A stream of water pouring into and splashing around a tall glass with ice against blue background; concept is water and weight

If you’ve ever tried to lose excess weight, you’ve probably gotten this advice: drink more water. Or perhaps it was more specific: drink a full glass of water before each meal.

The second suggestion seems like a reasonable idea, right? If you fill your stomach with water before eating, you’ll feel fuller and stop eating sooner. But did that work for you? Would drinking more water throughout the day work? Why do people say drinking water can help with weight loss — and what does the evidence show?

Stretching nerves, burning calories, and thirst versus hunger

Three top theories are:

Feel full, eat less. As noted, filling up on water before meals has intuitive appeal. Your stomach has nerves that sense stretch and send signals to the brain that it’s time to stop eating. Presumably, drinking before a meal could send similar signals.

Burning off calories. The water we drink must be heated up to body temperature, a process requiring the body to expend energy. The energy spent on this — called thermogenesis — could offset calories from meals.

You’re not hungry, you’re thirsty. This explanation suggests that sometimes we head to the kitchen for something to eat when we’re actually thirsty rather than hungry. If that’s the case, drinking calorie-free water can save us from consuming unnecessary calories — and that could promote weight loss.

Exercise booster, no-cal substitution, and burning fat demands water

Being well-hydrated improves exercise capacity and thus weight loss. Muscle fatigue, cramping, and heat exhaustion can all be brought on by dehydration. That’s why extra hydration before exercise may be recommended, especially for elite athletes exercising in warm environments.

Swapping out high calorie drinks with water. Yes, if you usually drink high-calorie beverages (such as sweetened sodas, fruit juice, or alcohol), consistently replacing them with water can aid weight loss over time.

Burning fat requires water. Dehydration impairs the body’s ability to break down fat for fuel. So, perhaps drinking more water will encourage fat breakdown and, eventually, weight loss.

The bottom line

So, should you bump up hydration by drinking water before or during meals, or even at other times during the day?

Some evidence does suggest this might aid weight loss, at least for some people. But those studies are mostly small or short-term, or based on animal data. Even positive studies only found modest benefits.

That said, if you think it’s working for you, there’s little downside to drinking a bit more water, other than the challenge of trying to drink if you aren’t particularly thirsty. My take? Though plenty of people recommend this approach, it seems based on a theory that doesn’t hold water.

About the Author

photo of Robert H. Shmerling, MD

Robert H. Shmerling, MD, Senior Faculty Editor, Harvard Health Publishing; Editorial Advisory Board Member, Harvard Health Publishing

Dr. Robert H. Shmerling is the former clinical chief of the division of rheumatology at Beth Israel Deaconess Medical Center (BIDMC), and is a current member of the corresponding faculty in medicine at Harvard Medical School. … See Full Bio View all posts by Robert H. Shmerling, MD

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