As summer is winding down and going out like a lion in terms of some hot days yet, I began to think about hydration. It is pretty common to see students walking around campus carrying large water bottles in their backpacks. I bet if I stopped these students and asked how much water they should drink in a day I’d mostly hear, “eight 8 ounce glasses”. So where did that dietary advice come from and is it even accurate?
The origin behind this fluid advice seems to reside with a 1945 Food and Nutrition Board recommendation that American adults required 2.5 L of fluid daily. The caveat to this, and one that’s been widely ignored, is the subsequent sentence to this recommendation. It read, “Most of this quantity is contained in prepared foods'' (Carroll, 2015). According to Dr. Carroll, this is a medical myth and one he wrote about both in the British Medical Journal in 2007 and again in a book on medical myths in 2009. Water is present in many of our foods and beverages including fruits, vegetables, soups, juices, tea, milk, and coffee and easily contributes to what the body needs. In terms of worrying about coffee dehydrating you, Dr. Carroll says that research shows that needn’t be a concern either. He and other medical experts argue that the thirst mechanism is normally quite good in terms of matching our fluid needs to our intake and we need no formal recommendation for a daily amount of water (Phillips et al., 1984). But you say extra water is good for the skin and kidney function, right? The research doesn’t bear this out either. Drinking 8 glasses of water each day did not help in keeping the skin more hydrated, healthier, or making it less prone to wrinkles (Wolf et al., 2010). Likewise, drinking all this water has not been shown to improve kidney function or reduce all-cause mortality (Palmer et al., 2014).
For athletes, the fluid question becomes a different story because hydration levels definitely have an impact on performance. In sports nutrition, we discuss hydration in terms of two strategies, “planned drinking” and “drinking to thirst”. With planned drinking, predetermined amounts of fluid are consumed with the purpose of preventing dehydration while preventing over-drinking (Kenefick, 2018). It is based on sweat rates and sweat electrolyte concentrations and is thus customized to the individual athlete. It’s also an investment in time. Drinking to thirst is also “ad libitum drinking” meaning the consumption of fluid whenever and in whatever volume is desired. The idea here is to let one’s thirst be the driving force into how much fluid is consumed. At rest and in normal daily living and average healthy exercise, the sensation of thirst works really well; however, during more intense exercise, thirst is less sensitive/accurate (Greenleaf & Sargent, 1965). Planned drinking is used more for conditions of exercise or competition lasting 90 min to 2 h, are of high intensity, and takes place in warm or hot environments.
So while some long-standing medical myths may have been busted, what’s the bottom line here when it comes to water? With most things in nutrition, as well as in life, there isn’t a one-size-fits-all approach. How much fluid you need will depend on your size, the weather, your activity level, and overall health condition. A good rule of thumb is 4 to 6 cups a day (Health.Harvard.edu, 2021). You can let your thirst dictate beyond that. So there is no need to guilt yourself into drinking several water bottles throughout the day. Your body has an elegant and innate wisdom to know what it needs and more importantly has the means to let you know.
Carroll, A., MD. (2015, August 24). No, You Do Not Have to Drink 8 Glasses of Water a Day. New York Times. Retrieved from NYTimes.com
Phillips, P. A., Rolls, B. J., Ledingham, J. G., & Morton, J. J. (1984). Body fluid changes, thirst and drinking in man during free access to water. Physiology & Behavior, 33(3), 357-363. doi:10.1016/0031-9384(84)90154-9
Wolf, R., Wolf, D., Rudikoff, D., & Parish, L. C. (2010). Nutrition and water: Drinking eight glasses of water a day ensures proper skin hydration—myth or reality? Clinics in Dermatology, 28(4), 380-383. doi:10.1016/j.clindermatol.2010.03.022
Palmer, S. C., Wong, G., Iff, S., Yang, J., Jayaswal, V., Craig, J. C., . . . Strippoli, G. F. (2014). Fluid intake and all-cause mortality, cardiovascular mortality and kidney function: A population-based longitudinal cohort study. Nephrology Dialysis Transplantation, 29(7), 1377-1384. doi:10.1093/ndt/gft507
Kenefick, R. W. (2018). Drinking Strategies: Planned Drinking Versus Drinking to Thirst. Sports Medicine, 48(S1), 31-37. doi:10.1007/s40279-017-0844-6
Greenleaf, J. E., & Sargent, F. (1965). Voluntary dehydration in man. Journal of Applied Physiology, 20(4), 719-724. doi:10.1152/jappl.19220.127.116.119
How much water should you drink? (2020, March 25). Retrieved from Health.Harvard.edu