About 60 percent of the average adult human body is made of water, according to a National Institutes of Health report. This includes most of your brain, heart, lungs, muscles and skin, and even about 30 percent of your bones. Besides being one of the main ingredients in the recipe for humankind, water helps us regulate our internal temperature, transports nutrients throughout our bodies, flushes waste, forms saliva, lubricates joints and even serves as a protective shock absorber for vital organs and growing fetuses. [How Long Can A Person Survive Without Water?]
There's no dispute that water is crucial to a healthy life (or any life at all, for that matter). And yet, there's little scientific consensus about the exact amount of the stuff an individual should consume each day. So how much water do you actually need to drink to be healthy?
You may have heard that you should drink eight 8-ounce (237 milliliters) glasses of water a day (totaling 64 ounces, or about 1.9 liters). That's the wrong answer. Despite the pervasiveness of this easily remembered rule, there is no scientific evidence to back it up, according to a 2002 review of studies. In fact, numerous studies suggest that this is far more actual drinking water than is necessary for most healthy adults.
The problem with this rule, researchers say, is that drinking water by the glass is not the only way that humans hydrate. Yes, it's true that guzzling H2O is an inexpensive and calorie-free way to whet your whistle, but the "8 x 8" rule crucially overlooks two big sources of daily water consumption.
Food and drink
One such source is food. Everything you eat contains some water. Raw fruits and vegetables have a lot; fruits such as watermelons and strawberries, for example, are more than 90 percent water by weight, according to the U.S. Department of Agriculture. Different diets naturally contain different amounts of water, but it adds up. According to a 2004 report by the National Academies of Sciences, the average North American gets about 20 percent of his or her daily water intake through food, and that counts toward healthy hydration.
The other key water sources that the "8 x 8" rule overlooks are other beverages. Non-alcoholic drinks such as coffee, tea, milk, juice and soda contain mostly water, and all contribute to your hydration. Contrary to another popular myth, studies show that coffee does not dehydrate you and is a suitable form of H2O intake. (Just remember that there can be adverse side effects of drinking too much caffeine, including headaches and disrupted sleep.)
So, between all the food, water, and other fluids you consume in a day, how much water should you aim to imbibe? The National Academies of Sciences suggests that women consume a total of approximately 2.7 liters (91 ounces) of water from all beverages and foods each day and that men get approximately 3.7 liters (125 ounces) daily. But these are just general guidelines and are not supported by firm scientific studies.
The truth is, there is no magic formula for hydration — everyone's needs vary depending on their age, weight, level of physical activity, general health and even the climate they live in. The more water you lose to sweating, the more water you'll need to replace with food and drink. So, naturally, a person doing strenuous physical work in a hot, tropical climate would need to drink more water than a person of identical weight and height who spent the day sitting in an air-conditioned office.
If you are looking for concrete advice, though, the best place to look is within.
"The vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide," according to the National Academies of Sciences. Your body naturally feels thirsty when your hydration levels are dropping, and water is the best medicine. (On the other end of the digestive spectrum, your urine can also tell you whether you're getting enough to drink — dark yellow or orange urine usually indicates dehydration, while well-hydrated urine should look pale yellow or colorless.)
The bottom line: Drink up when you're thirsty, and drink more when you sweat more. Your body will take it from there.
Originally published on Live Science.