9 Water in Health

9.1 Overview

Water is the most abundant substance in the human body. Total intake includes drinking water as well as that in foods and beverages. For most adults, intake comfortably meets needs. The minimum daily requirement depends upon diet, activity level, and environment.

Water balance occurs when gain equals loss. Healthy people maintain balance by excreting excess in urine. Urine volume and color are indicators of hydration status. Dark-colored urine suggests that a person is dehydrated. Dehydration contributes to heatstroke. Changes in body weight over a short period point to hydration status.

Physiological and psychological factors prompt drinking. It is thought that increased water intake reduces the risk of kidney stones. Recent research suggests that contrary to previously held wisdom, caffeine and ethanol (alcohol) consumption does not contribute to dehydration over the period of a day, only in the immediate short term.

Objectives

• Describe water balance and explain thirst and hydration
• Review the function of water in body health
• Discuss ways to encourage water as a beverage of choice

9.2 Water and Hydration

Overview

Water is the most abundant substance in your body. It facilitates temperature regulation by absorbing body heat. Water is a solvent for electrolytes and can dissociate into hydrogen ions and hydroxide ions, although in very small amounts.¹ Adequate water is required to maintain blood volume and provide transport for nutrients and wastes. A minimum amount of water is needed for the kidney to excrete urea and creatinine along with other waste products.

Key Concepts

• The homeostatic system and how it helps maintain adequate hydration
• Water balance and urine production
• Weight change as a rough estimate of hydration

Adequate Intake

How much water do you drink? When you think about it, do you include drinking water as well as that found in beverages and foods? Recommendations on how much water you need each day are based upon US survey data and the amount needed to prevent acute dehydration.²

As adequate hydration is possible over a range of daily intakes, the AI is not a specific requirement but rather a general target with values below and above normal. In 2004 the Institute of Medicine recommended 3.7 liters per day for men and 2.7 liters per day for women.³ In the US, approximately 80 percent of dietary water is provided by beverages, with the remainder acquired from foods.

For most healthy adults, water intake comfortably meets the need. A homeostatic system prompts drinking when a state of dehydration is reached.⁴ Throughout the day, people consume fluids for reasons other than hydration, such as taste enjoyment, thereby adding to total consumption. If adequate amounts are routinely available, fluid consumption matches your body’s need for water. The minimum daily water requirement is unique to the individual and depends upon diet, activity level, and environment.

Water Balance

In a state of water balance, gain equals loss. In the presence of adequate fluid, healthy people maintain balance by drinking water (increasing intake) or urinating (excreting excess). The kidneys have the capacity to excrete 0.7 to 1.0 liters of water per hour. Urine output ranges from one to three liters per day and varies inversely with hydration status. As you drink more, you urinate more. In rare and extreme circumstances connoted by rapid fluid consumption and low sodium intake, kidney capacity is overwhelmed, and acute water toxicity results.

Urine and Hydration

Urine volume and appearance point to hydration status. Urine color darkens and lightens based upon solute load (substances dissolved in the water) per unit of water, with a darker color indicating less volume and a higher concentration of solute.⁵ Color, however, is a crude measure of hydration, as other factors, including diet, medications, and vitamins, influence appearance. These factors should be considered when estimating hydration status.

For healthy adults, a urine output of one hundred milliliters per hour signals adequate hydration. If for an extended time, output falls to fewer than thirty milliliters per hour, dehydration is likely. Exercise and heat exposure can reduce urine output by up to 60 percent. In hot climates, sweat loss of over a liter a day is common, further concentrating urine and increasing the risk of dehydration.

In a state of dehydration, heat exhaustion occurs at a lower core temperature, and the resting heart rate remains elevated. Blood pressure is maintained with difficulty, and the potential for fainting increases. Dehydration contributes to heatstroke, a life-threatening condition of elevated body temperatures and reduced cardiac output.

While healthy adults tolerate a 7-10 percent change in body weight with only transient effects, losses beyond that quickly become dangerous.

