Does drinking water before meals really help you lose 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."

This advice is grounded in the belief that water can play a significant role in weight management by making one feel fuller, thereby reducing food intake, among other proposed benefits. 

But how effective is this strategy, and what does scientific evidence say about the relationship between hydration and weight loss? Here's what Harvard Health Publishing says...

Feeling full to eat less

The rationale behind drinking water before meals is straightforward: it's thought to stretch the stomach and trigger nerves that signal fullness to the brain, potentially leading to reduced food intake. 

Some small, short-term studies lend support to this idea, showing that older adults who drank water before meals consumed less food, and participants on a low-calorie diet who increased their water intake before meals experienced decreased appetite and greater weight loss over 12 weeks. 

However, the long-term impact of this strategy on weight loss remains unassessed and unclear.

Thermogenesis: Burning calories by drinking water

Another theory suggests that the body expends energy to heat ingested water to body temperature, a process known as thermogenesis, which could potentially offset calorie intake from meals. 

While early studies hinted at this possibility, more recent research has found little evidence to support the idea that drinking water significantly boosts calorie burning, casting doubt on this explanation for water-induced weight loss.

Misinterpreting thirst for hunger

It's also been suggested that we sometimes confuse thirst with hunger, leading us to eat when we should simply drink water. 

This theory posits that by drinking water, we can avoid unnecessary calorie consumption. 

Yet, the regulation of thirst and hunger is complex, varies by age, and lacks convincing human studies to substantiate the claim that misinterpreting thirst for hunger contributes to 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.

For most people, hydrating before exercise seems unnecessary, and I could find no studies specifically examining the role of hydration in exercise-related weight loss.

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.

A dramatic reduction in calorie intake by substituting water for higher-calorie beverages could certainly lead to long-term weight loss. While it's hard to design a study to prove this, indirect evidence suggests a link between substituting water for high-calorie beverages and weight loss.

Even so, just as calorie-restricting diets are hard to stick with over the long term, following a water-only plan may be easier said than done.

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.

Though some animal studies support the idea, I could find no compelling evidence from human studies that drinking extra water helps burn fat as a means to lose excess weight.

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.