Optimal Use of Fluids of Varying Formulations to Minimise Exercise-Induced Disturbances in Homeostasis

Summary

The rationale underlying the development of various formulations of beverages for consumption before, during, and/or after physical exercise is that such formulations should minimise some of the disturbances in physiological homeostasis that occur during exercise and thereby prevent injury and/or enhance performance. Exercise- and dehydration-induced increases in core temperature, body fluid osmolality, heart rate, losses of plasma and other body fluid volumes, and carbohydrate depletion are probably the most important homeostatic disturbances that can be ameliorated by fluid consumption. With the exception of athletes subject to hyponatraemia after consumption of ordinary water during prolonged activity, changes in electrolyte concentrations in the body fluids of most athletes do not justify the inclusion of electrolytes in fluid replacement beverages to be consumed during exercise. However, small amounts of sodium added to water does speed gastric emptying and fluid absorption from the intestine.

Recent evidence suggests that a precompetition meal high in easily digested carbohydrates should be consumed not later than 5 to 6 hours before competition. There is little published research on the optimal composition of this meal. Water ingestion 30 to 60 minutes before exercise seems to be of benefit to temperature regulation and cardiovascular homeostasis if the exercise is of moderate intensity (50 to 65% V̇_2max), but probably has little effect at the higher intensities of athletic performance. There is no systematic evidence to support the inclusion of calcium or sodium chloride in drinks consumed an hour or 2 before exercise. Furthermore, if glucose solutions are fed 15 to 45 minutes before prolonged exercise, they will probably cause a fall in blood glucose during exercise and may adversely affect performance. These adverse effects are not present when fructose is consumed before exercise. Contrary to the adverse effects of glucose feedings 15 to 60 minutes before exercise, the consumption of 18 to 50% solutions of glucose or glucose polymers 5 minutes before prolonged exercise seems to have potential for improving endurance performance. Similarly, the inclusion of caffeine in beverages consumed 60 minutes before prolonged exercise improves athletic performance for many subjects. Others may be hypersensitive to the effects of caffeine and are adversely affected by its use.

For exercise leading to exhaustion in less than 30 minutes, neither caffeine nor carbohydrate ingestion is effective in minimising homeostatic perturbations or improving exercise performance. On the other hand, the addition of bicarbonate to precompetition drinks has been shown to favourably affect plasma pH and enhance exercise performance in events lasting from 1 to 10 minutes if the exercise test is preceded by a warmup exercise period lasting 10 to 30 minutes. Attempts to rehydrate with various beverages after self-imposed severe dehydration are often effective in restoring cardiovascular function to near normal levels, but they are almost always ineffective, regardless of beverage formulation, in restoring performance to the level achieved with normal hydration.

Although much has been made of the critical importance of the gastric emptying rate for establishing the value of various beverage formulations for fluid replenishment during prolonged exercise, recent evidence suggests that differences in gastric emptying rates among beverages are not particularly important in determining the efficacy of various drinks for minimising homeostatic disturbances and enhancing performance. In spite of presumably slower rates of gastric emptying, beverages containing simple sugars or glucose polymers with or without small amounts of electrolytes minimise disturbances in temperature regulation and cardiovascular function as well as ordinary water, maintain blood glucose levels better than water, and are likely to enhance athletic performance compared with water. Therefore, although optimal formulations for prolonged exercise are still unknown, it seems appropriate to conclude that beverages containing 5 to 10% glucose or sucrose, or 5 to 20% glucose polymers in volumes of 150 to 250ml consumed every 15 to 20 minutes are preferable to ordinary water. The more palatable these beverages are for a given athlete, the more likely it is that enough liquid will be consumed during exercise to optimally affect homeostasis and performance.

Little research has been published on the question of optimal beverage formulations for rehydration during recovery periods following exercise. From what is known, it appears that fluids containing glucose, sucrose, or glucose polymers in concentrations of 5 to 20% might be appropriate if rapid rehydration is a reasonable objective. To quickly restore electrolytes lost during exercise, small amounts of sodium, chloride, and potassium may be added to the recovery beverages; if speed of recovery is not critical, the normal athlete’s diet can usually meet the electrolyte replacement needs.