Body Weight and Hydration

Over short periods of time, hydration can be estimated by measuring one’s change in body weight. Total body water, including that inside and outside of cells, averages 60 percent of body weight, with a normal range of 45-75 percent. Regardless of gender, those with a greater lean body mass (muscle tissue) exhibit a higher percentage.

Studies suggest that a water deficit of 1-2 percent of body weight impairs mental function and adversely affects work performance.⁵ During endurance exercises, a 2 percent loss alters cardiovascular, thermoregulatory, and central nervous system functions. In addition, core body temperature regulation is compromised.

Carbohydrate loading increases pre-exercise weight, as glycogen in muscles has an osmotic hold (binding effect) on water. Conversely, a portion of post exercise weight loss reflects depletion of glycogen stores with associated water and is not indicative of dehydration.

Thirst

What about thirst? Both physiological and psychological factors prompt drinking. An increase in plasma osmolality (the concentration of particles per unit of serum) stimulates drinking at a short-term body weight loss of 3 percent or a plasma volume reduction of 5 percent. Most people do not even feel thirsty until plasma volume has decreased by 7 percent! So remember, a drink before your feel thirsty helps you avoid dehydration.

Beverage temperature influences the rate at which fluids are consumed. Even though cooler liquids are viewed as more desirable, those at a temperature of 59°F induce the greatest volume consumption in the shortest period of time.

Summary

Adequate water intake is key to good health. The body has mechanisms—including thirst, which stimulates drinking when more is needed, and urination when excess water is expelled—that help it maintain a desirable water balance. Urine output and appearance are rough estimates of hydration, with low output and dark color indicating dehydration. Over a short period of time, changes in body weight also reflect hydration status. Thirst reflects blood plasma osmolality, and the best temperature for quickly drinking water after your workout is 59°F.

9.3 Go for Water!

Overview

Adequate daily water intake is a central part of a healthy lifestyle. Water is critical to a range of body functions, from temperature regulation to waste removal. A range of environmental conditions influences the need for water—in particular, exposure to hot weather. Both food and drink contribute to daily water intake, from high water content fruits and vegetables to low content dried nuts and seeds. The MyPlate web page has tips for making water your number-one fluid source and ways to minimize sugar drinks.

Key Concepts

• Water and how it is integral to a range of body functions
• How both external and internal conditions influence water needs
• Water content of foods and how that contributes to daily water intake
• How to manage water intake and limit sugar drinks

Water and a Healthy Body

• Water helps your body regulate its temperature.
• Water lubricates and cushions joints.
• Water protects your spinal cord.
• Water aids in digestion.
• Water helps prevent constipation.
• Water helps maintain electrolyte balance.
• Water is a universal solvent that helps your body get rid of wastes.

Conditions That Increase Needs

• Hot environments and physical activity increase sweating and water needs
• Fevers increase body temperature and water needs
• Diarrhea and vomiting increase loss and increase need for additional waster

Did you know that once the air temperature around you reaches 80°F (27°C), your water needs increase? This is the temperature where you start sweating. Couple this with physical exercise, and water needs increase even more.⁶

Water in Food and Drink

Did you know that many of the foods that you eat contain water? In fact, when you estimate your daily water intake, you can include water from foods. You might expect fruits and vegetables to contain water, but what about sliced cheese or boiled rice? Many of the foods that we eat contain water. Typically, foods that require refrigeration, like dairy products and meats, are high in water and require refrigeration to prevent bacterial growth. Conversely, oils and dried foods—like raisins, sun-dried tomatoes, or beef jerky—are so low in water, you can store them on a shelf.

Interested in learning more about the water content of your favorite foods? Go to FoodData Central, a website managed by the US Department of Agriculture, and explore different foods that you eat.⁷ Information on nutrient content includes the number of grams of water per one hundred grams of product so you can calculate a percentage. Table 9.1 lists some common foods and their water content as an example of where to find the water in your food, for example, 91% of the weight of a watermelon is water.

