this should be the target for symptom reduction.

Food-dependent, exercise-induced gastrointestinal distress

Among athletes strenuous exercise, dehydration and gastric emptying (GE) delay are the main causes of gastrointestinal (GI) complaints, whereas gut ischemia is the main cause of their nausea, vomiting, abdominal pain and (blood) diarrhea. Additionally any factor that limits sweat evaporation, such as a hot and humid environment and/or body dehydration, has profound effects on muscle glycogen depletion and risk for heat illness. A serious underperfusion of the gut often leads to mucosal damage and enhanced permeability so as to hide blood loss, microbiota invasion (or endotoxemia) and food-born allergen absorption (with anaphylaxis). The goal of exercise rehydration is to intake more fluid orally than what is being lost in sweat. Sports drinks provide the addition of sodium and carbohydrates to assist with intestinal absorption of water and muscle-glycogen replenishment, respectively. However GE is proportionally slowed by carbohydrate-rich (hyperosmolar) solutions. On the other hand, in order to prevent hyponatremia, avoiding overhydration is recommended. Caregiver's responsibility would be to inform athletes about potential dangers of drinking too much water and also advise them to refrain from using hypertonic fluid replacements.

Hydration and Health

Daily adequate water intake to maintain euhydration is arguably the most important nutrient requirement for humans. Within a margin of error, the body regulates the maintenance of body fluid balance and especially that of the plasma volume, through mechanisms that stimulate thirst and/or modify the rate of urine production. However, there are circumstances such as with excessive sweating during exercise in the heat, osmotic diarrhea, or excessive fluid consumption, or water intoxication where normal mechanisms of regulation may be inadequate to compensate for acute changes in hydration status and result in life threatening consequences. Health and onset of disease may be affected by the chronic hydration state of individual. The risks of colorectal cancer, nephrolithiasis in those with a history of kidney stones, and bladder cancer may be reduced by more frequent water consumption. Recent research suggests that appropriate timing of water intake around meal occasions may help reduce energy intake and contribute to maintenance of body weight in overweight individuals. Definitive benefits of hydration on cardiovascular and oral health and general immune system function remain to be determined. It is also unclear whether the health benefits of water and fluid ingestion are a function of the process of frequent fluid intake or the maintenance of a potentially expanded state of hydration.

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.

Electrolyte and plasma changes after ingestion of pickle juice, water, and a common carbohydrate-electrolyte solution

CONTEXT

Health care professionals advocate that athletes who are susceptible to exercise-associated muscle cramps (EAMCs) should moderately increase their fluid and electrolyte intake by drinking sport drinks. Some clinicians have also claimed drinking small volumes of pickle juice effectively relieves acute EAMCs, often alleviating them within 35 seconds. Others fear ingesting pickle juice will enhance dehydration-induced hypertonicity, thereby prolonging dehydration.

OBJECTIVE

To determine if ingesting small quantities of pickle juice, a carbohydrate-electrolyte (CHO-e) drink, or water increases plasma electrolytes or other selected plasma variables.

DESIGN

Crossover study.

SETTING

Exercise physiology laboratory.

PATIENTS OR OTHER PARTICIPANTS

Nine euhydrated, healthy men (age = 25 +/- 2 years, height = 179.4 +/- 7.2 cm, mass = 86.3 +/- 15.9 kg) completed the study.

INTERVENTION(S)

Resting blood samples were collected preingestion (-0.5 minutes); immediately postingestion (0 minutes); and at 1, 5, 10, 15, 20, 25, 30, 45, and 60 minutes postingestion of 1 mL/kg body mass of pickle juice, CHO-e drink, or tap water.

MAIN OUTCOME MEASURE(S)

Plasma sodium concentration, plasma magnesium concentration, plasma calcium concentration, plasma potassium concentration, plasma osmolality, and changes in plasma volume were analyzed. Urine specific gravity, osmolality, and volume were also measured to characterize hydration status.

RESULTS

Mean fluid intake was 86.3 +/- 16.7 mL. Plasma sodium concentration, plasma magnesium concentration, plasma calcium concentration, plasma osmolality, and plasma volume did not change during the 60 minutes after ingestion of each fluid (P >or= .05). Water ingestion slightly decreased plasma potassium concentration at 60 minutes (0.21 +/- 0.14 mg/dL [0.21 +/- 0.14 mmol/L]; P <or= .05).

CONCLUSIONS

At these volumes, ingestion of pickle juice and CHO-e drink did not cause substantial changes in plasma electrolyte concentrations, plasma osmolality, or plasma volume in rested, euhydrated men. Concern that ingesting these volumes of pickle juice might exacerbate an athlete's risk of dehydration-induced hypertonicity may be unwarranted. If EAMCs are caused by large electrolyte loss due to sweating, these volumes of pickle juice or CHO-e drink are unlikely to restore any deficit incurred by exercise.

