How much water should you drink in a day?

art illustration of a person drinking from a bucket.

The answer isn’t as simple as you might think.

There are many luxuries in life, but water is decidedly not one of them. Most of us are aware that we all need the liquid to survive, but exactly how much of it is necessary is surprisingly complex. 

There’s a popular notion that we all need to be chugging eight cups of water everyday for optimal health. While it is true that staying hydrated will certainly help contribute to your body working at its best, there’s no evidence to suggest that consistently drinking eight glasses of water a day is needed. 

In reality, each person’s water intake needs vary, and they depend on a number of factors, including how much exercise you get, the weather conditions of where you are, what you eat, and other health conditions you might have. Taking all these factors into account, the purported eight glasses a day just doesn’t work for most people. And our bodies already have an easy way to tell us if we need water: thirst. You can quickly replenish your lost fluids with a good helping of water. The human body has a carefully calibrated system for deciding when it needs more hydration and by listening to its cues, you can ensure you stay on top of your hydration needs. 

The amount of water you need depends on your body size. According to a 2018 review, infants need less water (in the form of breastmilk or formula, of course) than young children, who need less water than teens and adults who generally need the same amount of the liquid, on average. There are other factors to consider in this equation, too. For instance, people who are lactating need the most baseline water than most other groups. 

Your activity level also plays a large role. If you are exercising a lot, then you are more likely to sweat more, which forces you to need more water to replenish that lost amount. This is especially true if you are exercising in a particularly hot or humid environment, or your workout is long or intense. 

Additionally, water is not the only source of hydration. In fact, according to a highly-cited 2005 report from the ​​Institute of Medicine, we get about 20 percent of our hydration from the food we eat. Some foods, like watermelon, are almost exclusively water. It also might surprise you that consuming caffeinated beverages, such as tea or coffee, which are often considered diuretics, don’t actually dehydrate you. While caffeinated beverages will likely make you have to pee, that effect is temporary, and won’t have a considerable effect on your overall hydration balance. In other words, all those cups of coffee you down every day are indeed contributing to your daily hydration needs.

Given that you don’t need to be chugging all those glasses throughout the day, how can you tell if you are staying on top of your hydration needs? The answer is surprisingly simple. Follow your thirst.  

While it is true that you can drink water to excess, it is incredibly hard to do. Hyponatremia occurs when your body does get too much water and the amount of water in your blood becomes so high that it throws your electrolytes, particularly sodium, off balance. The condition, whose symptoms include headaches, confusion, nausea, and muscle weakness, is incredibly dangerous, but thankfully rare. Listening to your body’s thirst—and noting when weather, exercise intensity, or other factors, might make you more likely to need extra hydration—will ensure you are getting all the water you need, no counting required. 

All Rights Reserved for Claire Maldarelli

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