Does a Glass of Water Before Meals Help With Weight?

Drink a big glass of water before you eat. It is one of the oldest weight-loss tips, and also one of the few that has actually been tested in randomized trials. The results are modest but interesting. Here is what was done, what was found, and how to think about it. This is general education, not medical advice.

The idea

Water has no calories, but it takes up room in the stomach. The thinking is that a large drink about half an hour before a meal may make you feel fuller sooner, so you eat a little less. It may also replace a sugary drink you would otherwise have had.

Two trials worth knowing

• Adults aged 55 to 75. In a 2010 trial in the journal Obesity, 48 middle-aged and older adults with overweight or obesity followed the same reduced-calorie diet for 12 weeks. One group also drank 500 millilitres (about 17 ounces) of water before each main meal. That group lost about 2 kilograms (roughly 4 pounds) more than the diet-only group.
• Adults with obesity in primary care. A 2016 trial in Obesity randomized 84 adults to drink 500 millilitres of water about 30 minutes before main meals or to a comparison strategy (imagining a full stomach), and all received weight-management advice. After 12 weeks the water group had lost about 1.3 kilograms more on average. Those who did it before all three meals did best.

What these results do and do not say

• The effect is small. About 1 to 2 kilograms over three months is a helpful nudge, not a transformation.
• The trials were short and small. Neither followed people for long, and the benefit beyond 12 weeks is not well established.
• Everyone was also doing something else. Participants were getting dietary advice or following a reduced-calorie diet, so water was an addition, not a replacement.
• It may work best for some people. The strongest evidence was in middle-aged and older adults; evidence in younger adults is thinner.

Making it practical

• Try it before your largest meals. A glass of water 20 to 30 minutes before eating is simple to test.
• Swap, do not add. Water in place of a sweetened drink may matter as much as the preload itself. Our guide to everyday food choices and the other food guides show how small swaps add up.
• Pair it with the basics. Fiber-rich foods and regular meals do more for fullness. See fiber.
• Listen to your body. If it feels uncomfortable, or you are already full, skip it.

When to be careful

Large amounts of water are not right for everyone. If you have heart failure, kidney disease or another condition with a fluid limit, or you take medicines that affect sodium or fluid balance, follow your clinician’s advice. Drinking very large quantities in a short time can be unsafe.

Where it fits

A glass of water before meals is cheap, easy and low-risk for most healthy adults, which makes it a reasonable experiment. Just keep expectations realistic: it is a small helper, not a method on its own.

Sources

• Dennis EA, Dengo AL, Comber DL, et al. Water consumption increases weight loss during a hypocaloric diet intervention in middle-aged and older adults. Obesity, 2010;18(2):300-307.
• Parretti HM, Aveyard P, Blannin A, et al. Efficacy of water preloading before main meals as a strategy for weight loss in primary care patients with obesity: RCT. Obesity, 2016.