eight-by-eight rule 1945 accepted wisdom
But the 64-ounces-in-a-day directive has been abandoned and replaced with the Board’s 2004 recommendation that we figure out how much to drink each day by “letting our thirst be [our] guide.” It turns out that between the water we get from beverages (yup, even caffeinated ones), water-rich fruits and vegetables, and our bodies’ super sophisticated mechanism for regulating water balance, maintaining hydration is fairly simple for most healthy adultsWater, Hydration and Health, Popkin, B., D’Anci, K., Rosenberg, I. Annual Review of Nutrition, 2010 August; 68(8): 439–458.. Under certain circumstances (like when we get sick, work out extra hard, or exert ourselves in the heat) our bodies have a harder time keeping up with fluid loss and we can become dehydrated.But what exactly does that mean?
Our Bodies, Our Cells
The importance of water to our bodies can’t be overstated. After all, it makes up more than two thirds of our body weight and is responsible for a variety of functions, including digestion, blood flow, and body temperature regulation, as well as for overall cell health. Fortunately, the fact that we lose between roughly four to nine cups of water per day through breathing, sweating, peeing, and pooping isn’t a problem for most healthy adults because the systems that regulate hydration are so sensitive. According to CamelBak Hydration Advisor Doug Casa, PhD, evidence shows that the body will compensate for a loss of just one to two percent of the total amount of water in the body by triggering the sensation of thirst and the cue to drinkWater as an essential nutrient: the physiological basis of hydration. Jéquier E, Constant F. European Journal of Clinical Nutrition, 2010 Feb;64(2):115-23.Water, Hydration and Health, Popkin, B., D’Anci, K., Rosenberg, I. Annual Review of Nutrition, 2010 August; 68(8): 439–458.. These cues stay on track and properly-timed because our brains, kidneys, various glands, and hormones work in concert to monitor the amount of water that we’re taking in versus how much we’re losingHow much water is lost during breathing? Zieliński J, Przybylski J. Pneumonologia I Alergologia Polski, 2012;80(4):339-42.Water, Hydration and Health, Popkin, B., D’Anci, K., Rosenberg, I. Annual Review of Nutrition, 2010 August; 68(8): 439–458..
It all begins with the hypothalamus, the gland responsible for regulating our body temperature and triggering the processes that balance the fluids in our bodies. When the hypothalamus detects too little water in our blood, it signals the release of an anti-diuretic hormone that causes the kidneys to remove less water from the blood. The result? We pee less, and when we do, our urine is more concentrated and darker in color. At this point the brain also tells us we’re thirsty, and once we sip on some water or consume something hydrating our water levels return to normalWater intake and the neural correlates of the consciousness of thirst. McKinley MJ, Denton DA, Oldfield BJ, et al. Seminars in Nephrology, 2006 May;26(3):249-57.. Similarly, when our body temperature rises either from fever, working out, or being in a warm environment, our bodies try to lower our temperature by sweating: When sweat evaporates from our skin, it takes some heat with it, helping to cool us off.
But although we lose water when we sweat, pee, and breathe, our bodies are so darn good at triggering thirst and cueing urination that it’s only when we’re losing more water than we can replace—think sweating like crazy, throwing up, having diarrhea, or peeing excessively, or maybe even spending a long night drinking—that we experience dehydrationWater, Hydration and Health, Popkin, B., D’Anci, K., Rosenberg, I. Annual Review of Nutrition, 2010 August; 68(8): 439–458..
Hot, Tired, and Thirsty: How Dehydration Feels
When dehydration occurs, we experience a range of symptoms from dry mouth, headaches, dizziness, and fatigue to lethargy, cool skin, and the inability to pee (because our kidneys have been told not to excrete scarce fluid). Here’s how it works: Water is integral to regulating blood volume, which in turn affects blood pressure and heart rate. When we’re dehydrated our blood volume drops, and so does our blood pressure, causing our hearts to beat fasterDehydration markedly impairs cardiovascular function in hyperthermic endurance athletes during exercise, González-Alonso J, Mora-Rodríguez R, et al. Journal of Applied Physiology, 1997 Apr;82(4):1229-36.Salt and fluid loading: effects on blood volume and exercise performance. Mora-Rodriguez R, Hamouti N. Medicine & Science in Sports & Exercise, 2012;59:113-9. For some, dehydration causes headaches or triggers migraines. Though the link between water balance and headaches is still being researched, one theory is that as our bodies work to maintain fluid levels during dehydration our blood vessels narrow, reducing the supply of oxygen and blood to the brain, which causes headachesWater-deprivation headache: a new headache with two variants. Blau JN, Kell CA, Sperling JM. Headache, 2004 Jan;44(1):79-83.Water deprivation: a new migraine precipitant. Blau JN. Headache, 2005 Jun;45(6):757-9.. So basically, when our cells are deprived of the water they need to function optimally, all systems are especially taxed and must work harder to power us, causing us to feel fatigued or lethargic.
