How much water should people drink each day? Is it true that we need 8 glasses per day, and if so, how big would those ‘glasses’ have to be? Actually, your water needs depend on many factors, including your health, how active you are and the climate you live in.

Why water is important

Water is your body’s main component and makes up about 60 % of your body weight. Every system in your body depends on water; for example, water flushes toxins out of vital organs such as your kidneys and liver, carries nutrients to your cells and provides a moist environment for ear, nose and throat tissues. A 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. Severe dehydration can be significantly more serious; resulting in hospitalization and in some cases, death.

How much water do you need?

Every day, you lose water as you exhale, perspire, and of course, pass urine and have bowel movements. For your body to function properly, you must replenish lost water by consuming beverages and foods that contain water. How much water does the average, healthy adult living in a temperate climate such as southern Canada or the northern USA, need? The Institute of Medicine in the US has determined that an adequate intake (AI) for adult men is roughly 3 liters (about 13 cups) of total beverages a day (not just water). The AI for women is 2.2 liters (about 9 cups) of total beverages a day.

What about the advice to drink eight glasses a day?

Everyone has heard the advice, “drink eight 8-ounce glasses of water a day.” That’s about 1.9 liters, which isn’t that different from the Institute of Medicine recommendations. Just remember, it doesn’t have to be just water; all fluids count (except caffeinated ones, which increase urine output and thus, dehydrate you).

Factors that influence your water needs

You may need to modify your total fluid intake depending on how active you are, the climate you live in and how hot it is out, and your health status.

• Exercise. If you exercise or engage in any activity that makes you sweat, you need to drink extra water to compensate for the fluid loss. An extra 400 to 600 milliliters (about 1.5 to 2.5 cups) of water should suffice for short bouts of exercise, but intense exercise lasting more than an hour (for example, running a marathon) requires more fluid intake. How much additional fluid you need depends on how much you sweat during exercise, and the duration and type of exercise. During long bouts of intense exercise, it’s best to use a sports drink that contains sodium, as this will help replace sodium lost in sweat and reduce the chances of developing hyponatremia, which can be life-threatening. Also, continue to replace fluids after you’re finished exercising.

• Environment. Hot or humid weather can make you sweat and requires additional intake of fluid. Heated indoor air also can cause your skin to lose moisture during wintertime. Further, altitudes greater than 8,200 feet (2,500 meters) may trigger increased urination and more rapid breathing, which use up more of your fluid reserves.

• Illnesses or health conditions. When you have fever, vomiting or diarrhea, your body loses additional fluids. In these cases, you should drink more water and sometimes it is helpful to drink oral rehydration solutions, such as Gatorade or Powerade. Also, you may need increased fluid intake if you develop certain conditions, including bladder infections or urinary tract stones. On the other hand, some conditions such as heart failure and some types of kidney, liver and adrenal diseases may impair excretion of water and even require that you limit your fluid intake.

• Pregnancy or breast-feeding. Women who are expecting or breast-feeding need additional fluids to stay hydrated. Large amounts of fluid are used especially when nursing. The Institute of Medicine recommends that pregnant women drink 2.3 liters (about 10 cups) of fluids daily and women who breast-feed consume 3.1 liters (about 13 cups) of fluids a day.

Beyond the tap: Other sources of water

Although it’s a great idea to keep water within reach at all times, you don’t need to rely only on what you drink to meet your fluid needs. What you eat also provides a significant portion of your fluid needs. On average, food provides about 20 percent of total water intake. For example, many fruits and vegetables, such as watermelon and tomatoes, are 90 % or more water by weight.

In addition, beverages such as milk and juice are composed mostly of water. Remember though, while beer, wine and caffeinated beverages such as coffee, tea or cola and root beer contribute to fluid intake, these are best not be a major portion of your daily total fluid intake, as they increase fluid loss. Water really is your best bet because it’s calorie-free, inexpensive and readily available.

Staying safely hydrated

Generally if you drink enough fluid so that you rarely feel thirsty and produce 1.5 liters (6.3 cups) or more of colorless or light yellow urine a day, your fluid intake is probably adequate. If you’re concerned about your fluid intake or have health issues, check with your doctor or a registered dietitian. He or she can help you determine the amount of water that’s right for you.

To ward off 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.

Is it possible to drink too much water?

Although uncommon, it is possible to drink too much water. When your kidneys are unable to excrete the excess water, the electrolyte (mineral) content of the blood is diluted, resulting in low sodium levels in the blood, a condition called hyponatremia. Endurance athletes, such as marathon runners, who drink large amounts of water, are at higher risk of hyponatremia. In general, though, drinking too much water is rare in healthy adults who eat an average North American diet.

To learn more about staying properly hydrated while exercising, recovering from an injury or healthy eating in a variety of conditions, please feel free to contact our Registered Dietitian, Joy Kiddie, MSc, RD.