One of the most familiar pieces of weight-loss advice is also among the simplest: Drink lots of water. It makes you feel fuller, for one, so you don't eat as much junk later on (and maybe you won't drink sodas or other sugar-filled drinks).

Now, a new study adds some scientific heft to that advice. People who drank more water, researchers found, stayed slimmer than those who drank less. The study followed more than 120,000 people for about two decades, checking in on their lifestyle habits and weight every four years.

Overall, people in the study gained a small amount weight during each four-year span. But each cup of water a person drank per day meant they gained, on average, nearly three-tenths of a pound less during that time. The more water they drank, the less weight they put on – a noteworthy finding for a study that, for most participants, spanned a large chunk of middle age.

Fruit juice and sugar-sweetened beverages like soda, on the other hand, made participants gain more weight; each cup per day led to half a pound and four-fifths of a pound more weight gain in four years, respectively. Subbing water for a sugary drink, then, would likely have an even bigger impact, says An Pan, a public health researcher at the National University of Singapore who helped lead the study. Swapping a 12-ounce soda for a large glass of water every day would lead to a 1.25-pound smaller weight gain over four years.

If you tire of constantly sipping plain water, the researchers found that coffee had a similar slimming effect, too – though unlike water, it's easier to drink too much joe, which can have some downsides.