Humans are approximately 60% water. From lubricating joints, to delivering oxygen throughout your body, water is essential for life. At the same time, water is lost through breathing, urine and bowl movements, as well as when you sweat. As a result, you need to constantly replace lost water.

The Role of Sweat

How much water you need each day is a topic of much debate. I grew up being told to drink eight glasses of water per day. That’s about 2 litres. And that’s still a common recommendation, but not the only one. In 2004, The Institute of Medicine of the National Academy of Sciences suggested men who are adequately hydrated consumed 3.7 litres, and women 2.7 litres per day. However, there’s no evidence to support one ideal amount of water for everyone as daily needs vary from person to person, and day to day. One such variable is how much you sweat.

When you exercise, your muscles generate heat. This raises your body’s temperature. If it gets too high, your body needs to cool off and you start sweating. When sweat evaporates, it absorbs heat. This cools the surface of your skin and cools you down. How much you sweat depends just as much on the weather conditions as how hard you’re exercising. On a hot humid day, a light walk, or even sitting can cause you to sweat. While on a cool dry day with a breeze you might not even notice yourself sweating.

Why We Need Water During Exercise

When I was training for my marathon in 1990 it wasn’t common for people to carry water while running. I would often run 20 km along main roads in the summer heat and stop at gas stations to sip from the hose. Nowadays carrying water while exercising is easy. From backpacks that have a tube for your mouth, to belts and harnesses that look like something Rambo would wear. But we might not always need all that water and these devices can add between 3 to 15 pounds of extra weight.

There are two concerns when it comes to water intake and exercise. The first is for health and safety reasons. If too much water is lost, your body can’t cool itself, which can lead to heat exhaustion. Images of athletes crawling across a finish line after a marathon may come to mind. But this can happen even at low levels of activity in hot and humid conditions. Symptoms can include exhaustion, fatigue, poor mental functioning (dizziness, confusion, irritability), nausea, vomiting and fainting. If heat exhaustion isn’t treated, it can lead to long-term disability and even death.

The second, is regarding exercise performance. If you don’t start to replenish lost water, you can become dehydrated. When this happens your blood volume decreases. This can lead to lower blood pressure. And in turn, an increased heart rate. As a result, it takes more effort to maintain the same level of work. It’s believed that mental and physical performance suffers when sweat loss is greater than 2% of body weight. For someone who’s 150 pounds that would end up being 3 pounds or 5-6 cups of water loss.

Effects of Dehydration on Exercise

The effects of dehydration on exercise performance have been tested in numerous studies without reaching a consensus. Conducted in trained subjects in climate-controlled heat rooms, these studies often require participants to undergo a dehydration protocol before doing the exercise test. Some of these studies have shown exercise performance suffers below a weight loss of 3%. While a number of studies have shown no impact.

However, lab research don’t always match real-world conditions. A study of 643 marathon finishers reported body weight loss to be negatively associated with finishing times. Those runners with the fastest times had the greatest weight loss. And elite runners have been found to lose 5% or more of weight, even when consuming water, without affecting their performance.

One limitation of these studies is equating changes in body weight with water loss, which isn’t true. When exercising, you’re using fat and glycogen (carbohydrate) for energy. Their breakdown results in weight loss, and water is released into the body as a by-product of that process. This water is used to replenish stores just as if you drank it.

The other important point is when one sweats, it’s more than just water. Sodium is excreted too. Replacing sweat loss with just water, can dilute your blood. This is unlikely to be a problem during short bouts of exercise, but during longer bouts, such as ultramarathons, it can be problematic. At the extreme, it can lead to hyponatremia- low sodium concentration in the blood. For these events, a sports drink may be helpful.

The Perfect Balance?

So how do you come up with the perfect balance? It’s been suggested we should drink one cup of water for every 20 minutes of exercise and to drink before we’re thirsty. This is due to a belief that if you wait until you’re thirsty, it’s already too late. If you’re aiming to prevent weight loss, or fully replenish water loss, you’ll need to do that. But that doesn’t mean you’re performance will improve.

And research doesn’t seem to back it up. When cyclists were allowed to drink as they wished, their time trial performance was better than when told how much to drink. Of note, the cyclists drank less when it was their choice. This has led to the theory thirst is linked with blood concentration (osmolality) as opposed to total water volume. This would explain why body weight could decrease due to sweating but performance doesn’t suffer when people are able to drink freely.

Given what we know, it seems the best may be to drink when you want to. Using thirst as an indicator seems totally reasonable. However, after your workout, it’s important to rehydrate and refuel.

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