The lesson to learn from these observations is that, during exercise, we should keep well hydrated by ingesting fluids.6 This is not only for the sake of a better performance, which is not the goal of everyone. Most people exercise regularly as part of a healthy lifestyle for their well-being and, for a large segment of the population, to help maintain body mass. For these people, the goal should be to minimize the pain and increase the pleasure.
Hydration Before, During, and After Exercise
The amount of fluid that can reasonably be ingested during moderate exercise is around 0.6 L/h for a 70-kg subject. This is enough to fully compensate for water loss in sweat only a low workload (eg, brisk walking, slow jogging) in a cool environment. For a higher workload, it becomes difficult to compensate for the sweat lost during the exercise period, particularly in warm conditions. Thus, the individual should be well hydrated before the beginning of exercise and should ingest fluids not only during but also after the exercise period. A recent position paper from the American College of Sports Medicine7 suggests to ingest slowly, 5 to 10 mL/kg of fluid, 4 hours before exercise (350-700 mL for a 70-kg subject); 6 to 12 mL/kg per hour during exercise (400-800 mL/h for a 70-kg subject, in 3-4 fractions); and 1.5 L of fluid after exercise for each 1-kg reduction in body mass (Table).
Minerals and Carbohydrate in Hydrating Fluids
Except for people engaging in very prolonged endurance exercise (>2-3 hours) in warm environment, there is no need to add large amounts of minerals to the fluids that are ingested before, during, and after exercise. In fact, because the amount of minerals in sweat is much lower than in the plasma (eg, sodium 20-90 mmo/L in sweat vs 140 mmol/L in plasma),8-10 sweating increases the osmolality of plasma. Unlike the horse, which produces sweat that is very rich in salts,11 when humans sweat, more water is lost than minerals. A proper replacement of the fluid lost thus should focus on water, not on minerals. Interestingly, sport drinks do not contain a lot of mineral salts, as reviewed by Coombes and Hamilton.12 The concentrations of the mineral salts in these drinks range only between 4 and 19 mmol/L for sodium (vs 140 mmol/L in plasma), 0 to 9 mmol/L for chloride (100 mmol/L in plasma), and 3 to 16 mmol/L for potassium (5 mmol/L in plasma). When compared with plasma, however, these drinks are iso-osmotic or hyperosmotic because they contain large amounts of carbohydrates. Indeed, consistent evidence shows that carbohydrate ingestion postpones fatigue and increases endurance performance for exercise lasting about 90 minutes and more,13 the beneficial effect of carbohydrate being larger during exercise lasting longer than 2 hours.14 However, for shorter periods of exercise, such as 60 minutes or less, there is no convincing evidence that ingestion of carbohydrates either before or during exercise has any effect on fatigue and performance. A study by Clark et al15 actually showed that, for a 60-minute exercise period, the beneficial effect of carbohydrate ingestion, when present, could be a placebo effect. No improvement in performance was observed when (a) subjects were told that they received a placebo, but actually consumed carbohydrate or the placebo and (b) when they were told that they received carbohydrate and actually consumed carbohydrate. An improvement in performance was observed only when they were told that they received carbohydrate but were given the placebo. A recent study by Chambers et al16 using functional magnetic resonance imaging suggests a neurophysiological basis for the placebo effect of glucose. In this study, performance for an exercise period of about 60 minutes was better when a glucose or glucose polymer solution (vs a saccharin solution) was swished in the mouth but not swallowed. This effect of the mere presence of carbohydrate in the mouth was associated with an activation of reward zones in the brain.
Maximizing Energy Deficit During Exercise
A large number of people exercise mainly for 1 hour or less as part of a healthy lifestyle, to stay in shape, help delaying some effect of aging, and prevent some degenerative disease. For them, unlike endurance athletes who exercise for a longer period, there is no need to include carbohydrate in the fluids ingested before, during, or after exercise. If they do so, the added calories will counteract one of the effects of exercise, which is to contribute to the reduction or the maintenance of body weight-often one of the main objectives of a regular exercise program. In this type of program, for an average person, the amount of energy spent during an exercise session generally does not exceed about 400 to 500 kcal. If this individual ingests a drink with about 65 g of carbohydrate per liter,12 following the recommendation of the American College of Sports Medicine to drink before, during, and after exercise (Table), the total amount of energy provided by fluids will be about 650 kcal. This is ∼35% more than spent during the exercise period. Thus, instead of helping create the energy deficit needed to lose weight or maintain a healthy body weight, because of a poor choice of the rehydration fluid ingested, the exercise session can actually further contribute to the excess caloric intake, without providing any benefit to the participant in terms of reduction in fatigue. In this situation, water offers a clear advantage to sport drinks in terms of energy balance.
1. Gonzalèz-Alonso J, Crandall CG, Johnson JM. The cardiovascular challenge of exercising in the heat. J Physiol
2. Gonzalèz-Alonso J, Mora-Rodriguez R, Below PR, Coyle EF. Dehydration reduces cardiac output and increases systemic and cutaneous vascular resistance during exercise. J Appl Physiol
3. Kenefick RW, O'Moore KM, Mahood NV, Castellani JW. Rapid IV versus oral rehydration: responses to subsequent exercise heat stress. Med Sci Sports Exerc
4. Rehrer NJ. Fluid and electrolyte balance in ultra-endurance sport. Sports Med
5. Sawka MN, Montain SJ. Fluid and electrolyte supplementation for exercise heat stress. Am J Clin Nutr
6. Ganio MS, Casa DJ, Armstrong LE, Maresh CM. Evidence-based approach to lingering hydration questions. Clin Sports Med
7. Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc
8. Boisvert P, Brisson GR, Peronnet F. Effect of plasma prolactin on sweat rate and sweat composition during exercise in men. Am J Physiol
9. Morgan RM, Patterson MJ, Nimmo MA. Acute effects of dehydration on sweat composition in men during prolonged exercise in the heat. Acta Physiol Scand
10. Saat M, Sirisinghe RG, Singh R, Tochihara Y. Effects of short-term exercise in the heat on thermoregulation, blood parameters, sweat secretion and sweat composition of tropic-dwelling subjects. J Physiol Anthropol Appl Human Sci
11. Jansson A, Lindholm A, Dahlborn K. Effects of acute intravenous aldosterone administration on Na(+), K(+), and water excretion in the horse. J Appl Physiol
12. Coombes JS, Hamilton KL. The effectiveness of commercially available sports drinks. Sports Med
13. Jeukendrup AE. Carbohydrate intake during exercise and performance. Nutrition
14. Karelis AD, Smith JW, Passe DH, Péronnet F. Carbohydrate administration and exercise performance: what are the potential mechanisms involved? Sports Med
15. Clark VR, Hopkins WG, Hawley JA, Burke LM. Placebo effect of carbohydrate feedings during a 40-km cycling time trial. Med Sci Sports Exerc
© 2010 Lippincott Williams & Wilkins, Inc.
16. Chambers ES, Bridge MW, Jones DA. Carbohydrate sensing in the human mouth: effects on exercise performance and brain activity. J Physiol