The myth of 8 glasses a day

Nutritional beliefs go in and out of style. One of the most tenacious is the fixation on drinking large quantities of water.

In the 1980s, someone suggested that people should drink eight large glasses of water, 8 ounces each, every day. The idea stuck, and 8 x 8 became household shtick with nutritionists and around much of North America. My patients frequently boasted that water consumption was the main reason for their good health.

What was the appeal of eight glasses of water, 8 ounces each, per day? Was it a feng shui thing? Was it that water is the ultimate in “natural”? More likely, it was that most of us just like to believe in things that sound good.

What does drinking eight glasses of water per day do, besides ensure a certain familiarity with the nearest bathroom?

I’ve witnessed significant problems. One patient drank lots of water to flush out impurities. Unfortunately, he also had Crohn’s disease, an intestinal illness in which nutrients don’t get absorbed well. He lost serious weight and became progressively weaker until finally his astute doctor discovered the water jag and advised drinking liquids with calories and nutrients.

Another patient with kidney stones started drinking large quantities of water as prevention. One week after consuming 2 liters of water every day (about eight glasses, 8 ounces each), he developed severe fatigue and weakness. Blood tests showed dangerously low sodium levels.

The eight-glasses-per-day mantra, it turns out, is a misinterpretation of normal physiological requirements. Daily fluid loss in adults is about 2 liters. The loss comes from sweating, breathing and excretion of waste by kidneys and bowels. The most important concept is replacement, not flushing impurities. Normal eating and drinking should suffice since water is a major ingredient in not only beverages but also nearly all foods -- fruit, meat, dairy, breads, everything except dried foods. Concerning healthy skin, drinking swimming pools of water does not help at all. If your kidneys are healthy, extra water does not end up in skin: You excrete it.

What then, is the evidence for the 8-x-8 rule?

In 2002, the American Journal of Epidemiology published an exhaustive literature search conducted by a scientist at Dartmouth University that found no scientific studies in support of 8 x 8. It concluded that large quantities of water are not needed and that thirst regulates water intake quite effectively.

However, in another study -- same year, same journal -- researchers at Loma Linda University reported that daily water intake of five glasses or more was associated with nearly a 50% reduction in the risk of fatal coronary events.

Surprisingly, soft drinks and juices increased coronary risk. The authors could not fully explain this, but proposed that dehydration and the effect of sugared drinks on blood viscosity (thickness) were possibilities.

Maybe there is more to water than replacement. I now subscribe to 5 x 5.

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James Channing Shaw is a dermatologist at the University of Toronto.