SB: The Rule of Drinking Eight Glasses of Water a Day Has No Scientific Basis

The popular advice to drink eight glasses of water a day has no scientific basis — there are no specific studies confirming that everyone needs to consume this amount. The recommendation most likely originated from a 1945 U.S. report that was later repeatedly quoted without its full context. This was explained by physiologist Heinz Valtin of Dartmouth Medical School, as cited by ScienceBlogs.

According to the expert, a document from the U.S. Food and Nutrition Board did indeed recommend consuming about 2.5 liters of water per day — roughly eight glasses. However, immediately after this figure, the document clarified that a significant portion of the required fluid comes from food. Over time, this explanation disappeared from popular accounts, and an approximate estimate of total daily water intake was transformed into a mandatory recommendation for drinking plain water.

The scientist emphasized that more recent recommendations also do not establish a universal “eight glasses” rule. In 2004, the U.S. Institute of Medicine of the National Academies stated that most healthy people can maintain normal hydration by responding to their sense of thirst. Daily fluid requirements include water obtained from both beverages and food.

“A study of more than 5,600 people from 23 countries, published in Science in 2022, demonstrated just how much fluid requirements vary from person to person. They depend on body weight, physical activity, temperature and humidity, altitude, pregnancy, and other factors. Therefore, there is no single recommended amount that applies to everyone,” Valtin explained.

At the same time, drinking excessive amounts of water can also be dangerous. For example, after the Boston Marathon, 13% of the runners examined were found to have hyponatremia — a potentially dangerous drop in blood sodium levels. The main risk factor was weight gain during the race, which indicated excessive fluid consumption.

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