Introduction
Research over the past 20 years has clearly documented that the type, amount, composition, and timing of food intake can dramatically affect exercise performance, recovery from exercise, body weight and composition, and health. As individuals increase their physical activity (PA) or work activity to 1 hour per day or more, the importance of adequate energy and nutrient intakes become more critical. With our growing obesity epidemic and the call for the American public to eat better and exercise more, the interest in nutrition, PA, and weight loss has increased. Thus, providing active individuals with accurate and precise nutritional recommendations will reduce the use of unhealthy dietary practices or sport and weight loss supplements not grounded in science.
Over the past 8 years, the Food and Nutrition Board (FNB) of the Institutes of Medicine (IOM) has published new dietary reference intake (DRIs) for micronutrients (vitamins and minerals) and related compounds [1–4], energy, macronutrients [5•], and fluids and electrolytes [6•]. For the first time, the specific needs of active individuals were considered for energy and some nutrients and recommendations were given for the level of PA (60 min/d) needed to maintain body weight. Brooks et al. [7•] and Blair et al. [8] provide a detailed discussion of these PA recommendations and how the IOM recommendations differ from those given by the American College of Sports Medicine (ACSM). The dietary recommendations summarized in this paper are based on extensive research in the area of nutrition and PA, and five current national and international publications related to the nutritional needs of active individuals. These five publications are the Position Statement on Nutrition and Athletic Performance [9], the IOM report for energy and macronutrients [6•], the IOM report on fluids and electrolytes [6•], and the 2003 International Olympic Committee's (IOC) Consensus Statement on Sport Nutrition [10] and the accompanying research articles [11–19] recently published in the Journal of Sport Science in 2004. These recommendations are general in nature, because energy and nutrient needs will vary depending on the age, sex, exercise training intensity and duration, health issues, and the sport in which one participates. This paper addresses the current energy, macronutrient, and fluid needs of active individuals.
Energy Needs
Determining the energy needs of an active individual requires that a number of factors be considered, including age, sex, body size, sport, level and intensity of PA, and activities of daily living. The IOM report [5•] provided some general energy recommendations for active and very active individuals based on age and body size. They categorized an active individual as walking between 6 and 10 miles per day, although a very active individual walks more than 10 miles per day at 2 to 4 mph. For example, an active or very active 30-year-old man weighing 72.2 kg (160 lbs) and having a body mass index (BMI) of 24.99 (kg/m2), would have estimated energy needs of 2959 and 3434 kcal/d, respectively. An active or very active 30-year-old woman weighing 68 kg (150 lbs) and having a BMI of 24.99 (kg/m2), would have estimated energy needs of 2477 and 2807 kcal/d, respectively. For the active individual with a healthy body weight, the goal is to maintain adequate energy intake to assure that weight is maintained. The dietary plan needs to focus on timing of meals and snacks throughout the day and insure that these individuals are well fueled before training or practice sessions. For most individuals, energy needs decrease with age, even if activity levels remain constant; thus, the amount of energy required to maintain body weight will decrease. For this reason, body weight typically increases with age even in active individuals.
Maintaining Body Weight and Body Composition
Weight loss and dieting are popular topics, even among active individuals who typically have body weights within normal ranges for their height (BMI 19–25 kg/m2). Many active individuals want to change their body composition (eg, get leaner) or lose weight to improve their sport performance or appearance. If weight loss is the goal, it should be accomplished slowly during a period when the individual is not participating in competitive events [20]. Conversely, if weight gain is the goal, this can be accomplished by adding about 500 to 1000 kcal/d into the diet per day, while the athlete participates in appropriate strength training exercises. [9,21]. It is important that these steps be done simultaneously to assure that the extra energy consumed is contributing to increased muscle mass and not fat mass. The athlete needs to understand that increases in muscle mass usually occur slowly and will depend on one's genetic make-up, degree of positive energy balance achieved, and the type of strength training program being used [22].