Dehydration affects us cognitively and psychologically, too. Studies have shown that even mild dehydration can cause dips in concentration, memory, and moodMild dehydration affects mood in healthy young women. Armstrong LE, Ganio MS, Casa DJ, et al. Journal of Nutrition, 2012 Feb;142(2):382-8.Mild dehydration impairs cognitive performance and mood of men. Ganio MS, Armstrong LE, Casa DJ. British Journal of Nutrition, 2011 Nov;106(10):1535-43.
The amount of water we need varies from person to person and depends on what we’re eating and drinking, the climate, and our activity level. Healthy adults can pretty much rely on their body’s thirst mechanism to keep them hydratedWater as an essential nutrient: the physiological basis of hydration. Jéquier E, Constant F. European Journal of Clinical Nutrition, 2010 Feb;64(2):115-23. And if you’re eating fruits and vegetables and drinking fluids you’re probably getting the water you need.
However, there are circumstances when we need to pay more attention to how hydrated we are: when we’re exercising hard, in the heat, or for long periods of time (or in some combination of these conditions), or when we’re sick with fever or a stomach bug that causes diarrhea or vomitingThe importance of good hydration for work and exercise performance. Shirreffs SM. Annual Review of Nutrition, 2005 Jun;63(6 Pt 2):S14-21.Dehydration and rehydration in competitive sport. Maughan RJ, Shirreffs SM. Scandinavian Journal of Medicine & Science in Sports, 2010 Oct;20 Suppl 3:40-7.
Our bodies are actually so smart about maintaining water balance that when we sweat excessively and are losing salt and other electrolytes, we’re cued to crave drinks that both quench thirst and contain sodiumWater, Hydration and Health, Popkin, B., D’Anci, K., Rosenberg, I. Annual Review of Nutrition, 2010 August; 68(8): 439–458.Sweat rate and sodium loss during work in the heat. Bates GP, Miller VS. Journal of Occupational Medicine and Toxicology, 2008 Jan 29;3:4.. During prolonged workouts, Casa recommends going for drinks that have carbohydrates and electrolytes (like sports drinks or coconut water) which will keep you hydrated even when you’re sweating like a champFluid and electrolyte balance in ultra-endurance sport. Rehrer, N.J. School of Physical Education and Department of Human Nutrition, Otago University, Dunedin, New Zealand. Sports Medicine, 2001;31(10):701-15.Rehydration after exercise with fresh young coconut water, carbohydrate-electrolyte beverage and plain water. Saat, M., Singh, R., Sirisinghe, R.G., et al. Department of Physiology, School of Medical Science, Universiti Sains Malaysia, Malaysia. Journal of Physiological Anthropology and Applied Human Science, 2002 Mar;21(2):93-104.. Plus, they’ll can help take the edge off post-workout fatigueFluids and hydration in prolonged endurance performance. Von Duvillard SP, Braun WA, Markofski M, et al. Nutrition Journal, 2004 Jul-Aug;20(7-8):651-6..
In helping to replenish valuable electrolytes, which could happen if we’re guzzling plain water, sports drinks prevent us from getting overhydratedSweat rate and sodium loss during work in the heat. Bates GP, Miller VS. Journal of Occupational Medicine and Toxicology, 2008 Jan 29;3:4.Too much of a good thing? Angus H N Whitfield. British Journal of General Practice, 08/2006; 56(528):542-5.. When we drink way more water than we need, our kidneys can’t keep up and we’re unable to urinate enough to get our water level back in balance. As a result, the sodium in our blood becomes diluted, and water intoxication occurs, causing symptoms like headache, fatigue, nausea or vomiting, confusion, or even seizures.Fatal water intoxication. D J Farrell and L Bower. Journal of Clinical Pathology, 2003 Oct, Volume 56(10); 803-804. There is no set amount of water that causes intoxication. Instead, Casa says we can avoid overdoing it by drinking “according to the sensation of thirst.” He also recommends getting a handle on how much fluid we’re losing while working out by determining our sweat rate.
To make sure we’re hydrated, our brains, organs, and hormones do the heavy lifting, keeping our fluids balanced without us having to think much about itWater, Hydration and Health, Popkin, B., D’Anci, K., Rosenberg, I. Annual Review of Nutrition. 2010 August; 68(8): 439–458.. But the better we know our bodies and how we handle exercise, sickness, and climate changes, we can be sure that we’re not doing anything to sabotage the precise and involuntary processes that keep everything flowing and functioning.And if you’re planning for workouts or trying to ballpark how much water you’re likely to want over the course of your day, you can always get a estimate by using a hydration calculator.