Make Better Beverages Choices (US Department of Agriculture) has some advice for improving your water intake while managing added sugars from beverages.⁸

1. Drink water instead of sugary drinks. Regular soda, energy or sports drinks, and other sweet drinks usually contain a lot of added sugar, which provides more calories than needed.
2. How much water is enough? Let your thirst be your guide. Water is an important nutrient for the body, but everyone’s needs are different. Most of us get enough water from the foods we eat and the beverages we drink. A healthy body can balance water needs throughout the day. Drink more if you are very active, live or work in hot conditions, or are an older adult.
3. Water is a thrifty option: it is usually easy on the wallet. You can save money by drinking water from the tap at home or when eating out.
4. Manage your calories—drink water with and between your meals. Adults and children take in about four hundred calories per day as beverages, and drinking water can help you manage your calories.
5. Make water, low-fat or fat-free milk, or 100 percent juice an easy option in your home. Have ready-to-go containers filled with water or healthy drinks available in the refrigerator. Place them in lunch boxes or backpacks for easy access when kids are away from home. Depending on age, children can drink a half to one cup, and adults can drink up to one cup of 100 percent fruit or vegetable juice each day.
6. When you choose milk or milk alternatives, select low-fat or fat-free milk or fortified soymilk. Each type of milk offers the same key nutrients, such as calcium, vitamin D, and potassium, but the number of calories is very different. Older children, teens, and adults need three cups of milk per day, while children four to eight years old need two and a half cups, and children two to three years old need two cups.
7. Enjoy the beverage of your choice, but just cut back on sweetened drinks. Remember to check the serving size and the number of servings in the can, bottle, or container to stay within calorie needs. Select smaller cans, cups, or glasses instead of large or supersized options.
8. Water is always convenient. Fill a clean, reusable water bottle, and toss it in your bag or briefcase to quench your thirst throughout the day. Reusable bottles are also easy on the environment.
9. Use the nutrition facts label to choose beverages at the grocery store. The food label and ingredients list contain information about added sugars, saturated fat, sodium, and calories to help you make better choices.
10. Make your own flavored water by adding a squeeze or wedge of lemon or lime to your water. Avoid added sugars and extra calories in water flavored with sugar.

Summary

Good health requires adequate water intake. Water needs are influenced by a range of factors that are both internal to the body, such as an elevated temperature, and external, such as exercising in a hot climate. Both food and drink contribute to daily water intake, with water-rich foods such as fruits and vegetables contributing more water per serving than dried foods. Health recommendations from the MyPlate web page provide strategies for optimizing benefits from water while managing added sugars via sweetened beverages.

References

1. Chaplin, M. Water dissociation and pH. http://www1.lsbu.ac.uk/water/water_dissociation.html. Updated April 2020. Accessed January 16, 2021.
2. Popkin BM, D’Anci KE, Rosenberg IH. Water, hydration, and health. Nutr Rev. 2010;68(8):439-458. doi:10.1111/j.1753-4887.2010.00304.x.
3. Want to Stay Hydrated? Drink before You’re Thirsty. Mayo Clinic. https://www.mayoclinic.org/want-to-stay-hydrated-drink-before-youre-thirsty/art-20390077. Published July 2018. Accessed January 16, 2021.
4. Daily Water Intake among U.S. Men and Women, 2009-2012. National Center for Health Statistics, Centers for Disease Control and Prevention. https://www.cdc.gov/nchs/products/databriefs/db242.htm. Reviewed April 2016. Accessed January 16, 2021.
5. Riebl SK, Davy BM. The hydration equation: Update on water balance and cognitive performance. ACSMs Health Fit J. 2013;17(6):21-28. doi:10.1249/FIT.0b013e3182a9570f.
6. Greenleaf JE. Environmental Issues That Influence Intake of Replacement Beverages. National Center for Biotechnology Information. https://www.ncbi.nlm.nih.gov/books/NBK231133/. Accessed January 16, 2021.
7. 7. FoodData Central. US Department of Agriculture. https://fdc.nal.usda.gov/. Accessed January 16, 2021.
8. Make Better Beverage Choices. US Department of Agriculture. https://www.nutritionnc.com/edres/doc/DGTipsheet19MakeBetterBeverageChoices_1.pdf. Revised October 2016. Accessed January 16, 2021.