The advice to drink at least eight glasses of water a day can be found throughout the popular press.w1-w4 One origin may be a 1945 recommendation that stated: A suitable allowance of water for adults is 2.5 litres daily in most instances. An ordinary standard for diverse persons is 1 millilitre for each calorie of food. Most of this quantity is contained in prepared foods.w5 If the last, crucial sentence is ignored, the statement could be interpreted as instruction to drink eight glasses of water a day.w6
Another endorsement may have come from a prominent nutritionist, Frederick Stare, who once recommended, without references, the consumption “around 6 to 8 glasses per 24 hours,” which could be “in the form of coffee, tea, milk, soft drinks, beer, etc.”w7 The complete lack of evidence supporting the recommendation to drink six to eight glasses of water a day is exhaustively catalogued in an invited review by Heinz Valtin in the American Journal of Physiology.w8 Furthermore, existing studies suggest that adequate fluid intake is usually met through typical daily consumption of juice, milk, and even caffeinated drinks.w9 In contrast, drinking excess amounts of water can be dangerous, resulting in water intoxication, hyponatraemia, and even death.
“There is very little evidence that drinking water promotes weight loss; it is one of those self-perpetuating myths,” said Beth Kitchin, Ph.D., R.D., assistant professor of nutrition sciences. “I’m not saying drinking water isn’t good; but only one study showed people who drank more water burned a few extra calories, and it was only a couple of extra calories a day.”
Kitchin says another water myth is the consumption rule: eight 8-ounce glasses of water per day.
“Yes, people do need to get fluids; but it does not have to be water,” Kitchin said. “There’s no evidence that it melts away fat or makes you feel fuller, so if you don’t like water it’s OK.”
Although food is often overlooked as a source of water, foods supply a significant proportion of our water requirements, especially in sedentary people. It can be seen from Table 3 that the water content of foods varies widely from over 90% in some fruits and vegetables to less than 5% in savoury snacks and confectionery. Dietary guidelines focus on the provision of nutrients rather than on the hydration requirements. However, following advice to eat five portions of fruit and vegetables a day, to base meals around starchy foods and to consume foods that are high in fat and sugar in small quantities is likely to increase the water content of the diet. Consuming foods that have a high water content may be particularly important for those looking to control or reduce their body mass as the water content of foods is inversely proportional to their energy density (the amount of energy provided per gram) (Drewnowski 1998). Consuming a lower energy density diet is associated with a lower energy intake and lower body mass in epidemiological studies, and experiments where subjects have consumed a lower energy density diet have found that weight loss is enhanced, even when subjects eat ad libitum (Benelam 2009). Thus, although there is little research investigating the effect of dietary water on hydration status, consuming plenty of water-rich foods may be beneficial both for hydration and for dietary quality. Some studies investigating rehydration after exercise have found that consuming water-rich foods as well as drinks may aid rehydration after exercise-induced water loss, as this approach delivers both water and sodium (Maughan et al. 1996; Sharp 2007).