Studies Lead You to Water, But How Much to Drink?
By MELINDA BECK
JULY 1, 2008
Lately it's been in vogue to dismiss the advice to drink eight glasses of water a day as a "medical myth."
Books and medical-journal articles have declared there's no scientific evidence for claims that "8 x 8" -- eight ounces of water, eight times a day -- can bring a wide range of benefits, from speeding weight loss to ridding the body of toxins, fighting constipation, fatigue, dry skin and hastening recovery from colds and flu. Headlines have jeered that 8 x 8 "doesn't hold water" and "water advice doesn't wash."
It's really more a dispute over whether the glass is half empty or half full. Many studies have linked drinking extra water with health benefits, but critics generally dismiss them as statistically insignificant, inconclusive or not widely applicable.
For instance, a 10-year study of nearly 48,000 men published in the New England Journal of Medicine in 1999 found that the risk of bladder cancer fell 7% for every cup subjects drank per day. Other studies have found that the more water subjects drank, the fewer precancerous colon polyps they had. And a study of 20,000 Seventh-day Adventists in California in the American Journal of Epidemiology found that women who drank at least five glasses of water a day had a 41% lower risk of fatal heart disease, and men had a 54% lower risk, compared with those who drank just two glasses a day.
Physiologist Heniz Valtin of Dartmouth Medical School discussed these studies and more in a seminal 2002 article in the American Journal of Physiology -- and still concluded that they didn't support a universal recommendation that everyone drink 8 x 8, just people "known to have a propensity for the disease(s) in question." Six years later, Dr. Valtin says, "I haven't seen a single scientific report that disagrees with my conclusion."
Similarly, an editorial in the Journal of the American Society of Nephrology in April brushed off small studies suggesting that water increases thermogenesis (calorie-burning), reduces migraines and increases blood flow to the skin. The piece repeated Dr. Valtin's conclusion: "There's no clear evidence of benefit from drinking increased amounts of water." But, it added, "we concede there is also no clear evidence of lack of benefits."
That frustrates researchers in the field. "There's enough data to suggest that we should go get more data," says Jodi Stookey, a scientist at Children's Hospital and Research Center in Oakland, Calif. Her studies have found that substituting water for sweetened beverages helped female dieters cut overall calories (whereas diet drinks seem to stimulate more eating) and that women on four popular diets who drank at least one liter (34 ounces) of water a day lost more weight than those who didn't, regardless of their diet.
It's impossible to sort out cause and effect from casual associations in such studies, and participants aren't always accurate when asked to remember what they ate or drank. So what would satisfy skeptics as evidence? "Only large and expensive randomized trials could settle these questions definitely," the April editorial notes. "Given that water cannot be patented, such trials seem unlikely."
Urologists do agree that extra water can reduce the recurrence of kidney stones. But they don't know if water will prevent them in the first place. "Kidney stones occur in 4% to 5% of the population, so it's not practical to tell everyone to drink that much," says Stanley Goldfarb, a kidney specialist at the University of Pennsylvania Health System and co-author of the editorial.
He also disputes the notion that drinking extra water improves the function of various organs -- because it doesn't stick around. "You don't accumulate those eight glasses of water. They're in the toilet," he says, noting that the body has an exquisite system of self-regulation, excreting anything it doesn't need. And that doesn't mean the body is flushing out extra toxins, either, he says; the same toxins are just diluted in more water.
Experts also agree that people's water needs vary considerably: You'll need more if you're playing in the U.S. Open than if you're sitting at a computer in an air-conditioned office. But most nephrologists, and the National Academy of Sciences, say that thirst alone is a perfectly sufficient guide to how much you need. "Thirst is one of the most powerful human motivators -- it's way above sex," says Dr. Goldfarb.
Actually, that's a point of contention, too. Fereydoon Batmanghelidj, a controversial researcher who extolled the benefits of water in two books, called relying on thirst to regulate hydration "the greatest tragedy in medical history." He and other water aficionados believe that by the time people feel thirsty, they are already dehydrated. This camp believes that the body needs at least two liters of water a day to replace what it loses, and that other beverages, particularly caffeinated ones, don't count.
That's another big area of contention: In 2004, The Institute of Medicine reported that U.S. women who appeared to be adequately hydrated consumed the equivalent of about 91 ounces of fluids each day, and men about 125 ounces -- far more than the 64 ounces in eight glasses of water -- but stated that beverages other than water and the fluid in solid foods also counted toward the total. "Even a slice of white bread is more than 30% water," says Dr. Valtin.
Jacquelin Chan, an epidemiologist at Loma Linda University and the lead investigator on the Adventist heart study, says that some fluids like juice have been shown to increase blood viscosity, whereas water decreases it, at least temporarily. "That's why we tell people to drink water frequently during the day," says Dr. Chan, whose studies have also linked a lowered risk of stroke to drinking at least five glasses of water a day, which she also believes is due to reduced blood viscosity.
Dr. Chan also notes that fluid intake has already increased over the years: The mean intake of plain water is now six cups a day, up from just 2.6. in 1976, according to a big government study. But intake of caloric beverages and food has also increased sharply; She's now examining data from 97,000 Americans to further assess water and health links.
Whether the average human needs that much, many diet advisers -- from the late Robert Atkins to Oprah Winfrey -- firmly believe that drinking extra water helps people feel fuller and makes the body retain less fluid, even though some concede the benefit may be as much behavioral as metabolic.
"If your usual habit is to have a bowl of popcorn or peanuts on your desk, sipping on a glass of water instead is satisfying and keeps your hands and mouth busy," says Karen Miller-Kovach, chief scientific officer at Weight Watchers, which recommends six glasses a day.
"So many of my patients -- including physicians -- tell me that water improves their weight loss and reduces constipation and cuts fluid retention," says Manhattan weight-loss coach Stephen Gullo. "Try it yourself and see if it makes a difference. There's no downside."
Some marathoners have died from downing large quantities of water very quickly. The human body can absorb only about one quart of water per hour, max, and after that, the brain can swell dangerously, says Dr. Goldfarb.
Hyponatremia, in which the sodium concentration in the blood falls dangerously low, is another hazard in drinking too much water. But it's rare in healthy people. Drinking eight glasses a day "is not likely to harm you, but it's very unlikely to help you in any way that the conventional wisdom has been claiming," says Dr. Goldfarb.
Is there a bottom line in this debate? For one thing, more research is in order to verify the intriguing existing data. (Could it be that people who drink more water are just apt to be more thirsty because they exercise more?) In the meantime, if you don't overdo it, drinking water may possibly give you an edge in a number of health areas -- particularly if you drink it instead of caloric beverages. Even Dr. Goldfarb says, "We didn't say you shouldn't drink eight glasses of water a day, just that you don't have to...
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