Hydration shouldn’t be that complicated—just drink something when you’re thirsty, right? But since there are so many ways in which the environment, exercise, and individual health can affect one’s hydration status, there are numerous theories about the best ways to avoid dehydration. When is an electrolyte drink better than water? Will cold water really help cool someone down on a hot day? How much is too much water? Since I’m often exercising outdoors in the heat, I decided to investigate some of the truths and half-truths behind hydration.
1. If You’re Thirsty, You’re Already Dehydrated
People used to think that once you felt thirsty, the damage was done—you’re already dehydrated. But thirst can be triggered by slight physiologic changes, some of them, like a high salt meal, are not necessarily related to a severe lack of liquid. Even small changes in plasma osmolality—the concentration of salts and substances in our bloodstream—can trigger thirst. For instance, a less-than-2-percent rise in plasma osmolality can trigger the thirst sensation. But for true dehydration to occur, the osmolality must rise by about 5 percent.
Things like a really salty or sweet meal can make us thirsty even though we’re not dehydrated. But thirst is still a good indicator that your body wants something to drink—and if heeded, drinking when you’re thirsty can prevent future problems, like true dehydration. It’s also one of the best mechanisms for reminding yourself to stop and sip.
2. Eight Glasses of Water a Day Is Ideal
The eight glasses of water a day average is widely used as a general guideline for how much water to consume, but it doesn’t have much scientific backing. The Institute of Medicine recommends that men ingest about three liters (around thirteen cups) of liquid a day and women ingest just over two liters (about nine cups) of liquid a day. That sounds like a lot, except that we get liquid in many forms, not just from a glass of water. Food usually accounts for about 20 percent of our water intake, and possibly more, if you eat high water fruits and vegetables frequently. And most drinks, including tea, coffee, and juice, can count toward your liquid requirement. The eight glasses a day average also says nothing about those who exercise strenuously in hot weather, those who stay on the couch, the elderly, or people who have chronic diseases, so it doesn’t apply to everyone.
3. Caffeinated Beverages Are Dehydrating
It’s generally thought that caffeinated drinks, especially coffee, make you dehydrated. But in his review, “Caffeine, Body Fluid-Electrolyte Balance, and Exercise Performance,” Lawrence E. Armstrong, a professor of exercise physiology at the University of Connecticut disproves the notion that caffeinated beverages rob us of our precious fluids. He concludes that although caffeine, like water, is a mild diuresis (it increases excretion of urine), moderate caffeine consumption does not produce a “fluid-electrolyte imbalance” that can affect health or exercise performance.
Other findings support his conclusions. A small study done at the University of Nebraska tested the body weight, urine output, and blood of eighteen subjects after they consumed caffeinated and non-caffeinated beverages. They determined that there were “no significant differences in the effect of various combinations of beverages on hydration status of healthy adult males.” The Institute of Medicine’s expert panel on water and electrolyte intake asserts that the diuretic effects of caffeine are transient, and that coffee, tea, and colas can contribute to total water intake.
4. Cold Water Cools You Down
There is some debate over whether drinking cold water in hot weather helps bring down core body temperature. A 2007 study at the University of Exeter in England found that drinking fluids did not prevent body temperature rise or improve performance. However, a 2008 study in Medicine and Science in Sports and Exercise found that compared with a drink at room temperature, a cold drink before and during activity reduced physiological strain, including heat accumulation, and led to an improved endurance capacity. Other studies have found that water temperature may influence skin temperatures but not core temperatures.
But while the effects of cold water cooling us or preventing body temperature from rising may need more research, there are other reasons to have that drink on ice. Cold water can leave the stomach of an athlete faster than room temperature water, meaning it’s absorbed quicker, and most people find cold drinks more palatable and thus drink more of them, helping to promote liquid intake.
5. Sports Drinks Hydrate Better Than Plain Water
It depends. During short amounts of exercise or simply being in hot weather, water is a fine mode of hydration. Sports drinks, however, can be useful during exercise sessions that last longer than an hour, when people lose sodium and other electrolytes in their sweat. For some endurance athletes, hyponatremia, or low sodium, can occur when too much water is consumed over a short period. Sports drinks or other electrolyte supplements can help offset this risk.
Studies have also found that people tend to drink more liquid when it’s a sport drink compared to just plain water. But if you’re just lounging around on the couch, then sports drinks are just extra calories that you may not need, so water is a better bet.
When it comes down to it, staying hydrated can, and should be, simple. Drink when you’re thirsty, pay attention to your environment and body, and imbibe what you like.