Eight Glasses of Water Per Day
By John Reggars

Ever wondered where the oft quoted “you must drink 8 X 8oz glasses of water per day” comes from?

Well I did, and so did Professor Emeritus, Heinz Valtin MD, a kidney specialist from the Department of Physiology, Dartmouth Medical School, who has authored two widely used texts on the kidney and water balance.

My literature search failed to find the source, of what appears to be a classic urban myth, and Dr Valtin’s investigations also failed to find a scientific basis for its origins¹. It appears that this recommended amount of water per day, roughly 1.9 litres, is over and above the water contained in the food we consume and that doesn’t include caffeinated or alcoholic beverages. What always troubled me was that from an evolutionary perspective it didn’t seem to make sense. As a species, if we required that amount of water per day we would have to stay or make frequent trips to a permanent water source. This would obviously place us in great danger, due to large predators lingering about those water sources in the hope of an easy meal and therefore be counter-productive for the proliferation and evolution of our species.

So where did this now accepted fact come from? Valtin searched the electronic data bases, manually searched the older literature and also made enquires with several nutritionists who specialize in the field of thirst and drinking fluids. What he found was two unsubstantiated recommendations which may have been the origins of this myth. In a 1974 book on nutrition the authors state “How much water each day? ……….for the average adult, somewhere around 6 to 8 glasses per 24 hours and this can be in the form of coffee, tea, milk, soft drinks, beer etc. Fruits and vegetables are also a good source of water.” Another unsubstantiated reference to the 8 X 8 requirement appears in the 1945 recommendations of the Food and Nutrition Board of the National Research Council, “A suitable allowance of water for adults is 2.5 liters daily in most instances…… Most of this quantity is contained in prepared foods.” Valtin feels that the inclusion of caffeinated and alcoholic beverages and water contained in food as part of the daily intake, mentioned in these articles, has been omitted over time resulting in the ubiquitous 8 X 8 recommendation.

So if the 8 X 8 intake is not supported by any scientific evidence how much water does the average adult, living in temperate climate, performing at most mild exercise need on a daily basis? Well apparently there is no scientifically validated answer to this question but according to Valtin 8 X 8 is far too much. Interestingly, two surveys conducted before and after the 8 X 8 rule became folklore, showed a significant increase in water intake after its acceptance. In 1977-8 the average American consumed 1695ml of fluid daily, 674 in water and the rest in other beverages, while in 1998 it was 2188 in total fluid, 841ml being water, the rest being other beverages like tea, coffee, milk, alcohol etc. Another study on healthy medical students showed a water intake of 1222ml as fluid, 1000ml in food and 300ml in metabolic water.

The next question answered in this research paper was, is there any harm or benefit in consuming large amounts of water? Apparently, there is some evidence, albeit inconclusive, that high water intake can reduce the incidence of bladder cancer, colorectal cancer and coronary heart disease. Other claimed benefits include weight loss, relief of constipation, fatigue, arthritis, migraine, hypertension, to name just a few, but the evidence for these benefits is either limited research trials or anecdotal speculation. Conversely, there is definitely some harm from excess water consumption. Not long ago a young scientist collapsed unconscious in Alice Springs after consuming 8-10 litres of water to prevent dehydration². Hyponatremia is a well documented condition which often affects endurance athletes when they consume too much water and in some cases can be fatal. One two year research study on marathon runners reported 26 cases of hyponatremia with three cases requiring intensive care hospitalisation³. Other cases have been reported in teenagers who take the popular party drug Ecstasy, which apparently promotes an intense thirst. Further, Valtin states that even modest increases in fluid intake can result in water intoxication in the presence of certain diseases and or drug therapies and that excessive water consumption may expose the individual to increased risk of disease from pollutants within the water supply.

Other myths about water intake were dispelled by Valtin in his paper, such as by the time you are thirsty it’s too late. Valyin points out that there is voluminous scientific literature on the efficacy of the osmoregulatory system to maintain water balance through the antidiuretic hormone and thirst. Thirst begins when the concentration of blood has risen by less than 2%, whereas most experts define dehydration as beginning when the concentration rises above 5%. However, the regular consumption of fluid during endurance sports or in high temperatures is essential, regardless of thirst indicators⁴. “Dark urine indicates dehydration.” No so says Valtin. The depth of colour of the urine will vary inversely with the urinary volume and urinary volume varies greatly among individuals and in most instances the warning that dark urine reflects dehydration is alarmist and false.

In summary, there is no scientific basis for healthy adults in a temperate climate performing mild exercise to consume 8 X 8 glasses of water per day and that the available scientific evidence would suggest the contrary and that we do not require this amount of water per day to maintain health.

References:

1. Valtin H. “Drink at least eight glasses of water a day.” Really? Is there scientific evidence for “8 X 8”? Am J Physiol Regul Integr Comp Physiol. 2002; 993-1004.
2. Notaras L. Excess water a drinking problem. The World Today 28/2/2003 http://www.abc.net.au/worldtoday/s795217.htm.
3. Davis DP, et al. Exercise-associated hyponatremia in marathon runners: a two-year experience. J Emerg Med 2001; 21:47-57.
4. Noakes T. Fluid replacement during marathon running. Clin J Sports Med 2003; 13:309-18.