Weight loss: drinking a gallon of water a day probably won’t help you lose weight

An older man pours a glass of water to drink with his meal.

It’s often said that if you’re trying to lose weight, one of the things you should do each day is drink lots of water – with some internet advice even suggesting this by as much as a gallon (about 4.5 liters). The claim is that water helps burn calories and reduces appetite, which in turn leads to weight loss.

But while we might all wish it was this easy to lose weight, there’s sadly little evidence to back up these claims.

Myth 1: water helps burn calories

One small study, of 14 young adults, found drinking 500ml of water increased resting energy expenditure (the number of calories our bodies burn before exercise) by about 24%.

While that sounds good, this effect only lasts an hour. And it won’t make a big difference at all. For an average 70kg adult, they will only use an additional 20 calories – a quarter of a biscuit – for every 500ml of water they drink.

Another study of eight young adults looked only at an increase in energy expenditure when the water was cold – reporting a 4% increase in calories burned. This may be because the body needs to use more energy to raise the temperature of the water to body temperature, or because the body needs more energy to filter the increased volume of fluid through the kidneys. Again, this effect is only visible for about an hour.

So while it’s scientifically possible, the actual net increase in calories burned is minuscule. For example, even if you drink an extra 1.5 liters of water per day, it will save you fewer calories than you would get from a slice of bread.

It should also be noted that all of these studies were conducted in healthy young adults. More research is needed to see if this effect is also seen in other groups (such as middle-aged and older adults).

Myth 2: water while eating reduces appetite

This claim again seems plausible, that if your stomach is at least partially full of water there is less room for food – so you end up eating less.

A number of studies actually support this, especially those in middle-aged and older adults. That is also the reason people who are unhealthy or have a poor appetite are advised not to drink before eating because it can lead to undereating.

But for people who want to lose weight, the science is a little less straightforward.

One study showed middle-aged and older adults lost 2 kg over a 12 week period when they drank water before meals compared to people who didn’t drink water with meals. Younger participants (aged 21-35) on the other hand did not lose weight, regardless of whether they drank water before eating or not.

Water before meals only caused weight loss in some groups.
theshots.co/ Shutterstock

But because the study didn’t use incognito (where information that could influence participants is kept private until the experiment is complete), it means that participants may have realized why they drank water before eating. This may have led some participants to deliberately change how much they ate in hopes that it would increase their weight loss changes. However, this does not explain why the effect was not seen in young adults, so it is important that future studies investigate why this is the case.

Another challenge with many studies of this kind is that the study focused solely on whether participants ate less at one of their meals after drinking water. While this may indicate potential for weight loss, there is little good-quality evidence to suggest that reducing appetite in general leads to weight loss over time.

Perhaps this is due to our body’s biological drive to maintain its size. It is for this reason that no claims can be made legally in Europe about foods that help you feel full longer in relation to weight loss.

So while there may be some appetite-stunning effects of water, it appears it may not lead to long-term weight change — and may be due to making conscious changes to your diet.

Water alone is not enough

There’s a pretty good reason why water alone isn’t very effective at regulating appetite. If they did, prehistoric humans probably would have starved.

But while appetite and satiety – feeling full and not wanting to eat any more – are incompatible with the ability to lose weight, they can be a helpful starting point.

The part that helps us feel full is our stomach. When food enters the stomach, it triggers stretch receptors which in turn cause the release of hormones that tell us that we are full.

But because water is a liquid, it is quickly emptied from our stomachs – meaning it doesn’t actually fill us up. Even more interesting, because of the shape of the stomach, liquids can pass through the contents of any semi-solid food that is digested at the bottom of the stomach. This means that water can still be quickly emptied from the stomach. So, even if it is consumed at the end of a meal, it doesn’t necessarily extend your feeling of fullness.

If you’re trying to eat less and lose weight, drinking copious amounts of water may not be a good solution. But there’s evidence to show that when water is mixed with other substances (such as fiber, soup or vegetable dip) it can delay how quickly your stomach empties its contents – meaning you feel full longer.

But even though water may not help you lose weight directly, it can still help you lose weight considering it is the healthiest drink we can choose. Replacing high-calorie drinks like soda and alcohol with water may be an easy way to reduce the calories you consume each day, which can help you lose weight.

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