How much and what should I drink to stay hydrated?
by Mary B Grosvenor, MS, RD and
As our climate continues to change, we are experiencing more and more sweltering days each summer. When temperatures rise, we sweat. The evaporation of sweat is cooling but also causes water to be lost from our body. If we do not replace what we lose, we become dehydrated; blood volume decreases and with it the body’s ability to function. Dehydration symptoms can occur with a water loss of as little as one percent of body weight and a loss of ten percent of water can be deadly. [1] As heat waves become hotter and more frequent more people are affected, both in terms of comfort and health. To stay healthy in the summer heat, stay hydrated by knowing how much and what to drink.
Why Is Staying Hydrated So Important?
Water is vital for life. All the chemical reactions that occur in our bodies take place in water. Water delivers oxygen and nutrients to cells and removes waste products. It is needed to regulate acid-base balance, provide lubrication and protection, and regulate body temperature. Normal body temperature is around 98.6°F (37°C). If it rises above 108°F, death is likely. Water helps maintain a normal body temperature in a number of ways. It holds heat and changes temperature slowly, helping to keep body temperature constant when the outside temperature fluctuates. The water in blood affects the amount of heat lost from the surface of the body. When body temperature rises, the blood vessels in the skin dilate, causing blood to flow close to the surface where it can release heat to the environment. The water lost in sweat helps to cool the body by evaporation; excess body heat is used to convert beads of sweat into vapor.
Water Ins and Outs
The human body is about 60% water, but this fluctuates as we gain and lose water throughout the day. We take in water from water and other fluids we drink as well as from foods we eat. Juices, sports drinks, broth, soup, and milk contribute to fluid needs; even coffee, tea, and other caffeinated beverages contribute to your daily water intake. [2] Although it is often suggested that caffeinated beverages have a negative impact on hydration status, the diuretic effects are minimal in individuals who regularly consume caffeine, so the net amount of water that caffeinated beverages contribute is similar to noncaffeinated beverages. [3] Alcohol is also a diuretic so alcoholic beverages are not recommended for hydration, but the overall effect of alcohol consumption on fluid balance depends on the relative amounts of water and alcohol consumed. So, on a hot day, enjoying beer with less than 4% alcohol and alternating your beer with water is unlikely to negatively impact your hydration. [4,5] Whole fruits and vegetables also contribute a significant amount of water; watermelon, lettuce, and tomatoes are over 90% water, apples and pears are about 85% water.
We lose water from the body in urine, about 4 to 8 cups per day, and feces, about ½ cup per day. We also lose about four cups of water daily through evaporation from our lungs and skin. When we are active, or the temperature is high, we lose water in sweat. An average adult doing light work at an ambient temperature of 84°F loses about ten cups of water per day in sweat. Strenuous activity in a hot environment can dramatically increase these sweat losses to more than six cups per hour. [6]
Dehydration
Early symptoms of dehydration include fatigue, dry mouth, thirst, and headache. As the kidneys work to conserve water, urination becomes less frequent, and the urine becomes darker in color. The fatigue and discomfort of dehydration can lead to irritability and difficulty with cognition, memory, and concentration. [7] Chronic mild dehydration can impair health by increasing the risk of kidney stones, urinary tract infections, high blood pressure, and stroke. [8] If dehydration becomes severe confusion, fainting, an inability to urinate, rapid breathing, and increased heart rate occur. These symptoms signal a potentially life-threatening condition, and oral fluids may not be enough to restore hydration; medical attention and intravenous fluids are needed.
The risk of dehydration increases as the environmental temperature rises. Risks are particularly high in infants, older adults, and in those who are active in the heat. Infants are at risk because their kidneys cannot concentrate urine efficiently and, due to their small body size, they lose proportionately more water through evaporation. In older adults, thirst sensation and kidney function decrease, potentially reducing water intake and increasing urinary losses. The risk is further increased in those with conditions such as diabetes and kidney disease that can increase urinary water losses and in those taking diuretics or other medications that increase urination. Individuals who exercise or work outdoors in the heat are at risk because they may lose large amounts of water in sweat.
How Much and What Should You Drink?
The recommended fluid intake for adults is about eleven cups per day for women and sixteen cups per day for men. (This represents overall fluid intake per day, including anything you eat or drink that contains water, such as fruits or vegetables.)
[9] But this amount can vary greatly depending on body size, environmental temperature, and activity level. Larger individuals need more water than smaller ones. Men typically have higher water needs than women because they have larger bodies and more muscle, which requires more water to maintain than fat. Temperature has a significant impact on water needs. When it is hot we need more water to replace the water we lose in sweat and through evaporation. Activity further increases sweat losses; water needs can triple with activity in very hot weather. [9] To stay hydrated when working or exercising in a hot environment, you need to drink about one cup every 15 to 20 minutes (3-4 cups per hour) for the duration of the work or exercise.