Because the goal of weight loss is to maximize fat loss while minimizing lean tissues losses, weight should be lost gradually (∼ 1–2 lbs/wk or 0.5–1.0 kg/wk) [9,20] with an energy deficit from both diet and exercise of approximately 500 to 1000 kcal/d [20]. A number of problems arise that defeat the goals of a weight-loss program if an active individual restricts energy too severely. Severe energy restriction typically means the nutritional quality of the diet is compromised, lean tissue is lost, and the ability to exercise decreases. Severe energy restriction can be extremely stressful and lead to a preoccupation with food, loss of motivation, and the inability to stay on the diet [21,23–25]. For these reasons, it is not recommended that energy be restricted to less than 1800 kcal/d for active women and less than 2000 kcal/d for active men. In order to remain physically active while dieting, the active individual needs a diet that provides adequate carbohydrate for glycogen replacement, adequate protein for the maintenance and repair of lean tissue, and enough fat to meet essential fatty acid needs and to make the diet palatable. Depending on the food preferences of the individual, a multivitamin and mineral supplement may also be necessary while dieting for weight loss.
Determining a weight loss goal for an active individual can be difficult. First, a realistic healthy body weight for his or her level of PA must be established. This decision should be made jointly with the active individual and based on past dieting and weight loss experiences, type of PA currently engaged in, the eating environment in which they work and live, genetics (family size and shape), health risk factors, and psychologic issues. If an unrealistic weight goal is set, there is a high probability of failure, which has a number of emotional and psychologic outcomes. A healthy weight is one that can be realistically maintained, allows for positive advances in exercise performance, minimizes the risk of injury or illness, is consistent with long-term good health, and reduces the risk factors for chronic disease [5•,9]. Unfortunately, failure to meet weight loss goals in some sports can result in severe consequences, such as being cut from the team, restriction from participation, or elimination from competition. These situations can result in active individuals chronically dieting to maintain a lower than healthy body weight, causing loss of lean mass, decreased ability to perform at high intensities, loss of muscle strength and endurance, and poor nutritional status. Some of these individuals may develop disordered eating, which in severe cases lead to a clinical eating disorder [26]. Nutritional strategies for identification, intervention, and treatment of eating disorders in active individuals and athletes have been presented elsewhere [27,28].
Chronic dieting in the active woman with normal body weight can have devastating consequences. Energy restriction can result in poor intakes of bone-building nutrients, weight loss, increases in catabolic hormones, and menstrual dysfunction [14,29–31]. The decrease in reproductive hormones can lead to loss of (or failure to gain) bone mass in young female athletes and active adult women [32,33]. Finally, a pattern of low energy intake, while exercise energy expenditure is high, can put active women at risk for one or more of the disorders in the female athlete triad (amenorrhea, disordered eating, and osteoporosis) [33].
Macronutrient Requirements for Exercise
Not only do active individuals need adequate energy to maintain weight and repair damaged tissues, they also need the right proportion of carbohydrate, protein, and fat in their diets. The exact proportions of these nutrients will depend on an individual's total energy needs, the intensity, duration and frequency in which they participate in PA, the type of exercise they engage in, and their health, body size, age, and sex. The recommendations for macronutrients for those engaged in daily PA are given below and in Table 1.
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Table 1:
Dietary guidelines for Americans and dietary reference intakes and for macronutrients and recommendations for active individuals
Carbohydrate
The exact mix of fuels (protein, fat, carbohydrate) that is burned during exercise depends on a number of factors including prior nutritional status (eg, when and what was last eaten), the intensity and duration of the exercise performed, and one's level of fitness. In general, as exercise intensity increases the use of carbohydrate for energy will also increase [34,35]. Duration of exercise can also impact fuel use. When exercise duration exceeds 60 minutes, muscle glycogen becomes depleted; thus, circulating blood glucose becomes as a principle source of carbohydrate. As blood glucose decreases during exercise, the ability to perform intensity exercise will decrease [36]. Although fat can be used as a source of energy over a wide range of exercise intensities, the proportion of energy contributed by fat decreases as exercise intensity increases. Thus, in long-endurance activities, carbohydrate becomes the dominant fuel source [37]. The contribution of protein as a fuel source during exercise is typically small (eg, < 5% of the energy expended), but depends on the nutritional status of the individual prior to exercise and whether carbohydrate is being consumed during exercise [38,39]. When glycogen stores become depleted, protein can also be used as a glucoenogenic substrate to help maintain blood glucose. Thus, the amount of protein used during exercise is dependent on a number of other factors. For example, when subjects are tested after an overnight fast, the contribution of fat to the energy pool is greater than when these same individuals are tested after a meal [40]. In both situations, the exercise performed was moderate (∼ 50% of VO2max). For higher intensity exercise (> 65% of VO2max) neither prior feeding nor exercise training significantly altered fuel used [40].
