Two liters of water a day is a myth, major study finds

The common recommendation to drink two liters, or about eight glasses, of water each day is not supported by scientific evidence, according to a comprehensive study conducted by researchers at the University of Wisconsin-Madison. This extensive research, published in the journal Science, analyzed fluid needs in over 5,600 individuals across 26 countries, ranging in age from 8 days to 96 years.

Dale Schoeller, a professor of nutritional sciences at the University of Wisconsin-Madison and a co-author of the study, stated that the "eight glasses a day" rule has never been scientifically validated. He explained that this popular guideline often confused total water turnover in the body with the amount of water consumed solely from beverages, overlooking the significant contribution of water from food.

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Science never supported the eight glasses rule as an appropriate guideline, partly because it confused total water turnover with water from beverages, while a large part of fluids comes from food.

The study employed a precise method using isotope-labeled water to track how efficiently the human body processes and eliminates fluids. By measuring the rate at which stable isotopes are excreted in urine over a week, researchers could determine the body's water turnover and, by tracking oxygen isotopes, estimate calorie expenditure. This advanced technique allowed for a more accurate understanding of individual hydration requirements.

Findings indicate that daily water needs can fluctuate dramatically, typically ranging from 1 to 6 liters, with extreme cases, such as those of elite athletes in challenging weather conditions, potentially reaching up to 10 liters per day. Schoeller emphasized that a single average figure is not very informative due to this wide variability. However, the research identified key factors influencing these differences, including age, sex, body mass, and environmental conditions like temperature and humidity. For instance, water turnover is highest in men around age 20, remains stable for women between 20 and 55, and newborns process the largest proportion of water daily.

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By measuring the rate at which a person eliminates these stable isotopes in their urine over a week, the hydrogen isotope tells us how much water is being exchanged, and the elimination of the oxygen isotope allows us to determine how many calories are burned.