Are 8 Glasses Of Water A Day Too Much?

It has become accepted wisdom: "Drink at least eight glasses of water a day!"

But that's not necessarily true, according to Dartmouth Medical School physician Heinz Valtin. The universal advice that has made guzzling water a national pastime is more urban myth than medical dogma and appears to lack scientific proof, he said.

In review published online by the American Journal of Physiology on Aug. 8, Valtin reported that he found no supporting evidence to back this popular counsel, commonly known as "8 x 8" (for eight, 8-ounce glasses). The review will also appear in a later issue of the journal.

Valtin, a kidney specialist and author of two textbooks on the kidney and water balance, sought to find the origin of this commonly held advice and to examine the scientific evidence, if any, that might support it.

But after his research, Valtin said he finds it "difficult to believe that evolution left us with a chronic water deficit that needs to be compensated by forcing a high fluid intake."

Valtin found no scientific studies in support of 8 x 8. Rather, surveys of fluid intake on healthy adults of both genders strongly suggest that such large amounts are not needed. His conclusion is supported by published studies showing that caffeinated drinks, such as most coffee, tea and soft drinks, may be counted toward the daily total. He also found many published experiments that attest to the capability of the human body to maintain proper water balance.

Valtin warns, however, that his conclusion is limited to healthy adults in a temperate climate leading a largely sedentary existence. He said large amounts of fluid, equal to and greater than 8 x 8, are advisable for the treatment or prevention of some diseases, such as kidney stones, as well as under special circumstances, such as strenuous physical activity, long airplane flights or hot weather.

But barring those exceptions, Valtin concluded that we are currently drinking enough and possibly even more than enough.

But really, what's the harm?

"The fact is that, potentially, there is harm even in water," Valtin said. Even modest increases in fluid intake can result in "water intoxication" if one's kidneys are unable to excrete enough water through urine. Such instances are not unheard of, and they have led to mental confusion and even death in athletes, in teenagers after ingesting the recreational drug Ecstasy, and in ordinary patients.

And he lists other disadvantages of a high water intake:

• possible exposure to pollutants, especially if sustained over many years;
• frequent urination, which can be both inconvenient and embarrassing;
• expense, for those who satisfy the 8 x 8 requirements with bottled water; and
• feelings of guilt for not achieving 8 x 8.

But what about all the arguments we hear that support drinking high levels of water? Valtin discusses them:

Thirst is too late. It is often stated that by the time people are thirsty, they are already dehydrated. On the contrary, thirst begins when the concentration of blood (an accurate indicator of our state of hydration) has risen by less than 2 percent, Valtin said, whereas most experts would define dehydration as beginning when that concentration has risen by at least 5 percent.
Dark urine means dehydration. At normal urinary volume and color, the concentration of the blood is within the normal range and nowhere near the values that are seen in meaningful dehydration, Valtin said. Therefore, the warning that dark urine reflects dehydration is alarmist and false in most instances.

The 8 x 8 rule is widely followed. Everywhere, people carry bottles of water, constantly sipping from them; it is acceptable to drink water anywhere, anytime. A pamphlet distributed at one southern California university even counsels its students to "carry a water bottle with you. Drink often while sitting in class ..."

Valtin thinks the notion may have started when the Food and Nutrition Board of the National Research Council recommended approximately "1 milliliter of water for each calorie of food," which would amount to roughly 2 to 2.5 quarts per day (64 to 80 ounces). Although in its next sentence, the board stated "most of this quantity is contained in prepared foods," that last sentence may have been missed, so that the recommendation was erroneously interpreted as how much water one should drink each day.