The 2002 IOM report entitled Dietary Reference Intakes: Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein and Amino Acids estimated an acceptable macronutrient distribution range (AMDR) range for carbohydrate (45%–65% of energy), protein (10%–35% of energy), and fat (20%–35% of energy). For most active individuals, these proportions of energy from carbohydrate, protein, and fat adequately refuel the athlete and provide adequate protein for muscle building and repair. The AMDR is defined as a range of intakes associated with reduced risk of chronic disease, while providing adequate intake of essential nutrients. The AMDRs are broad and allow for developing flexible dietary recommendations across a variety of activity levels, body sizes, food preferences, and health-related dietary issues.
Because active individuals vary greatly in the type of sport they perform and their body size, it is easiest to make macronutrient recommendations for protein and carbohydrate based on body size (eg, g/kg body weight). For individuals engaged in exercise of moderate duration and low intensity, 5 to 7 g of carbohydrate/kg body weight should be sufficient to replenish glycogen and refuel the athlete for the next day [11]. As training intensity and duration increase, the amount of carbohydrate required to refuel the body also increases. For those engaged in heavy endurance exercise, 7 to 12 g of carbohydrate may be required [11]. Thus, carbohydrate recommendations vary depending on the individual's particular training program and competition schedule.
Protein
It is generally acknowledged by exercise physiologists and sport nutritionists that active individuals have higher protein requirements than their sedentary counterparts (g/kg) because they typically have more lean mass to support, a greater need for protein for repair of muscle tissue damaged during exercise, and use more protein for energy during exercise [9,18,41,42]. Exactly how much additional protein is needed for an active individual will depend on the type of exercise performed (endurance vs resistance), the intensity and duration of the activity, body composition (kg of lean tissue mass), and whether weight loss is being attempted. Although this higher protein need is not specifically reflected in the 2002 IOM report on energy and macronutrients, the levels of protein recommended for active individuals fall within the AMDR for protein (10%–35% of energy intake).
The research literature indicates that protein intakes of 1.2 to 1.4 g/kg/d for individuals participating in endurance sports and 1.6 to 1.8 g/kg/d for those involved in resistance or speed exercise is more than adequate to cover needs [41,42]. The higher protein recommendations for individuals participating in resistance training allows for the accumulation and maintenance of lean tissue [41–43]. Although these recommendations are higher than the current recommended dietary allowance (RDA) for protein (0.8 g/kg/d) [5•], they do not exceed the habitual protein intakes of most active individuals. Thus, because most athletes consume well above the current RDA for protein (0.8 g/kg/d) in their typical diets, there is usually little need to recommend that active individuals consume more protein or use protein supplements. Those individuals at greatest risk for low protein intakes are active individuals, especially active women, who restrict energy intake for weight loss or follow vegetarian diets [30,31].
Fat
As outlined above, dietary recommendations for active individuals typically focus on getting adequate intakes of carbohydrate and protein, with fat making up the rest of the energy required for weight maintenance. When this is done, the fat intake is usually 25% to 30% of energy intake, which is within the AMDR for fat (20%–35% of energy). Fat is a necessary component of a normal diet and provides energy and essential elements for cell membranes and is associated with the intakes of the fat-soluble vitamins E, A, and D. Low fat intakes (< 15%–17% of energy) are generally not recommended for active individuals [44,45], because it may decrease energy and nutrient intake and exercise performance. In healthy individuals, there appear to be no health benefits to consuming a very low-fat diet (< 15% of energy from fat) compared with a moderate-fat diet [46]. Thus, unless there is some medical reason for restricting fat, dietary intakes should be within the AMDR for fat.
