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Departments: Health & Fitness A to Z

Principles of Hydration

Amin, Sarah Ph.D., MPH; Liguori, Gary Ph.D., FACSM

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ACSM's Health & Fitness Journal: March/April 2019 - Volume 23 - Issue 2 - p 5-7
doi: 10.1249/FIT.0000000000000459
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Though water has been around since the beginning of time, our understanding of this essential nutrient and the role it plays in health promotion and disease prevention is relatively new. In fact, the dietary guidelines for water, or Dietary Reference Intakes (DRIs), were only first released in 2005 (1). What is known is that water is vital to our survival through supporting and maintaining the function of every bodily system (2). Given that our brains and muscles are 75% to 80% water by weight (3), water literally dictates our cognitive abilities and bodily movements. We also know that without water, we would perish in merely a few days. In this Health & Fitness from A to Z article, we deconstruct the topic of hydration from its historical roots of dehydration to technological trends in monitoring water intake.


The U.S. Centers for Disease Control and Prevention define daily fluid intake or total water as the amount of water consumed from foods, plain drinking water, and other beverages (4). Water is indispensable to health and health promotion, yet its value as a nutrient is complex, beginning at the cellular level, and acting primarily as a solvent that dissolves and delivers necessary nutrients. Water also functions to remove waste from our cells through the kidney’s filtration system and excretion in urine. It further prevents overheating through sweat, especially during physical activity. The physical properties of water lubricate our joints and form the mucus that lines our respiratory and digestive tracts (2,3).


Our understanding of the history of hydration dates back to the 17th century when Italian physician Sanctorius measured the “insensible perspiration” of voluntary participants, one of whom was Galileo. However, the importance of hydration did not re-emerge until 200 years later when cholera became a global public health issue. Through the seminal work of William Brooke O’Shaughnessy, who analyzed cholera patients’ blood serum, dehydration first became recognized as a “physiological disturbance” that if left untreated could lead to death. The cholera epidemic of 1831 to 1832 lead to the first intravenous solutions of salt and water being tested on the most severely ill patients; however, since the patients being treated were too sick to drink, the mechanism for oral rehydration therapy wasn’t perfected. It wasn’t until the 1970s (5), when searching for a solution to treat diarrhea, that the mix of water, sugar, potassium, and sodium became the standard for oral rehydration therapy, now used routinely to prevent and treat dehydration (6). In 2004, the Institute of Medicine of the National Academies set DRIs for water, salt, and potassium with an eye toward maintaining health and preventing chronic disease (1).


Using thirst as a primary indicator to drink, especially during activity, has long been proven ineffective, if not dangerous. Thirst is actually our body’s first sign of dehydration, which may soon be followed by more severe symptoms including fatigue, headaches, confusion, and heightened heart rate. These symptoms will negatively impact activity and cognitive function and begin to occur with as little as a 1% loss of body weight as fluid (3). Practically speaking, this is only 1.5 lbs for a 160-lb person, hardly even noticeable.

So if thirst isn’t the best indicator, what is? The old adage “drink 8 glasses of water daily” no longer applies, and interestingly, few countries have developed water intake requirements because of challenges in collecting population-level measures of water intake and urine osmolality (2). In the United States, an adequate intake of 3.7 L for men and 2.7 L for women has been set for total water, which was informed by median intakes of total water using U.S. survey data that relied on self-reported food and beverage consumption (1). However, individual needs vary greatly based on foods consumed, weather conditions (i.e., temperature and humidity), altitude, physical activity level, and sweating. Furthermore, the elderly should closely monitor fluid intake because renal function tends to decline with age and these individuals tend to have diminished thirst cues and regulation of fluid intake (2,3).

