WASHINGTON -- The vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide, says the newest report on nutrient recommendations from the Institute of Medicine of the National Academies. The report set general recommendations for water intake based on detailed national data, which showed that women who appear to be adequately hydrated consume an average of approximately 2.7 liters (91 ounces) of total water -- from all beverages and foods -- each day, and men average approximately 3.7 liters (125 ounces) daily. These values represent adequate intake levels, the panel said; those who are very physically active or who live in hot climates may need to consume more water. About 80 percent of people's total water comes from drinking water and beverages -- including caffeinated beverages -- and the other 20 percent is derived from food.
"We don't offer any rule of thumb based on how many glasses of water people should drink each day because our hydration needs can be met through a variety of sources in addition to drinking water," said Lawrence Appel, chair of the panel that wrote the report and professor of medicine, epidemiology, and international health, Johns Hopkins University, Baltimore. "While drinking water is a frequent choice for hydration, people also get water from juice, milk, coffee, tea, soda, fruits, vegetables, and other foods and beverages as well. Moreover, we concluded that on a daily basis, people get adequate amounts of water from normal drinking behavior -- consumption of beverages at meals and in other social situations -- and by letting their thirst guide them."
Regarding salt, healthy 19- to 50-year-old adults should consume 1.5 grams of sodium and 2.3 grams of chloride each day -- or 3.8 grams of salt -- to replace the amount lost daily on average through sweat and to achieve a diet that provides sufficient amounts of other essential nutrients. Elevated blood pressure, which may lead to stroke, coronary heart disease, and kidney disease, is associated with increased sodium intake. On average, blood pressure rises progressively as salt intake increases. A tolerable upper intake level (UL) is set at 5.8 grams of salt per day; this is not a recommended intake level. Older individuals, African Americans, and people with chronic diseases including hypertension, diabetes, and kidney disease are especially sensitive to the blood pressure-raising effects of salt and should consume less than the UL. More than 95 percent of American men and 90 percent of Canadian men ages 31 to 50, and 75 percent of American women and 50 percent of Canadian women in this age range regularly consume salt in excess of the UL.
To lower blood pressure, blunt the effects of salt, and reduce the risk of kidney stones and bone loss, adults should consume 4.7 grams of potassium per day. However, most American women 31 to 50 years old consume no more than half of the recommended amount of potassium, and men's intake is only moderately higher. Canadians typically eat more potassium than their American counterparts. African Americans in the U.S. generally get less potassium than non-Hispanic whites, and because they have a higher prevalence of elevated blood pressure, increased potassium intake may have particularly significant benefits for them.
The typical Western diet is high in salt and low in potassium -- the opposite of what evidence shows is optimal for good health and reducing the risks of chronic disease, the report says. "Research is needed to find ways to help people select better food choices to reduce their salt intake and boost their potassium consumption," Appel said. In addition, because Americans and Canadians get the majority of their salt -- 77 percent, according to one study -- from prepared and processed foods, research should be done to help food processors develop alternative technologies that can reduce the amount of salt added during processing without impairing taste, shelf-life, or product qualities at an affordable cost.
Additional Findings on Water
This report refers to total water, which includes the water contained in beverages and the moisture in foods, to avoid confusion with drinking water only.
Total water intake at the reference level of 3.7 liters for adult men and 2.7 liters for adult women per day covers the expected needs of healthy, sedentary people in temperate climates. Temporary underconsumption of water can occur due to heat exposure, high levels of physical activity, or decreased food and fluid intake. However, on a daily basis, fluid intake driven by thirst and the habitual consumption of beverages at meals is sufficient for the average person to maintain adequate hydration.
Prolonged physical activity and heat exposure will increase water losses and therefore may raise daily fluid needs. Very active individuals who are continually exposed to hot weather often have daily total water needs of six liters or more, according to several studies.
While concerns have been raised that caffeine has a diuretic effect, available evidence indicates that this effect may be transient, and there is no convincing evidence that caffeine leads to cumulative total body water deficits. Therefore, the panel concluded that when it comes to meeting daily hydration needs, caffeinated beverages can contribute as much as noncaffeinated options.
Some athletes who engage in strenuous activity and some individuals with certain psychiatric disorders occasionally drink water in excessive amounts that can be life-threatening. However, such occurrences are highly unusual. Therefore, the panel did not set a UL for water.
Additional Findings on Salt and Potassium
The recommended intake levels for salt provide enough sodium to balance losses from sweat by people who are exposed to temperatures higher than what they are used to or who are moderately physically active. Endurance athletes and other very active individuals may need more sodium because they lose more in sweat from intense and prolonged physical activity.
High salt intake has been directly linked to elevated blood pressure, also known as hypertension. About 25 percent of American adults and more than half of those age 60 and older have hypertension. American men's median intake of salt is between 7.8 and 11.8 grams per day, and women consume between 5.8 and 7.8 grams every day. Canadian men and women consume 7.1 to 9.7 grams and 5.1 to 6.4 grams per day respectively. Because these figures do not include the salt that people add at the table, they are probably underestimates.
Studies indicate that reduced consumption of salt coupled with increased potassium intake can blunt the age-related rise in blood pressure. American men consume just 2.8 to 3.3 grams of potassium daily on average, and women on average get only 2.2 to 2.4 grams each day. Canadians consume slightly more, at 3.2 to 3.4 grams per day for men and 2.4 to 2.6 grams per day for women. Fruits and vegetables are both low in sodium and high in potassium. Among foods with the highest amounts of potassium per calorie are spinach, cantaloupes, almonds, brussels sprouts, mushrooms, bananas, oranges, grapefruits, and potatoes.
There was no evidence of chronic excess intakes in apparently healthy individuals to compel establishing a UL for potassium. However, people who have kidney dysfunctions that impair their ability to excrete potassium or who are on certain types of drug therapies -- such as ACE inhibitors -- should be under the supervision of a medical professional, who may recommend consuming less than the recommended 4.7 grams per day.
This study was sponsored by the U.S. Department of Health and Human Services' Office of Disease Prevention and Health Promotion and its National Institutes of Health; Health Canada; U.S. Environmental Protection Agency; U.S. Department of Agriculture; Institute of Medicine; the Dietary Reference Intakes Private Foundation Fund, including the Dannon Institute and the International Life Sciences Institute-North America; and the Dietary Reference Intakes Corporate Donors' Fund, contributors to which have included the Nabisco Food Group, Mead Johnson Nutritionals, and M&M Mars.
This study is one of a series undertaken by scientists from the United States and Canada under the auspices of the Institute of Medicine's Food and Nutrition Board to develop reference values for nutrients for use in both countries. The Institute of Medicine is a private, nonprofit organization that provides health policy advice under a congressional charter granted to the National Academy of Sciences. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate is available on the Internet at http://www.nap.edu. Copies of the report will be available for purchase later this year from the National Academies Press; tel. 202-334-3313 or 1-800-624-6242. Reporters may obtain a pre-publication copy from the Office of News and Public Information (contacts listed above).
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