H2O: Do you really need 8 glasses of water a day? Can too much water kill you?
We are commonly advised to “drink eight cups of water a day”, I did some digging to see if this advice is based on factual physiology and what I found was surprising. [Short on time? I got you covered, just read the bold passages]
There isn’t a one size fits all recommendation for water intake. The amount of water our body needs differ from person to person based on a number of different factors including; metabolism, diet, climate, clothing, activity, etc. Even our age and genders influence the amount of fluids we need, as we grow older we need to compensate for greater water losses, physical activity also tends to go down as does thirst drive and the fluid regulatory capacity due to “reduced renal concentrating and diluting capacity” (Sawka, Cheuvront & Carter, 2005). Daily water turnover meaning the replacement of water that is lost from our bodies in a given period of time is different between males and females (Shimamoto & Komiya, 2000). Studies indicate that for sedentary men daily water turnover is 3.3L, for active men it is 4.5 L and for more active men the turnover is even higher (>6L). Fewer data is available on the turnover for women for some reason but it has been reported about ~0.5-1.0 L less than males (Sawka, Cheuvront & Carter, 2005). Sweat loss depends on the level of physical activity and environmental climate, the researcher Adolph using water balance studies has determined that “daily body water varied narrowly between 0.22% and 0.48% in temperate and warm environments respectively” (Adolph, 1933). Even if these numbers move around a bit due to illness, expose or exercise “the fact that intakes are generally adequate to offset avenues of net loss from day to day is a reproducible phenomenon and a cornerstone basis for establishing water intake requirements from large population surveys” (Sawka, Cheuvront & Carter, 2005). This is some evidence to back up the general claims made the amount of water we need a day, yet the advertised eight glasses of 8 ounces of water per day (8 x 8) has been challenged. Heinz Valtin, a kidney specialist working at Dartmouth Med School decided to review peer-reviewed literature and consult several nutritionists to determine if this 8 x 8 recommendation holds scientific weight. No scientific studies from both electronic sources and non-electronic sources of older literature could provide evidence in support of 8 x 8. What he found instead was that this amount can be advisable for those preventing or battling a disease, or undergoing vigorous exercise and work in hot climates. Yet, for healthy adults in a temperate climate who do not get vigorous exercise, rather live a mostly sedentary life the 8 x 8 rule might be too large an amount. Valtin explains; “Rather, surveys of food and fluid intake on thousands of adults of both genders, analyses of which have been published in peer-reviewed journals, strongly suggest that such large amounts are not needed because the surveyed persons were presumably healthy and certainly not overtly ill.” Published studies also support the fact that drinks such as soda, alcoholic or caffeinated drinks may count towards the daily total, in addition to “the large body of published experiments that attest to the precision and effectiveness of the osmoregulatory system for maintaining water balance”(Valtin, 2002). Water balance, as spoken about earlier.
- Is it possible to drink too much water?
There are hazards to drinking too much water such as; water intoxication (fatal), non-fatal hyponatremia, exposure to pollutants, and general expense (Valtin, 2002). Hyponatremia is a word with Latin and Greek roots which means “insufficient salt in the blood”, numerically it is when your blood sodium concentration dips below 135 millimoles per liter (Ballantyne, 2007). Water intoxication can occur “if the renal excretion of water is limited by a sustained influence of the antidiuretic hormone (ADH), either endogenous or exogenous, on the kidney” (Ballantyne, 2007). Cases have been reported recently especially in teenagers and young adults using MDMA (Ecstasy) a drug that is known to deplete your body’s nutrients, so the consumer goes overboard drinking a heavy load of water to rehydrate and suffers from fatal hyponatremia. The reason being that the blood becomes too diluted, our kidneys control the amount of water, salts and other solutes that leave the body by “sieving blood through their millions of twisted tubules” (Ballantyne, 2007). When a person drinks a heavy load of water in a small amount of time, the kidneys are unable to flush it out fast enough and the blood becomes waterlogged (Ballantyne, 2007). Pollutants in water are also a concern because the quality of water has radically diminished in recent years, our tap water is recycled and undergoes vigorous chemical treatment which has people turning to bottled water for a solution which can actually lead “to the drinking of a poorer quality of water than would be the case with tap water” (Valtin, 2002). Reviews from the International Bottled Water Association and the Natural Resources Defense council differ, but it has been shown that some bottled water contains bacteria or carcinogens (Valtin, 2002).
- How can you tell if you are actually ingesting enough fluids?
So if the “right” amount of water varies from one person to the next, the best way to know if you are getting the adequate amount of fluids in my opinion would be to look for proof within yourself, by listening and observing the signs your body sends you. Certain signs are to monitor your urine, if it is not clear and you haven’t had any vitamins then you aren’t hydrated enough. Dehydration shows itself primarily through thirst, so if you are thirsty your body is asking for water, if that is ignored dehydration can lead to headache, fatigue and in severe cases the fatigue leads to lightheadedness, nausea, etc. (Holm, n.d.).
Adolph, E. (1933). The metabolism and distribution of water in body and tissues. Physiological Review, 13, 336-371. Retrieved from http://jn.nutrition.org/content/54/2/221.full.pdf
Ballantyne, C. (2007, June 21). Strange but true: drinking too much water can kill Scientific American, Retrieved from http://www.scientificamerican.com/article/strange-but-true-drinking-too-much-water-can-kill/
Holm, P. (n.d.). Water and hydration. Unpublished manuscript, Health Department, The University of Arizona, Tucson, AZ, . Retrieved from http://www.health.arizona.edu/health_topics/nutrition/general/waterhydration.htm
Valtin, H. (2002). "drink at least eight glasses of water a day.” really? is there scientific evidence for “8 × 8”?. American Journal of Physiology, 283(5), R993-R1004. doi: 10.1152/ajpregu.00365.2002
Sawka, M., Cheuvront, S., & Carter, R. (2005). Human water needs. Nutrition Reviews, 63(6), 30-39. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/16028570
Simamoto, H., & Komiya, S. (2000). The turnover of body water as an indicator of health. Journal of Physiological Anthropology and Applied Human Science, 19(5), 207-212. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/11155349