Do you really need eight glasses of water a day?

Not necessarily. The eight-glasses rule is easy to remember, and the Mayo Clinic calls it a good goal, but health sources agree that it is not a requirement that fits everyone. Cleveland Clinic describes the number as somewhat arbitrary and not rooted in scientific evidence, and Tufts says the standard is not supported by science. Individual needs depend on body size, activity, climate and health, and a good share of daily fluid comes from food and from drinks other than plain water. [1] [2] [3]

According to the Tufts Health & Nutrition Letter, the idea traces to a 1945 statement from the U.S. Food and Nutrition Board of the National Research Council, which encouraged adults to take in 64 ounces of water a day. That figure included water from food, which supplies roughly 20 percent of total water intake. The advice was therefore never meant to be eight glasses from the tap alone. [3]

The Harvard Nutrition Source reports that the National Academy of Medicine suggests an adequate daily fluid intake of about 13 cups for healthy men and 9 cups for healthy women, with higher amounts for active people or those in very warm climates. It stresses that these are general guides, not daily targets, and that drinking somewhat less will not necessarily harm an average person. Mayo Clinic cites studies suggesting that healthy adults are likely to get enough with about 2.7 to 3.7 liters of total fluid from all sources. The University of Rochester reports a figure of 92 to 124 ounces for the average adult in a temperate climate. These numbers differ in framing, but all refer to total fluid or water, not only drinking water. [4] [1] [5]

Tea, coffee and milk count toward fluid intake, as do juices, soda and sports drinks, though the last group can carry a lot of sugar, salt and calories. Water-rich foods such as cucumbers, lettuce, berries and melons contribute too. [1] [4]

A study of 5,604 people aged eight days to 96 years in 23 countries, described by Tufts, found that age, body size and composition, activity, climate, altitude, pregnancy and socioeconomic status were all linked to how much water people lost and took in. Hot or humid weather, heavy sweating, fever, vomiting, diarrhea, pregnancy and breastfeeding raise needs. Certain conditions may call for more or less water, so people with such conditions are advised to ask a doctor. [3] [1] [5]

Tufts nephrologists say the body tightly regulates water levels with hormones and receptors, and that most people can simply drink when thirsty. They note exceptions: infants and young children are sensitive to small fluid losses and cannot clearly express thirst, and older adults sense thirst less well, which raises the risk of both dehydration and over-hydration. Those groups are advised to pay special attention to fluid intake, especially in heat or when very active. [3]