Q: What made you look into the advice to drink eight glasses of water a day?
A: In July 2000, an article in “The New York Times” extolled the benefits of large amounts of water. And I turned to my wife and said, “What’s the evidence for this?” About nine months later an editor for the “American Journal of Physiology” asked if I would look into the recommendation. If he had asked me to write on any other subject I would have said no, but this one is dear to my heart. The system that keeps us in water balance has been the subject of my research for a lifetime.
Q: So there wasn’t much research?
A: There wasn’t ANY. I searched for 10 months with the help of a professional librarian. There wasn’t a single paper that gave any scientific support to this recommendation.
Q: Did you ever find out how the 8x8 advice started?
A: No. But what makes most sense to me is that as far back as the 1940s, the Food and Nutrition Board of the Institute of Medicine made recommendations on dietary intakes or nutrients, and one of them is water. The board wrote that a rough rule of thumb would be one milliliter of water per calorie eaten. That would mean about 2,000 milliliters, or two liters, of water a day. (Note: two liters is equal to approximately two quarts, or eight 8-ounce glasses). But then in the very next sentence, the board said, “and much of this can be gained from the solid food that we eat.” It always surprises people that white bread is more than 30% water. And I think people forgot about the second sentence, so it became the rule to DRINK two liters of water a day.
Q: And people decided that coffee,tea, and soft drinks also don’t count because they contain caffeine?
A: Yes. In large, pharmaceutical doses, caffeine is a diuretic - it increases urine flow. But in the smaller amounts that one gets in normal amounts of soft drinks, coffee, or tea, it is not diuretic. Alcohol in moderation - say, a cocktail or two with dinner - doesn’t lead to appreciable fluid loss either. So those beverages, which constitute one half or more of daily fluid intake for many adults, should count toward the total amount of liquid consumed.
Q: Do you think this emphasis on eight glasses of water has been encouraged by the bottled-water industry?
A: I am sure of it. The bottled-water industry is making billions of dollars on this myth. Bottled water is a huge money earner for Coca-Cola, which sells Dasani, and for Pepsi, which sells Aquafina. Both Dasani and Aquafina come out of the tap. Bottled water isn’t necessarily from pure springs.
Q: Do eight glasses of water a day have health benefits?
A: As of 2002, when I thoroughly reviewed the literature, there was still no conclusive scientific evidence that drinking large amounts of water helps prevent even kidney stones, urinary tract infections, or bladder cancer. And even if fluids might help, it’s not clear that you need eight glasses of water a day. Some studies found a lower risk of bladder cancer in men who drank only six glasses a day. But other studies found no benefit. As a physician, I certainly would not advise someone who has had kidney stones to dehydrate themselves. But nor do I advise them to drink volumes and volumes of liquid.
Q: Can drinking water help people lose weight?
A: Certainly if you fill your stomach with water, satiety may be reached earlier. But it is not scientifically proven that drinking a lot of water helps dieting. Barbara Rolls of Pennsylvania State University found that it might help if water is incorporated in the food - if you turned a casserole into a soup, for example - but not if you just drink a glass of water.
Q: Can water help prevent constipation?
A: There is no conclusive scientific evidence that it helps constipation. There is no good evidence that the water that one drinks is excreted with the stool rather than absorbed from the gut. But I would not be dogmatic about that. Some people have said to me, “Dr. Valtin, when I drink a lot I feel better, my migraine headaches are less, my bowel movements are better,” and so on. If water works for you, by all means keep it up, provided you realize that even plain water it potentially harmful.
Q: How?
A: Too much can cause water intoxication. When the amount of water we take in cannot be excreted by the body, the body retains water. That dilutes one of the important elements of blood, which is sodium. When the sodium concentration in blood goes down, water moves into the cells. Most cells in the body can tolerate that - they swell. But brain cells tolerate it very poorly because they cannot expand against the skull. That causes neurological symptoms like headaches, confusion, and soften seizures. It can even result in death.
Q: Aren’t those deaths rare?
A: Yes, but they’re terribly tragic when they occur. About a year ago there was a water drinking contest on a radio station in California. And a young woman drank herself to death.
Q: How much water would that take?
A: It usually involves large amounts - gallons. But it doesn’t have to be enormous. For example, in a contest, the excitement ma cause the secretion of what’s called the antidiuretic hormone, which prevents the kidneys from excreting water. The California woman drank only about two gallons. Water intoxication occurs when there’s an imbalance between the amount we drink and what the kidneys can excrete. If the kidneys are stimulated to reabsorb too much, then water is retained and water intoxication starts.
Q: Can it happen to athletes?
A: Yes. For a long time, endurance athletes - mainly marathoners - were told to drink whenever they can. Drink, drink, drink. And some marathoners drank themselves into water intoxication. Again, it’s not necessarily that they drink so much, as that they’re simultaneously excited and stressed, so the kidneys retain water.
Q: Are people right to worry about becoming dehydrated?
A: People have heard that once you’re thirsty, you can already be dangerously dehydrated. That’s baloney. The first line of defense against dehydration is the antidiuretic hormone. The moment our blood becomes slightly concentrated, the antidiuretic hormone reabsorbs more water from the kidneys and returns it to the blood. If that system fails and we don’t keep up with the water we need, then the blood concentrates a little more and thirst sets in. So then we drink in order to help restore fluid balance. It’s a marvelous system.
Q: So the kidneys conserve water before we get thirsty?
A: Yes. Thirst sets in when the blood is concentrated by about 2 percent. The widely accepted definition of dehydration is that the blood has been concentrated by 5 percent. So there’s a long way between when thirst sets in and when we are truly dehydrated.
Q: Are older people less able to sense thirst?
A: One can demonstrate a little less sensation of thirst in the elderly, so it may be correct to remind them to drink. But that doesn’t mean that they should force liquid intake. And I don’t deny that some people need more water - if they have certain diseases or engage in vigorous exercise or work in hot climates. What I have written applies to healthy people living in temperate climates, leading a largely sedentary existence.
Q: And all those plastic bottles of water damage the environment.
A: Absolutely right. It takes a lot of energy to make plastic bottles, and they’re not biodegradable.
Q: Do people think bottled water is healthier?
A: That’s the myth. In any civilized city in the world, certainly in the United States, the tap water in some cases may be purer than the bottled water. A few years ago, Coca-Cola started selling bottled water in London. The company had to withdraw the product in part because the press found out that it came from taps in one of the less desirable suburbs of the city.
Q: Is it true that if your urine isn’t clear you’re dehydrated?
A: Exercise magazines say that if your urine is colored, you’re dangerously dehydrated. That’s a myth. If your urine is very dark it probably means that you’re tending toward being dehydrated. That’s a signal that you should maybe drink a glass or half a glass of water, but I don’t consider it dangerous. The point is that urine doesn’t have to be clear.
Q: Is there any rule of thumb for how much to drink?
A: Drink what you usually drink with meals and in between meals plus when you’re thirsty. Five or six glasses a day has lasted me for 80 years or so. When I discuss the 8x8 rule with friends, relatives, and colleagues, the common response is, “I don’t come close to drinking eight 8-ounce glasses a day.” But of course that’s anecdotal. Actually there’s very good scientific evidence that it can be less than that. As long as the concentration of blood remains normal, that is the gold standard for our being properly hydrated. And population studies show that blood concentration is normal whether we drink 15 liters a day or only one or two liters a day.