The 2002 IOM report for macronutrients also gives recommendations for the types of fat to be included in the diet for all individuals, regardless of activity level. Diets should be limited in saturated and trans fats, while providing adequate amounts of essential fatty acids (linoleic and α-linolenic acids). The essential fatty acids are required to make a number of potent biologic compounds within the body that help regulate blood clotting, blood pressure, heart rate, and the immune response. Linoleic acid is found in vegetable and nut oils (sunflower, safflower, corn, soy, peanut oil), and it is recommended that adult men consume 14 to 17 g/d and adult women consume 11 to 12 g/d. The typical American appears to get adequate amounts of linoleic acid, due to the high content in salad dressings, salad oils, margarine, and mayonnaise-based foods. Linolenic acid is also found in leafy green vegetables, walnuts, soy oil and foods, canola oil, fish products, and fish oils. The recommended intakes for α-linolenic is 1.6 g/d for adult men and 1.1 g/d for adult women. Individuals who consume very low-fat diets (< 15% of energy) may have difficulty getting adequate amounts of the essential fatty acids in their diet [44].
Hydration
Extensive research has demonstrated that exercise performance is optimal when athletes and active individuals maintain fluid balance during exercise [12] and performance is impaired with progressive dehydration [47,48]. For the first time, the IOM set an adequate intake for water and recognized that PA and environmental conditions have substantial influence on the fluid needs of active individuals [6•]. The adequate intake for fluid (drinking water and other beverages) is approximately 3.0 L/d for men and 2.2 L/d for women 19 to 30 years of age. Higher water intakes are required for those who are physically active and exposed to hot environments [6•].
In addition to the 2004 IOM report on water and electrolytes that recommends baseline fluid intake for sedentary individuals, the ACSM and the National Athletic Trainers' Association (NATA) have position statements giving a comprehensive overview of the research and recommendations on maintaining hydration for physically active individuals in temperate and extreme environments [49–52]. These recommendations include consuming generous amounts of fluids 24 hours before exercise and 400 to 600 mL of fluid 2 hours before exercise. During exercise, active individuals should attempt to drink between 150 and 350 mL (6–12 oz) of fluid every 15 to 20 minutes. When exercise exceeds 1 hour and occurs in a hot environment, sport drinks containing carbohydrate and sodium should be used. After exercise, most active individuals have some level of dehydration. Research indicates that drinking enough fluids to cover 150% of the weight lost during exercise may be needed to replace fluids lost in sweat and urine [53]. The postexercise meal should contain fluids and foods with sodium, because diuresis occurs when only plain water is ingested [16]. Sodium helps maintain plasma osmolality and the desire to drink.
Conclusions
Dietary intake of macronutrients and fluids before, during, and after exercise can significantly impact health and exercise performance. Active individuals, especially those involved in competitive sports, need to make sure they consume enough energy to maintain a healthy body weight for their activity level and sport, while reducing risks of chronic disease. Carbohydrate and protein requirements are higher than those for sedentary individuals, and fat intake should not be severely restricted. Replacing fluids lost during exercise is also essential.
References and Recommended Reading
Papers of particular interest, published recently, have been highlighted as: • Of importance, •• Of major importance
1. Institute of Medicine:
Dietary Reference Intakes. Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride. Washington, DC: National Academy Press; 1997.
2. Institute of Medicine:
Dietary Reference Intakes. Thiamin, Riboflavin, Niacin, Vitamin B-6, Folate, Vitamin B-12, Pantothenic acid, Biotin, and Choline. Washington, DC: National Academy Press; 1998.
3. Institute of Medicine:
Dietary Reference Intakes. Vitamin C, Vitamin E, Selenium and Carotenoids. Washington, DC: National Academy Press; 2000.
4. Institute of Medicine:
Dietary Reference Intakes. Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodoine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium and Zinc. Washington, DC: National Academy Press; 2001.
5.• Institute of Medicine:
Dietary Reference Intakes. Energy, Carbohydrate, Fiber, Fat, Fatty acids, Cholesterol, Protein, and Amino acids. Washington, DC: National Academy Press; 2002.
Provides a detailed review of the macronutrients and gives specific recommendations for carbohydrate, protein, and fat intakes. Energy needs of active individuals are addressed.
6.• Institute of Medicine:
Dietary Reference Intakes. Water, Potassium, Sodium, Chloride, and Sulfate. Washington, DC: National Academy Press; 2004.
Provides a detailed review of water and electrolytes and specifically addresses the increased need for fluids during PA in temperate and extreme environments. Discusses electrolyte losses in sweat and gives recommendations for sodium and other electrolytes.
