• Gets rid of wastes through urination, perspiration and bowel movements
• Keeps your temperature normal
• Lubricates and cushions joints
• Protects sensitive tissues

Lack of water can lead to dehydration — a condition that occurs when you don't have enough water in your body to carry out normal functions. Even mild dehydration can drain your energy and make you tired. How much water do you need? Every day you lose water through your breath, perspiration, urine and bowel movements. For your body to function properly, you must replenish its water supply by consuming beverages and foods that contain water. So how much fluid does the average, healthy adult living in a temperate climate need? The National Academies of Sciences, Engineering, and Medicine determined that an adequate daily fluid intake is:   

• About 15.5 cups (3.7 liters) of fluids for men
• About 11.5 cups (2.7 liters) of fluids a day for women

These recommendations cover fluids from water, other beverages and food. About 20 percent of daily fluid intake usually comes from food and the rest from drinks. What about the advice to drink 8 glasses a day? You've probably heard the advice, "Drink eight 8-ounce glasses of water a day." That's easy to remember, and it's a reasonable goal. Most healthy people can stay hydrated by drinking water and other fluids whenever they feel thirsty. For some people, fewer than eight glasses a day might be enough. But other people might need more. Factors that influence water needs You might need to modify your total fluid intake based on several factors:  

If you do any activity that makes you sweat, you need to drink extra water to cover the fluid loss. It's important to drink water before, during and after a workout. If exercise is intense and lasts more than an hour, a sports drink can replace minerals in your blood (electrolytes) lost through sweat.

Hot or humid weather can make you sweat and requires additional fluid intake. Dehydration also can occur at high altitudes.

Your body loses fluids when you have a fever, vomiting or diarrhea. Drink more water or follow a doctor's recommendation to drink oral rehydration solutions. Other conditions that might require increased fluid intake include bladder infections and urinary tract stones.

Women who are pregnant or breast-feeding need additional fluids to stay hydrated. The Office on Women's Health recommends that pregnant women drink about 10 cups (2.4 liters) of fluids daily and women who breast-feed consume about 13 cups (3.1 liters) of fluids a day.

Beyond the tap: Other sources of water You don't need to rely only on what you drink to meet your fluid needs. What you eat also provides a significant portion. For example, many fruits and vegetables, such as watermelon and spinach, are almost 100 percent water by weight. In addition, beverages such as milk, juice and herbal teas are composed mostly of water. Even caffeinated drinks — such as coffee and soda — can contribute to your daily water intake. But water is your best bet because it's calorie-free, inexpensive and readily available. Sports drinks should be used only when you're exercising intensely for more than an hour. These drinks help replace electrolytes lost through perspiration and sugar needed for energy during longer bouts of exercise. Energy drinks are different from sports drinks. Energy drinks generally aren't formulated to replace electrolytes. Energy drinks also usually contain large amounts of caffeine or other stimulants, sugar, and other additives. Staying safely hydrated Your fluid intake is probably adequate if:  

• You rarely feel thirsty
• Your urine is colorless or light yellow

A doctor or registered dietitian can help you determine the amount of water that's right for you every day. To prevent dehydration and make sure your body has the fluids it needs, make water your beverage of choice. It's also a good idea to:   

• Drink a glass of water or other calorie-free or low-calorie beverage with each meal and between each meal.
• Drink water before, during and after exercise.
• Drink water if you're feeling hungry. Thirst is often confused with hunger.

Although uncommon, it's possible to drink too much water. When your kidneys can't excrete the excess water, the sodium content of your blood is diluted (hyponatremia) — which can be life-threatening. Athletes — especially if they participate in long or intense workouts or endurance events — are at higher risk of hyponatremia. In general, though, drinking too much water is rare in healthy adults who eat an average American diet.