How much water should you really drink?

If you’ve been toting a bottle of water around with you, worried about getting the magic eight to 10 glasses of water every day, there’s good news: You can relax. While it’s important to stay hydrated, recent scientific reviews suggest there are little basics for that well-known water goal. Proof of the purported benefits of drinking lots of water turns out to be equally elusive.

You’re probably getting plenty of water from regular beverage consumption and fluids in food. And, despite what you may have heard, yes, the water in caffeinated beverages, such as coffee and tea, does “count.”

For most people, in fact, the biggest worry shouldn’t be getting enough water. It’s getting too many “liquid calories” in the sweetened beverages they drink. The best thing about water — compared to a 250-calorie, 20-ounce non-diet soft drink — is that it quenches your thirst with zero calories.

Sound advice all wet?

The present reality check on the human body’s water needs began in 2007, when Indiana University School of Medicine researchers tested seven medical beliefs commonly accepted by doctors as well as the general populace. Writing in the British Medical Journal, the researchers challenged, among such notions, the belief that people should drink at least eight glasses of water daily.

“When we examined this belief, we found that there is no medical evidence to suggest that you need that much water,” said co-author Rachel Vreeman, MD. She speculated that the widely held belief about water consumption sprang from a 1945 finding by the US Nutrition Council that people need 64 ounces of fluids (eight eight-ounce glasses) daily. But Dr. Vreeman noted that an important part of the Council’s recommendation has gotten lost: Those 64 ounces include the fluids in food as well as coffee, tea, and soda.

A related misconception about water, Dr. Vreeman added, had to do with thirst: “In healthy people, thirst is an early sign of your body needing more fluids, not a late sign. We have often found people were surprised to hear this.”

The water prescription was further scrutinized in 2008 by two University of Pennsylvania researchers, Stanley Goldfarb, MD, and Dan Negoianu, MD, who published their work in the Journal of the American Society of Nephrology. Some people — athletes, people with certain diseases, and those living in desert climates, for example — do have special water needs, they found. (Others, such as those with some kidney diseases, may actually need to restrict fluid intake.) But most people don’t need to worry about getting enough water.

No single study, according to this review, shows that people need to drink eight eight-ounce glasses of water a day. The researchers note that a similar analysis in 2002 by Heinz Valtin, MD of Dartmouth Medical School — a kidney specialist and author of two widely-used textbooks on the kidney and water balance — found no scientific proof for the “8 x 8” rule of water intake. (Dr. Valtin also countered the notion that by the time you feel thirsty, it’s too late.)

The Penn researchers also looked at four major myths on benefits from drinking extra water:

1. Facilitates toxic excretion.

2. Improves skin tone.

3. Makes you feel less hungry.

4. Reduces the frequency of headaches.

“Our bottom line was that there was no good science — or much science at all — behind these claims, that they probably represent folklore,” Dr. Goldfarb, told Reuter news service.

The claims about toxin excretion were not validated by any scientific study, Dr. Goldfarb went on: “The kidneys clear toxins. This is what the kidneys do. They do it very effectively. And they do it independently of how much water you take in. When you take in a lot of water, all you do is put out more urine but not more toxins in the urine.”

Evidence for water’s benefits to the skin was similarly lacking, while there was no consistent evidence of its effect on hunger. Dr. Goldfarb explained, “What no one has looked at is whether anyone really loses weight over the long haul, if they go under this regimen of drinking lots of water.” Evidence that headache sufferers who drank more water experienced fewer headaches was not statistically significant, the researchers wrote.

They didn’t examine more extravagant claims, such as the so-called water cure promoted in books and web sites. And no research in peer-reviewed scientific journals has established that water can prevent or help cure asthma, heart disease, arthritis, diabetes or other chronic conditions. “Unintentional chronic dehydration,” purportedly the cause of pain and disease and even cancer, is not a term found in any medical literature.

Breaking A Sweat

None of this research, of course, is meant to dry up your enthusiasm for good old H2O. Your body does need plenty of water — straight or in foods and beverages — to regulate its temperature, lubricate and cushion your joints, protect your spinal cord and other sensitive tissues, remove wastes, and keep you breathing. Your blood, after all, averages 92-percent water, and even your muscles are 75-percent water — as is your brain.

As you grow older, it’s even more important to guard against dehydration. Older people have a reduced sensation of thirst, so it’s easier to miss the warning that they’re dehydrating. Your kidneys also don’t function as efficiently as you get older, which affects your fluid balance and your body’s ability to regulate your temperature. When older people exercise, they sweat less than younger athletes — another challenge to the body’s internal thermostat.

Exercise, however, can actually reduce your risk of dehydration. Regular exercise boosts the amount of water in your blood and your total body water content, meaning you can safely lose more water without becoming dehydrated. Fit people also sweat more, keeping the body cool, and have more diluted sweat, losing fewer electrolytes as they perspire.

But as you exercise more, you do need to drink more water. In an hour of exercise, your body can lose as much as a quart of water. The best hydration plan starts even before your workout: The American Council on Exercise (ACE) recommends drinking water two to three hours before exercise, and again a half-hour before or during warm-up. Then drink water every 10-20 minutes during your actual workout, followed by another drink within a half-hour after exercising.

The Best Beverages

While “sports drinks” may help replace electrolytes lost during exercise, they can also put right back on the calories you just burned off. Add sugared soft drinks, juices, alcoholic beverages, and coffee, and it’s no wonder that many people today get 21 percent of their daily calories from beverages — up from 17 percent in the 1970s. It’s easy to overlook these “liquid calories.” Because of serving-size sleight of hand on labels, they can be hard to calculate: A 20-ounce soft drink bottle might seem to contain only 100 calories — but that’s per eight-ounce serving. At 2.5 “servings,” you’re actually drinking 250 calories.

“Most people either forget or don’t realize how many extra calories they consume in what they drink, yet beverages are a major contributor to the alarming increase in obesity,” notes Barry Popkin, PhD, director of the University of North Carolina’s Interdisciplinary Obesity Center, who led the development of Healthy Beverage Guidelines to help counter that trend.

The guidelines, released in 2006 by a panel of experts, group beverages into six levels of recommended consumption. Here’s the countdown in brief:

6. Categorically sweetened beverages without nutrition. No more than one eight-ounce serving per day.

5. Caloric beverages with some nutrients. 0-8 ounces per day of juices. No whole milk. Sports drinks, consume sparingly except for endurance athletes, 0-16 ounces per day. Alcoholic beverages, 0-1 drink per day for women and 0-2 drinks per day for men (one drink is 12 ounces beer or 5 ounces wine or 1.5 ounces distilled spirits).

4. Non-categorically sweetened beverages. 0-32 ounces per day.

3. Low- and non-fat milk and soy beverages. 0-16 ounces per day.

2. Unsweetened tea and coffee. 0-40 ounces of tea, 0-32 ounces of coffee per day.

1. Water. 20-50 ounces per day.

Yes, ordinary water still comes out on top. Indeed, the Beverage Guidelines panel noted that all of an adult’s beverage needs can be met with water. Nothing beats water for health and hydration. When you’re thirsty, drink it. Now that’s a rule anybody can remember!

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To learn more: IOM Dietary Reference Intakes for Water, Potassium, Sodium, Chloride and Sulfate, www.nap.edu/openbook.php?isbn=0309091691; and American Journal of Clinical Nutrition, March 2006, with abstract at www.ajcn.org/cgi/content/abstract/83/3/529.