7.• Brooks GA, Butte NF, Rand WM,
et al.:
Chronicle of the Institute of Medicine physical activity recommendation: how a physical activity recommendation cam to be among dietary recommendations.Am J Clin Nutr 2004, 79(Suppl): 921S–930S.
Specifically address the methods used by the IOM to set the PA recommendation of 60 min/d for adults to prevent weight gain. Also discusses why PA recommendations were provided in a nutrition document.
8. Blair SN, LaMonte MJ, Nichaman MZ:
The evolution of physical activity recommendations: how much is enough?Am J Clin Nutr 2004, 79(Suppl): 913S–920S.
9. Manore M, Barr S, Butterfield G:
American Dietetic Association: Position of the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine. Nutrition and athletic performance.J Am Diet Assoc 2000,
100:1543–1556.
10. Burke LM:
The IOC consensus on sport nutrition 2003: new guidelines for nutrition for athletes.Int J Sport Nutr Ex Metab 2003,
13:549–552.
11. Burke LM, Kiens B, Ivy JL:
Carbohydrates and fat for training and recovery.J Sports Sci 2004,
22:15–30.
12. Coyle EF:
Fluid and fuel intake during exercise.J Sports Sci 2004,
22:39–55.
13. Hargreaves M, Hawley JA, Jeukendrup A:
Pre-exercise carbohydrate and fat ingestion: effects on metabolism and performance.J Sports Sci 2004,
22:31–38.
14. Loucks AB:
Energy balance and body composition in sports and exercise.J Sports Sci 2004,
22:1–14.
15. Maughan RJ, King DS, Trevor L:
Dietary supplements.J Sports Sci 2004,
22:95–113.
16. Shirreffs SM, Armstrong LE, Cheuvront SN:
Fluid and electrolyte needs for preparation and recover from training and competition.J Sports Sci 2004,
22:57–63.
17. Spriet LL, Gibala MJ:
Nutritional strategies to influence adaptations to training.J Sports Sci 2004,
22:127–141.
18. Tipton KD, Wolfe RR:
Protein and amino acids for athletes.J Sports Sci 2004,
22:65–79.
19. Powers SK, DeRuisseau KC, Quindry J, Hamilton KL:
Dietary antioxidants and exercise.J Sports Sci 2004,
22:81–94.
20. American College of Sports Medicine:
Position stand: appropriate intervention strategies for weight loss and prevention of weight regain for adults.Med Sci Sports Exerc 2001,
33:2145–2156.
21. Thompson JL, Manore MM:
Body weight regulation and energy needs: weight loss.Energy-Yielding Macronutrients and Energy Metabolism in Sports Nutrition. Edited by Driskell JA, Wolinsky I. Boca Raton, LA: CRC Press; 2000:291–308.
22. Manore MM, Thompson JL:
Sport Nutrition for Health and Performance. Champaign, IL: Human Kinetics; 2000.
23. Beals KA, Manore MM:
Behavioral, psychological, and physical characteristics of female athletes with subclinical eating disorders.Int J Sport Nutr Exerc Metab 2000,
10:128–143.
24. Beals KA, Manore MM:
Disordered eating and menstrual dysfunction in female collegiate athletes.Int J Sport Nutr Exerc Metab 2002,
12:281–293.
25. Manore MM:
Nutritional needs of the female athlete.Clinics in Sports Medicine: Nutritional Aspects of Exercise. Edited by Wheeler KB, Lombardo JA. Philadelphia: WB Saunders; 1999,18:549–563.
26. Sundgot-Borgen J, Torstveit MK:
Prevalence of eating disorders in elite athletes is higher than in the general population.Clin J Sports Med 2004,
14:25–32.
27. Beals KA:
Disordered Eating in Athletes: A Comprehensive Guide to Health Professionals. Champaign, IL: Human Kinetics; 2004.
28. American Dietetic Association:
Position of the American Dietetic Association: nutrition intervention in the treatment of anorexia nervosa, bulimia nervosa, and eating disorders not otherwise specified (EDNOS).J Am Diet Assoc 2001,
101:810–819.
29. Harber VJ:
Menstrual dysfunction in athletes: an energetic challenge.Exerc Sport Sci Rev 2000,
28:19–23.
