THE PHYSICIAN AND SPORTSMEDICINE - VOL 32 - NO. 4 - APRIL 2004
Dietary Guidelines for Water and Electrolytes
What Role for Thirst?
The traditional water intake recommendation of eight glasses a day never had a formal scientific basis or backing. To establish an evidence-based recommendation for water intake, an expert panel of US and Canadian scientists recently examined the issue, along with electrolyte intake, on behalf of the Institute of Medicine (IOM), and published their findings in February.1 The IOM is a division of the National Academy of Sciences, an independent, nonprofit organization that advises government agencies on scientific and technological matters.
What Counts as a Fluid?
The findings, highlighted in IOM press releases, indicate that most healthy people meet their daily hydration needs by letting thirst guide their intake. For healthy sedentary people in temperate climates, the panel set general recommendations of 91 oz (2.7 L) of water per day for women and 125 oz (3.7 L) per day for men from all dietary sources.
The group stopped short of offering a rule of thumb based on how many glasses of water people should drink each day. Lawrence Appel, MD, MPH, panel chair and professor of medicine, epidemiology, and international health at Johns Hopkins University in Baltimore, said the group avoided a water-glass-per-day recommendation, because daily hydration needs can be met through a variety of sources. "People also get water from juice, milk, coffee, tea, soda, fruit, vegetables, and other foods and beverages, as well," he said. The report notes that about 80% of water intake comes from water and beverages, and 20% is derived from food.
Some physicians have raised concerns in the past about the diuretic effect of caffeinated beverages; however, the panel found no convincing evidence that caffeine contributes to cumulative total-body water deficits.
Advice for Athletes
Prolonged physical activity and heat exposure increase water loss and raise daily fluid needs, the report notes, adding that research has shown that very active people who are continually exposed to hot weather have daily total water needs of 6 L or more. The report acknowledged excessive water consumption among endurance athletes as a cause of hyponatremia, but the group did not set an upper intake level for water, because the condition is relatively rare.
The American College of Sports Medicine (ACSM), in a response to the IOM recommendations, lauded the report but pointed out that ACSM experts believe thirst is not the best hydration indicator for active people and those who are exposed to heat. W. Larry Kenney, PhD, president of the ACSM and professor of physiology and kinesiology in the department of kinesiology at the Noll Physiological Research Center at The Pennsylvania State University in University Park, notes that athletes should heed the ACSM's current hydration guidelines that recommend that athletes replace fluids in an amount that approximates sweat loss.2 In those who participate in prolonged physical activity, Kenney says the sensation of quenched thirst, particularly with water, occurs before fluid replacement is achieved. "Relying on thirst to determine an active individual's fluid replacement needs is inadequate, especially so in older exercisers," he said.
The role of thirst in rehydration and the effect of mild dehydration on athletic performance are controversial and are the subject of continuing research.3
The IOM report recommends that healthy adults consume 1.5 g of sodium and 2.3 g of chloride each day—or 3.8 g of salt—to replace the amount lost through perspiration. The maximum amount should not exceed 5.8 g of salt each day (2.3 g of sodium). The report notes that older people, African-Americans, and patients who have elevated blood pressure, coronary heart disease, diabetes, and kidney disease avoid consuming at the upper level. Endurance athletes and other individuals who are involved in strenuous activities should consume more sodium to offset sweat losses, the IOM notes. The ACSM responded that while curbing sodium intake is sound advice for the general population, athletes need to liberally salt their foot and consume sports drinks when acclimatizing to or exercising in hot conditions.
Regarding potassium intake, the recommendations state that adults should consume 4.7 g per day to blunt the effects of salt, lower blood pressure, and reduce the risk of kidney stones and bone loss. The IOM report noted that American women and African-Americans would benefit most from increasing their potassium intake. Canadians had better potassium intakes than did their American counterparts.
ACSM Reviews Evidence on Exercise and Hypertension
Research on the influence of exercise on hypertension is copious; however, a panel of experts from the American College of Sports Medicine has analyzed the most recent research and has condensed its findings in a position stand on the topic.
Published in the March issue of Medicine & Science in Sports & Exercise, the position stand details strong evidence that exercise prevents and reduces hypertension. Even a single acute exercise session provides an immediate reduction in blood pressure that can last up to 22 hours.
The guidelines, which support exercise as a cornerstone for the prevention and treatment of hypertension, advise that, based on current evidence, adults who have hypertension should get at least 30 minutes of accumulated or continuous moderate-intensity endurance exercise on most days of the week. Resistance exercise is useful as an adjunct to endurance exercise.
The panel also recommended the following exercise safety tips for patients who have hypertension:
In addition to clinical recommendations, the position stand reviews the most recent research on the antihypertensive mechanism of action in exercise. The report acknowledges that both neural and vascular changes result from exercise and that genetics may play a role. The ACSM last presented its exercise and hypertension findings in 1993.