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.

Urine specific gravity in exercisers prior to physical training

Urine specific gravity (USG) is used as an index of hydration status. Many studies have used USG to estimate pre-exercise hydration in athletes. However, very little is known about the pre-exercise hydration status of recreational exercisers. The purpose of the present study was to measure the pre-exercise USG in a large sample of recreational exercisers who attended 2 different fitness centers in the United States. In addition, we wanted to determine if factors such as time of day, geographic location, and gender influenced USG. We tested 166 subjects in Chicago and 163 subjects in Los Angeles. Subjects completed a survey on their typical training regimen and fluid-replacement habits, and thereafter voided and delivered a urine sample to the investigators prior to beginning exercise. Samples were measured on site for USG using a hand-held refractometer. The mean (SD) USG was 1.018 (+/- 0.007) for all subjects. Males had a higher average USG (1.020 +/- 0.007) when compared with females (1.017 +/- 0.008; p = 0.001). Despite differences in climate, no difference in mean USG occurred based on location or time of day. Based on standards used for athletes (USG > or = 1.020), 46% of the exercisers were likely to be dehydrated.

Interaction of carbohydrate metabolism and rat liquid gastric emptying in sustained running

BACKGROUND

Moderate to severe running usually leads to gastrointestinal dysmotility and critical energy exhaustion. It is unknown whether the carbohydrate metabolism of runners can influence gastric emptying (GE). Using a running rat model, the present study explored the impact of exercise/carbohydrate metabolism on liquid GE.

METHODS

Rats were put on the runways of a moving treadmill for 1 h. Trained rats underwent daily running 5 days a week for 4 weeks. Untrained rats were those put on a quiet treadmill to serve as sham exercise. On the motility study day, trained and untrained rats ran for 45 min. After orogastric feeding of radiochromium marker, they resumed running for an additional 15 min and were then killed in order to measure GE. Another group of trained and untrained rats received lactate infusion for 1 h in the quiet condition to measure their GE. The third group of rats received glucose infusion during running to measure GE.

RESULTS

Running of untrained (P < 0.05) and trained (P < 0.01) rats enhanced GE compared to sham exercise. Running for the untrained rats rather than the trained counterparts had diminished plasma glucose level (P < 0.05). Running also elevated plasma lactate levels for both untrained (P < 0.001) and trained rats (P < 0.01). Lactate infusion delayed GE in untrained (P < 0.01) and trained rats (P < 0.05). Glucose infusion of untrained rats during running was not only to correct hypoglycemia (P < 0.01) but also to restore their enhanced GE (P < 0.01).

CONCLUSIONS

Running-induced hypoglycemia, rather than lactate accumulation, is one of the essential factors leading to enhanced liquid GE in untrained rats.

Nutritional considerations in triathlon

Triathlon combines three disciplines (swimming, cycling and running) and competitions last between 1 hour 50 minutes (Olympic distance) and 14 hours (Ironman distance). Independent of the distance, dehydration and carbohydrate (CHO) depletion are the most likely causes of fatigue in triathlon, whereas gastrointestinal (GI) problems, hyperthermia and hyponatraemia are potentially health threatening, especially in longer events. Although glycogen supercompensation may be beneficial for triathlon performance (even Olympic distance), this does not necessarily have to be achieved by the traditional supercompensation protocol. More recently, studies have revealed ways to increase muscle glycogen concentrations to very high levels with minimal modifications in diet and training. During competition, cycling provides the best opportunity to ingest fluids. The optimum CHO concentration seems to be in the range of 5-8% and triathletes should aim to achieve a CHO intake of 60-70 g/hour. Triathletes should attempt to limit body mass losses to 1% of body mass. In all cases, a drink should contain sodium (30-50 mmol/L) for optimal absorption and prevention of hyponatraemia.Post-exercise rehydration is best achieved by consuming beverages that have a high sodium content (>60 mmol/L) in a volume equivalent to 150% of body mass loss. GI problems occur frequently, especially in long-distance triathlon. Problems seem related to the intake of highly concentrated carbohydrate solutions, or hyperosmotic drinks, and the intake of fibre, fat and protein. Endotoxaemia has been suggested as an explanation for some of the GI problems, but this has not been confirmed by recent research. Although mild endotoxaemia may occur after an Ironman-distance triathlon, this does not seem to be related to the incidence of GI problems. Hyponatraemia has occasionally been reported, especially among slow competitors in triathlons and probably arises due to loss of sodium in sweat coupled with very high intakes (8-10 L) of water or other low-sodium drinks.