Research Throws Water on 8-Glasses-a-Day Advice

The advice to drink eight glasses of water a day is so engrained that it might be difficult to dislodge it. But some researchers are trying to, the Washington Post reports. A new study published in the journal Science finds issues with that guidance, which might have been misunderstood all along. The Food and Nutrition Board of the National Research Council in 1945 first suggested adults have about 64 ounces of water per day—translated as eight 8-ounce glasses, or 8 cups. But the goal of 64 ounces referred to total consumption, including what we get from food and other beverages. As the Mayo Clinic puts it, "you don't need to rely only on water to meet your fluid needs."

Experts and their research have questioned the scientific basis for the rule before. One study found no evidence that the 227 elderly adults among of 883 monitored were dehydrated despite regularly drinking less than six glasses of water a day. In the new research, data from 5,600 people in 26 countries ranging in age from 8 days to 96 years was analyzed. The study measured how much water the subjects lost and replaced each day, called water turnover. Researchers found water turnover was largely determined by a person's size and level of body fat. People with a more "fat-free" mass need more water. And men overall need more water because they have larger bodies and less body fat. "Men use more water every day because we have a bigger system to keep hydrated," said Herman Pontzer, a Duke University professor and study co-author.

A better guideline might be: Drink when you're thirsty. Water, coffee, and tea work, but sugary drinks are less help. The Mayo Clinic says you're probably getting enough water if you rarely feel thirsty and your urine is light yellow or colorless. There are complications, experts point out, including the fact that everyone will need to drink more as the climate warms. Researchers found no real harm in drinking 64 ounces of water a day, or more. "If you drink eight cups of water a day, you'll be fine—you're just going to be spending a lot more time in the bathroom," Pontzer said. (More health study stories.)