You've heard it before: Drink eight glasses of water a day to keep your body trim, healthy, and happy. Though water is vital in a variety of bodily functions and processes, you may not have to drink as much as you think.

Unlike other vital nutrients, water doesn't have specific daily requirements. Just like everyone has different caloric needs, a person's water intake is affected by age, weight, activity level, and environment. The Food and Nutrition Board at the Institute of Medicine offers a ballpark estimation: Women should drink approximately 2.7 liters (about 11 cups) and men approximately 3.7 liters (more than 15 cups) of total water a day.

The key word is total
The key word in this sentence is total. In fact, your morning coffee and that Gala apple you packed for lunch can count toward your total. Liquids can include water, juice, milk, plus all of the juicy fruits and veggies you eat each day. Eighty percent of our water intake comes from beverages, but the other 20% comes from food.

Fruits such as strawberries, cantaloupe, and grapefruit, and vegetables such as broccoli, cucumber, and spinach are more than 90% water. Including a lot of fresh produce in your diet will put you closer to your water goals without causing you to constantly circle the water cooler.

And although water is a cheap and calorie-free option, coffee, tea, and even beer and soda can count toward your total. But keep in mind that while you're getting water in beer, soda, and Frappachinos, you're also consuming calories and other not-so-nutritious ingredients like refined sugar.

Water for weight loss
Plenty of diet books will tell you that guzzling water is the secret to fighting the battle of the bulge. While this may be true, the research often conflicts. Two studies (one done in 2003 and the other in 2007) showed that consuming lots of water boosted the number of calories burned in a day, and a popular 2008 study published in Obesity suggested that drinking water may lower energy intake or alter metabolism.

However, a 2006 study cast doubt on this theory by showing that water had no effect on energy expenditure. And it can't be clear if the reason for weight loss can be attributed to drinking less calorie-laden beverages or the water itself.

It is true that certain individuals need more water—some endurance athletes can lose up to three quarts of sweat per hour! So if you are sick, live in a hot or high-altitude environment, have extended sweat sessions at the gym, or are pregnant or breast-feeding, you may have to adjust your fluid intake accordingly.

The bottom line
Drink plenty of water and other low-calorie, nutritious beverages, eat fresh fruits and veggies, and be sure to rehydrate after trips to the gym or time in the sun. If you do this, chances are you'll get enough fluids.