30. Manore MM:
Dietary recommendations and athletic menstrual dysfunction.Sports Med 2002,
32:887–901.
31. Manore MM:
Nutritional recommendations and athletic menstrual dysfunction.Int Sport Med J 2004,
5:45–55.
32. Roberts-McComb JJ, Lee N, Norman R:
Effect of bone mineral content of oligomenorrheic active women on body composition estimates.Am J Med Sports 2002,
4:278–283.
33. American College of Sports Medicine:
Position stand: female athlete triad.Med Sci Sports Exerc 1997,
29:i–ix.
34. Brooks GA, Mercier J:
Balance of carbohydrate and lipid utilization during exercise. The cross over concept.J Appl Physiol 1994,
76:2253–2261.
35. Brooks GA, Trimmer J:
Literature supports the cross over concept [letter].J Appl Physiol 1995,
80:1073–1075.
36. Coyle EF, Coggan AR, Hemmert MK, Ivy JL:
Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate.J Appl Physiol 1986,
61:165–172.
37. Bergman BC, Butterfield GE, Wolfe EE,
et al.:
Evaluation of exercise and training on muscle lipid metabolism.Am J Physiol 1999,
276:E106–E117.
38. El-Khoury AE, Forslund A, Olsson R,
et al.:
Moderate exercise at energy balance does not affect 24-h leucine oxidation or nitrogen retention in healthy men.Am J Physiol 1997,
273:E394–E407.
39. Phillips SM, Atkinson SA, Tarnopolsky MA, MacDougall JD:
Gender differences in leucine kinetics and nitrogen balance in endurance athletes.J Appl Physiol 1993,
75:2134–2141.
40. Bergman BC, Brooks GA:
Respiratory gas-exchange ratios during graded exercise in fed and fasted trained and untrained men.J Appl Physiol 1999,
86:479–487.
41. Lemon PWR:
Effects of exercise on dietary protein requirements.Int J Sport Nutr 1998,
8:426–447.
42. Lemon PWR:
Beyond the Zone: protein needs of active individuals.J Am Coll Nutr 2000,
19:513S–521S.
43. Tarnopolsky MA, Atkinson SA, MacDougall JD,
et al.:
Evaluation of protein requirements for trained strength athletes.J Appl Physiol 1992,
73:1986–1995.
44. Horvath PJ, Eagen CK, Ryer-Calvin SD, Pendergast DR:
The effects of varying dietary fat on the nutrient intake of male and female runners.J Am Coll Nutr 2000a,
19:42–51.
45. Horvath PJ, Eagen CK, Fisher NM,
et al.:
The effects of varying dietary fat on performance and metabolism in trained male and female runners.J Am Coll Nutr 2000b,
19:52–60.
46. Dreon DM, Fernstrom HA, Williams PT, Krauss RM:
A very low-fat diet is not associated with improved lipoprotein profiles in men with a predominance of large low-density lipoproteins.Am J Clin Nutr 1999,
69:411–418.
47. Barr SI:
Effects of dehydration on exercise.Can J Appl Physiol 1999,
24:164–172.
48. McConnell GK, Burge CM, Skinner SL, Hargreaves M:
Influence of ingested fluid volume on physiological responses during prolonged exercise.Acta Physiol Scan 1997,
160:149–156.
49. American College of Sports Medicine:
Position stand on exercise and fluid replacement.Med Sci Sports Exerc 1996a,
28:i–vii.
50. American College of Sports Medicine:
Position stand on heat and cold illnesses during distance running.Med Sci Sports Exerc 1996b,
28:i–x.
51. Brinkley HM, Beckett J, Casa DJ,
et al.:
National Athletic Trainers' Association position statement: exertional heat illness.J Athletic Train 2002,
37:329–343.
52. Casa DJ, Armstrong LE, Hillman SK,
et al.:
National Athletic Trainers' Association position statement: fluid replacement for athletes.J Athletic Train 2000,
35:212–224.
53. Shirreffs SM, Taylor AJ, Leiper JB, Maughan RJ:
Post-exercise rehydration in man: effects of volume consumed and drink sodium content.Med Sci Sports Exerc 1996,
28:1260–1271.
54. Food and Nutrition Board:
Recommended Dietary Allowances, edn 10. Washington, DC: National Academy Press; 1989.
