Folk wisdom says don't drink water before meals, for it could kill appetite. Precisely why you should do so if you want to lose weight, say researchers now.
Slimmers can lose an average of 5lb (2.3kg) extra if they drink two cups of water before meals, three times a day.
The study was funded by the charity the Institute for Public Health and Water Research.
Over the course of 12 weeks, those drinking water lost about 15.5lb (7kg) compared with the non-drinkers, who lost about 11lb (5kg). The participants were aged 55 to 75.
The experts presented their findings at the National Meeting of the American Chemical Society (ACS) in Boston.
Dr Brenda Davy, senior author of the study, from Virginia Tech, said it was the first randomised controlled trial looking at water consumption and dieting.
She said the reason water may be so effective is because it fills up the stomach with a liquid that has no calories.
"People should drink more water and less sugary, high-calorie drinks. It's a simple way to facilitate weight management," Dr Davy said.
Diet drinks and other drinks with artificial sweeteners may also help people reduce their calorie intake and lose weight, researchers said.
However, Dr Davy advised against drinks sweetened with sugar, because they are high in calories.
A regular can of fizzy drink contains about 10 teaspoons of sugar, she explained.