Those that are physically active should take into account the duration and intensity of exercise, environmental conditions (i.e., temperature), type of clothing worn, and individual characteristics (i.e., body weight), which collectively impact sweat rates. As a general rule of thumb, individuals should focus on three main areas of hydration. First, make an effort to rehydrate with beverages several hours before exercise, particularly when long and/or hot exercise is planned. The rate of fluid consumption for prehydration should be approximately 5 to 6 mL/kg. Second is to identify tailored fluid replacement plans to be used during activity, again, especially for those activities lasting more than 60 minutes and/or occurring in warmer weather. Fluid replacement beverages or “sports beverages” should contain sodium (~20 to 30 meq/L) and potassium (~2 to 5 meq/L) to replace electrolyte losses, and 5% to 10% carbohydrate, which helps maintain adequate levels of blood sugar/glucose for energy. The sodium in these beverages also has the added benefit of promoting thirst.

Finally, after exercise, individuals should consume the right types of beverages (i.e., sports beverages) and foods with a little extra salt to replace electrolytes when sweat sodium losses are substantial. To safely and sufficiently recover from postexercise fluid loss, ~1.5 L of fluid should be consumed for each kilogram of bodyweight lost and this should be done slowly to allow for proper fluid retention. Only in rare and severe cases (>7% body weight loss) is intravenous fluid replacement necessary (7).

Can we get too much of a good thing? Healthy individuals have an innate ability to maintain homeostasis, and when it comes to water consumption, our bodies are able to excrete water to achieve water balance (2,3). Therefore, unlike other nutrients, a Tolerable Upper Intake Level (UL) was not set for water (1), meaning it is OK to drink a lot of it. Though rare, when a high volume of water is consumed in a short period of time (i.e., several gallons in a few hours), the kidneys’ rate of excretion may not be able keep up, which may lead to blood dilution and acute water toxicity that is potentially fatal (1).


To stay hydrated, keeping track of fluid consumption throughout the day is paramount, and there are a variety of ways to monitor this, starting with a simple food diary or daily planner. However, it is no surprise that in our tech-savvy culture where we’ve become masters of collecting our own individual health data through smartphones, smartwatches, and other gadgets, there are burgeoning tech trends in tracking water intake. Smartwatches like the Apple Watch® and Fitbit® feature functionalities to log and track water consumption in cups or milliliters manually (8). More recently, a variety of “smart water bottles” geared toward promoting positive drinking habits have been developed. These bottles set individualized consumption goals (based on weight, sex, physical activity levels, etc.), feature prompts to drink by lighting up or sending a smartphone notification, and generate a log to help track and compare daily behaviors (9). Beyond tracking, knowing our bodies is important. A urine color of pale yellow, morning body weight that is close to your baseline (within 1%), and consistent urine volume all correspond with “euhydration” or normal body water content (7).


Water is indisputably the ideal source of hydration, although American’s dietary patterns indicate that fluid consumption varies dramatically. In fact, water is often eschewed for other beverages, including sugar sweetened beverages (SSBs). SSBs not only carry many calories, but also do not confer any essential nutrients. Fortunately, population-level data from the National Health and Nutrition Examination Survey show that SSB consumption is trending downward over the most recent 10-year stretch, 2003–2004 to 2013–2014, from 80% to 61% for children and 62% to 50% for adults (10,11). However, SSB consumption persists as a public health issue and heavy consumption (≥500 kcal/d) remains high in adolescents (16%) and young adults (20%) (10) despite numerous public health efforts aimed at reducing SSB consumption (12,13).

Finally, where and how we hydrate is imperative to consider from both a public health and sustainability lens. Efforts to ensure that communities have access to safe and free drinking water through hydration stations or water fountains (i.e., parks, schools, offices) must continue. This goes hand-in-hand with promoting the use of refillable water bottles versus single-use disposable bottles as an eco-friendly alternative (14).


The link between water and life as we know it is profound. The topic of hydration is complex and has attracted the attention of health practitioners, educators, researchers, environmentalists, and policymakers whose investment in this topic ranges from health and well-being to our food system and sustainability. Water plays many important roles in our bodies, and even slight deficits in this crucial nutrient can leave us with a headache or feeling confused. For the physically active, proper fluid intake is critical for peak performance and proper recovery. Therefore, being mindful of thirst cues and monitoring fluid consumption to align with individual needs is integral to healthy living.


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© 2019 American College of Sports Medicine.