How Much Water You Really Need
by ADA
The
American College of Sports Medicine Position Stand on Exercise and
Fluid Replacement and the National Athletic Trainers Association
Position Statement on Fluid
Replacement for Athletes provide comprehensive overviews of the
research
and recommendations on maintaining hydration
CARBOHYDRATES: How Much?; Nutrition and Performance:PART 1; Nutrition For Active Adults PART 2, Energy Needs: PART 3, Body Composition: PART 4, Carbs, Fat, Protein, REFERENCES
HYDRATION
Exercise performance is optimal when athletes maintain fluid balance during exercise; conversely, exercise performance is impaired with progressive dehydration (87-91). Moreover, dehydration increases the risk of potentially life-threatening heat injury such as heat stroke (92). Accordingly, athletes should attempt to remain well-hydrated before and during exercise.
The American College of Sports Medicine Position Stand on Exercise and Fluid Replacement (93) and the National Athletic Trainers Association (NATA) Position Statement on Fluid Replacement for Athletes (94) provide comprehensive overviews of the research and recommendations on maintaining hydration during exercise. The following information summarizes the key points from these position stands and provides recommendations for special environmental conditions.
Water and Electrolyte Balance
- Losses during exercise Athletes dissipate the metabolic heat produced during physical activity by radiation, conduction, convection, and by vaporization of water. In hot, dry environments, evaporation accounts for more than 80% of metabolic heat loss. Sweat rates will vary depending on variables such as body size, exercise intensity, ambient temperature, humidity, and acclimation, but can exceed 1.8 kg (approximately 1,800 mL) per hour (93). In addition to water, sweat also contains substantial amounts of sodium (an average of approximately 50 mmol/L, or about 1 g/L, although concentrations vary widely), modest amounts of potassium, and small amounts of minerals such as iron and calcium.
- Gastric emptying and intestinal absorption of fluids during exercise Euhydration (and theassociatedmaintenance of physiological function and performance) can be accomplished during exercise only if the rate of fluid ingestion and absorption equals the rate of fluid loss through sweating (and, in events of longer duration, urination). Fluid balance during exercise is not always possible because maximal sweat rates exceed maximal gastric emptying rates, which in turn limit fluid absorption. In most cases, however, rates of fluid ingestion by athletes during exercise fall short of amounts that could be emptied from the stomach and absorbed by the gut. For example, athletes often consume less than 500 mL per hour during competition (93), whereas gastric emptying rates of more than 1 L per hour are possible (93).
Gastric emptying is maximized when the amount of fluid in the stomach is high. It is reduced with hypertonic fluids or when carbohydrate concentration is greater than or equal to 8%; however, fluids containing 4% to 8% carbohydrate can generally be emptied at over 1 L per hour in most people when gastric volume is maintained at or above 600 mL (93,94).
- Dehydration, hypohydration, and hyponatremia Disturbances of fluid and electrolyte balance that can occur in athletes include dehydration, hypohydration, and hyponatremia (95). In their most severe forms, all can be life-threatening. Exercise-induced dehydration develops as a consequence of fluid losses that exceed fluid intake. In contrast, hypohydration occurs when athletes dehydrate themselves before beginning a competitive event, and can be induced by prior fluid restriction, exercise practices, diuretic use, or sauna exposure. In most cases, hypohydration is practiced by athletes competing in sports with weight categories (eg, wrestling, boxing, lightweight crew, weight lifting, and judo). Hyponatremia (low blood-sodium concentrations of less than 130 mmol/L) can develop either as a result of prolonged, heavy sweating with failure to replace sodium, or when excess water is retained in the body (96). Although endurance athletes are more likely to suffer from dehydration than from overhydration, the latter is not uncommon. For example, 11 of 605 athletes entered in the New Zealand Ironman triathlon developed severe hyponatremia, and 8 of these athletes were likely overhydrated, as they had either maintained or gained up to 5% of body weight during the race (97).
Fluid and Electrolyte Recommendations
- Before exercise Athletes should be well-hydrated when beginning to exercise. In addition to drinking generous amounts of fluid in the 24 hours before an exercise session, the ACSM and the NATA recommend drinking 400 to 600 mL of fluid 2-3 hours before exercise (93,94). Such a practice should optimize hydration status while allowing enough time for any excess fluid to be excreted as urine before beginning to exercise.
- During exercise Athletes should attempt to drink enough fluid to maintain fluid balance, as even partial dehydration can compromise performance. If fluid balance cannot be maintained, the maximal amounts that can be tolerated should be ingested. Optimal hydration can be facilitated by drinking 150 to 350 mL (6 to 12 oz) of fluid at 15- to 20-minute intervals, beginning at the start of exercise (93).
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