It is often stated that if you want to lose weight, one of the things you need to do every day is to drink plenty of water, and some advice on the internet even suggests drinking up to 4.5 litres. Water is claimed to help burn calories and reduce appetite, which in turn leads to weight loss.
But while we'd all like losing weight to be that easy, there's unfortunately little evidence to back up these claims.
One small study, conducted with 14 young adults, found that drinking 500 milliliters of water increased resting energy expenditure (the number of calories our bodies burn before exercising) by about 24%.
Although it may seem great, this effect only lasted for an hour. And this wouldn't translate to much of a difference at all. For an average 70kg adult, you would only expend an additional 20 calories – a quarter biscuit – for every 500 ml of water you drink.
In another study conducted with eight young adults, an increase in energy expenditure was only observed when the water was cold, with a modest 4% increase in calories burned. This may be because the body needs to use more energy to bring the water up to body temperature, or because the body needs more energy to filter the increased volume of fluid through the kidneys. And again, this effect was only seen for about an hour.
So while it might be scientifically possible, the actual net increase in calories burned is minuscule. For example, even if you were to drink 1.5 liters more water a day, you would save fewer calories than you would eat with a slice of bread.
Also keep in mind that all of this research was conducted in young, healthy adults. More research is needed to see if this effect is also seen in other groups (such as middle-aged and older adults).
This statement also makes sense, in the sense that if your stomach is at least partly full of water, there is less room for food, so you end up eating less.
In fact, several studies support this claim, especially those conducted in middle-aged and older adults. It is also one of the reasons why people who are unwell or have a poor appetite are advised not to drink before eating, as it can lead to undereating.
But for people who want to lose weight, the science is a little less straightforward.
One study showed that middle-aged and older adults lost 2kg. over a 12-week period if they drank water before meals, compared to people who did not drink water with food. In contrast, younger participants (aged 21 to 35) did not lose weight, regardless of whether or not they drank before meals.
But because the study did not use blinding (in which information that may influence participants is hidden until after the experiment is over), it means that participants may have been aware of why they drank water before their meal. This may have led some participants to intentionally change the amount they ate in the hope that it would increase their chances of losing weight. However, this does not explain why the effect was not seen in young adults, so it will be important for future studies to investigate why this is so.
The other problem with many studies of this type is that they only focus on whether participants eat less during one of the day's meals after drinking water. Although this might suggest the possibility of weight loss, there is very little good quality evidence to show that reducing appetite generally leads to weight loss over time.
Perhaps this is due to our body's biological drive to maintain its size. For this reason, in some areas no legal claims can be made about foods that help you feel fuller longer with reference to weight loss.
So while water may have some appetite-suppressing effects, it may not cause long-term weight change, and this may be due to conscious dietary changes.
There's a pretty good reason why water alone isn't terribly effective at regulating appetite. If it were, prehistoric humans would have starved.
But while appetite and satiety—feeling full and not wanting to eat again—aren't perfectly aligned with the ability to lose weight, they can be a helpful starting point.
Part of what helps us feel full is our stomach. When food enters the stomach, it activates stretch receptors which, in turn, trigger the release of hormones that tell us we are full.
But since water is a liquid, it empties quickly from the stomach, which means it does not fill us up. Even more interesting, because of the shape of the stomach, liquids can bypass any semi-solid food that is being digested in the lower part of the stomach. This means that water can be emptied quickly from the stomach. Therefore, even if it is consumed at the end of a meal, it will not necessarily prolong the feeling of satiety.
If you're trying to eat less and lose weight, drinking excessive amounts of water may not be a good solution. But there is evidence showing that when water is mixed with other substances (such as fiber, vegetable soups or sauces) it can slow down how quickly your stomach empties its contents, meaning you'll feel fuller for longer.
Although water does not help you lose weight directly, it can help you lose weight, since it is the healthiest drink we can choose. Switching from high-calorie drinks, such as soda and alcohol, to water can be a simple way to reduce the calories you consume daily, which can help you lose weight.
This article, translated in collaboration with the Lilly Foundation, was originally published on The Conversation.