The sensation of thirst helps motivate us to drink but relying on thirst alone may not be enough to maintain a healthy body water level in all situations. You can assess your hydration by checking the color of your urine. If you are consuming enough, your urine will be a pale-yellow color; darker urine can indicate dehydration. Another indicator of hydration is weight loss. The goal is to drink enough to prevent more than a 2% loss of total body weight as water; this would be about three pounds for a 150 lb. person. While most of us do not weigh ourselves over the course of the day, this method is often used by athletes exercising in the heat. [10,11]
The best fluid to keep you hydrated on a hot day is water. It is absorbed quickly and contains no calories or added sugars. When exercising or working in the heat for longer than 45 minutes, sports drinks may be helpful. They contain carbohydrates to fuel your activity and electrolytes to replace losses that may occur in sweat. While salt tablets were once promoted to maintain hydration, these are no longer recommended in most cases. Small snacks consumed with plenty of water can provide energy and replace lost electrolytes. It is uncommon for a healthy adult to drink too much water, but this can occur usually following extensive intense physical activity, such as military training or marathon running. Fluid intake should be limited to no more than 1.5 quarts per hour; excess fluid can cause a life-threatening electrolyte imbalance. [11]
The Bottom Line
When it is hot, we need to drink more to prevent dehydration, sometimes two to three times as much. This fluid can come from plain water and other fluids such as juice, sports drinks, milk, and even coffee or tea. However, in most situations water is the best hydration drink. [11] It is inexpensive and free of calories, sugar, and other additives. If you are exercising for a long period, working out in the heat, or tend to sweat a lot during physical activity, consider sports drinks to replenish electrolytes along with fluids.
Editor’s Note- For adults, the Institute of Medicine recommends men drink thirteen cups of water or other beverage every day and for women nine cups of water or beverage daily.
References
[1] Institute of Medicine (US) Committee on Military Nutrition Research; Marriott BM, editor. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington (DC): National Academies Press (US); 1993. 5, Water Requirements During Exercise in the Heat. Available from https://www.ncbi.nlm.nih.gov/books/NBK236237/#
[2] Maughan RJ, Watson P, Cordery PA, et al. A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index. The American Journal of Clinical Nutrition. 2015;103(3):717-723. doi:https://doi.org/10.3945/ajcn.115.114769
[3] Maughan RJ, Griffin J. Caffeine ingestion and fluid balance: a review. Journal of Human Nutrition and Dietetics. 2003;16(6):411-420. doi:https://doi.org/10.1046/j.1365-277x.2003.00477.x
[4] Shirreffs SM, Maughan RJ. Restoration of fluid balance after exercise-induced dehydration: effects of alcohol consumption. Journal of Applied Physiology. 1997;83(4):1152-1158. doi:https://doi.org/10.1152/jappl.1997.83.4.1152
[5] Wynne JL, Wilson PB. Got Beer? A Systematic Review of Beer and Exercise. International Journal of Sport Nutrition and Exercise Metabolism. 2021;31(5):438-450. doi:https://doi.org/10.1123/ijsnem.2021-0064
[6] Bates GP, Miller VS. Sweat rate and sodium loss during work in the heat. Journal of Occupational Medicine and Toxicology. 2008;3(1):4. doi:https://doi.org/10.1186/1745-6673-3-4
[7 Karim M, Alamgir H. High temperatures on mental health: Recognizing the association and the need for proactive strategies—A perspective. Health science reports. 2023;6(12). doi:https://doi.org/10.1002/hsr2.1729
[8] Hasegawa Y, Kato K, Kazuhiro Ogai, Konya C, Takeo Minematsu. Need for a consensus definition of chronic dehydration: A scoping review. Drug discoveries & therapeutics. Published online January 1, 2024. doi:https://doi.org/10.5582/ddt.2024.01014
[9] Institute of Medicine. 2005. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. Washington, DC: The National Academies Press. https://doi.org/10.17226/10925.
[10] Thomas DT, Erdman KA, Burke LM. American College of Sports Medicine Joint Position Statement. Nutrition and Athletic Performance [published correction appears in Med Sci Sports Exerc. 2017 Jan;49(1):222. doi: 10.1249/MSS.0000000000001162]. Med Sci Sports Exerc. 2016;48(3):543-568. doi:10.1249/MSS.0000000000000852
[11] Heat stress: Hydration. Accessed July 12, 2024. https://www.cdc.gov/niosh/mining/userfiles/works/pdfs/2017-126.pdf.
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