On average, food provides 20 percent of the fluid we need―and far more if you choose water-rich fare. When it's warm or you need to fill up, factor in food when considering your hydration needs.
Credit: Randy Mayor

If your water bottle travels with you everywhere, sip on this thought: Drinking water isn't the only way to stay hydrated. Food can significantly affect your daily fluid needs. In fact, you can obtain much of the liquid you need from the food and beverages (other than water) you eat and drink every day.

What you really need
Water is an important nutrient that comprises 50 to 60 percent of your body weight, and it also helps transport other nutrients within the body. Yet because the body doesn't store water, you have to replenish the supply. How much do you need? For years, we've been told to drink eight glasses of water a day for optimal health. But that one-size-fits-all prescription has given way to a more flexible approach. Health experts have found that fluid requirements vary from person to person, and for many of us, the best way to stay adequately hydrated is to use thirst as a guide.

Several factors influence your need for water, including climate, muscle mass, physical activity, and diet. (People who have more muscle need more water―that's why men generally have higher fluid requirements than women.)

Food, often overlooked as a water source, can be a rich supply of fluid. On average, it provides 20 percent of the fluid we need―and far more if you choose water-rich fare. That translates to about two cups for the typical female and three cups for most men each day; most people require a total of roughly 11 to 15 cups of water daily, according to the Institute of Medicine.

Food aids hydration in other ways, too. Since sipping a beverage helps moisten and wash down food, eating encourages us to drink more liquids. That's why we consume most of our beverages with meals. Food also provides minerals, like sodium and potassium, that help our bodies hold on to water, so the liquids we consume with―or in the form of―food are better retained than those we drink between meals.

In the kitchen
Obviously, cooking affects a food's moisture content. "People don't think about water as such, but it really is a major ingredient," says food scientist Shirley Corriher, author of CookWise: the Secrets of Cooking Revealed. "Water can be a vital part of a dish simply because it provides a medium for food to cook in without searing or burning." Just think about water's critical role in preparing grains and starches like pasta, couscous, rice, and hot cereals. When we cook these foods in boiling water, their starch granules soak up water, causing them to swell and soften (and become edible). This increases their fluid content as much as sixfold, transforming them from one of the driest foods to one of the wettest.

Cooking can make other foods lose small amounts of fluid. "Meat is packed with water, and that water is retained in a network of proteins," says Harold McGee, PhD, author of On Food and Cooking: The Science and Lore of the Kitchen. "But when you heat those proteins, they coagulate. That's the equivalent of squeezing a sponge, so you force moisture out of the food." Cooking meat causes it to release about 20 to 30 percent of its original water content.

But because foods like meat, poultry, and fish are inherently rich in water, they still contribute to our overall fluid needs, even when cooked. Consider a juicy sirloin steak. Raw, a four-ounce serving contains 2.5 ounces of water. After broiling, that steak supplies 2.2 ounces of water. Other foods are nearly immune to the effects of heat. For example, while vegetables often soften when cooked, they rarely loose a meaningful amount of water.

Caffeine considerations
You may have heard about the dehydrating effects of caffeine. But leading health authorities like the Institute of Medicine and the American College of Sports Medicine say that's a myth. While caffeine does signal our kidneys to rid our bodies of excess water, it does so for only a short time, so we still retain more fluid than we lose after sipping a caffeinated beverage.

The Institute of Medicine reports caffeinated beverages contribute to our daily water needs as much as non-caffeinated drinks. In a study published in 2007 in the International Journal of Sport Nutrition and Exercise Metabolism, when researchers asked cyclists to bike for two-plus hours under hot, humid conditions, they found a caffeine-infused sports drink was as hydrating as a traditional sports drink.

Thirst or dehydration?
You're thirsty. Does that mean you're dehydrated? Not necessarily. "Thirst kicks in when a person is approximately one percent dehydrated," says Ann Grandjean, EdD, FACN, executive director of the Center for Human Nutrition in Omaha, Nebraska. "At two percent dehydration, thirst becomes more intense, and dry mouth occurs at three percent."

Certainly, dehydration can make you thirsty. But true dehydration doesn't occur until you've lost more than two percent of body weight. That's about three pounds for a 150-pound woman. Dehydration makes blood more concentrated, causing sensors in the brain to signal the body to drink more. As we drink, our blood becomes more diluted and these signals subside. The bottom line: Thirst doesn't always equal dehydration, but it does mean it's time to drink up.