A quick consultation with Dr. Google will tell you that drinking lots of water — and staying well-hydrated — can help you lose weight.
But is there any truth to this? A new study published in the Annals of Family Medicine adds to the evidence that hydration may play a role in weight management.
"What we found was that people who were inadequately hydrated had increased odds of being obese," says study author Tammy Chang of the Department of Family Medicine at the University of Michigan.
The study was based on data collected by a federal health survey, in which researchers had documented weight and height of participants. They also conducted urine tests to establish an objective measure of participants' level of hydration.
Chang and her colleagues found the odds of being obese were 1.59 times higher for people who were not well-hydrated. And overall, they found that a lack of proper hydration was associated with higher body mass index.
"The idea that hydration could have this effect is interesting and important," says Chang. But there's still a lot that remains unknown. This study captures one snapshot in time, and it does not prove that staying hydrated helps people manage weight.
It is possible that people who stay well-hydrated also have other habits — say, a healthful diet — that keeps them slimmer.
The new findings do fit with a few, small studies that found dieters who drink water before meals are more successful at weight loss, at least in the short term.
"Over several months, people who are drinking water before their meal lose more weight," explains Eric Rimm of the Harvard School of Public Health. The idea here is that people feel less hungry after filling up on water. "But is it a lasting effect?" Rimm says it's not clear.
He says what is clear when it comes to the relationship between weight and what we drink is this: Swapping sugary drinks for water is beneficial.
In long-term studies that tracked people's habits over several decades, "we found that people who choose water instead of sugar-sweetened beverages gain weight at a much slower rate," says Rimm. It's not necessarily pointing to the importance of hydration, "just drinking fewer calories," he adds.
There's no hard and fast rule on how much water is ideal to drink. A gardener working outside or an athlete in training needs more than someone sitting in an air-conditioned office.
So, how do you know if you're well-hydrated? Chang says she tells her patients to notice the color of their urine. If it's dark, you probably need to drink more.
"If your urine is light — almost like the color of water — then you're probably well-hydrated," Chang says.
This may be particularly important for older people. Research suggests people over 65 years old don't feel as thirsty as younger people, even when they are dehydrated.