Do we really need 8 glasses of water a day?
You may be surprised to learn that many of the facts about health and nutrition that we take for granted aren’t facts at all. Despite health professionals having cited the ‘8 glass rule’ for decades there isn’t actually any documented scientific evidence to support it.
How much water should you drink each day? It's a simple question with no easy answer. For decades, health professionals have been recommending the daily consumption of 8 glasses of water. The Department of Health and NHS Choices also cite this in their guidance to the public (1). There is however, no documented scientific evidence that we need this amount of pure water.
In 2002, Dr. Heinz Valtin, a hydration expert and professor emeritus at Dartmouth Medical School, suggested that this recommendation most likely originated from the Food and Nutrition Board of the National Research Council in 1945 and was based on no actual research, but in fact a casual guess. It stated that "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." (2) By ignoring the last sentence, the advice can easily be misinterpreted to mean drinking eight glasses of water a day is recommended.
Extensive searches of published literature by Valtin and Tsindos found no scientific evidence for the notion that most people need to drink at least 8 glasses of water a day (2, 3). Although people with specific health concerns, such as kidney stones, benefit from drinking lots of water, there is no evidence to suggest that healthy individuals need to consume large quantities of water (2). Valtin also noted that"Despite the seemingly ubiquitous admonition to "drink at least eight 8-oz glasses of water a day" (with accompanying reminder that beverages containing caffeine and alcohol do not count), rigorous proof for this counsel appears to be lacking.
This review sought to find the origin of this advice (called "8x8" for short) and to examine the scientific evidence, if any, that might support it. The search included not only electronic modes but also cursory examination of the older literature that is not covered in electronic databases and, most importantly and fruitfully, extensive consultation with several nutritionists who specialise in the field of thirst and drinking fluids. No scientific studies were found in support of 8 x 8."
Other studies have come to similar conclusions that there is no evidence for drinking increased amounts of water (4). In fact, studies on Saharan desert nomads showed people can consume minimal amounts of water in harsh environments (5). Writing for the British Medical Journal in 2011, McCartney argued that the advice to drink 8 glasses of water a day is "thoroughly debunked nonsense", being spread by bottle water companies in order to make more profit (6).
Water is, of course, essential for human survival. The body loses water every day through breathing, perspiration, urination and bowel movement and these fluids need to be replenished. Activity levels, hot climates, pregnancy, breastfeeding and medical conditions all impact on fluid balance. Once the human body has lost between one and two percent of its total water supply, it recognises the need to replace this fluid through the mechanism of thirst. Another good measure to determine if the body needs fluid is assessing the colour of urine- if it's very dark, it's unlikely that enough fluids are taken. Ideally, urine should be a very light-coloured yellow.
The water lost during normal metabolism and activity needs to be replaced but this doesn't specifically have to be through drinking water. This can be in the form of other liquids and food such as soup, fresh fruits and vegetables, which can have 90 percent or more water by weight. It can also be in the form of coffee, carbonated drinks and juice although these fluids will contain calories, sugar and caffeine and should not be a major portion of daily total fluid intake. In terms of water source, there's nothing miraculous about water from a glass as opposed to water from food or any other beverage. This message needs to be communicated to the general public.
Staying hydrated is clearly essential but there are serious question marks over the current recommendations to drink eight glasses of water a day. Hydration needs vary from person to person and day to day. The vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide, and perhaps this should be the recommendation we communicate to the public.