It's a popular dieting secret: Drink more water, and you'll shed more pounds. Finally, science is adding weight to the practice.

After about three months, a new study found, obese dieters who drank two cups of water before each meal lost 5 pounds more than a group of dieters who didn't increase their water intake. A year later, the water-drinkers had also kept more of the weight off.

The study included only middle-aged and older adults, but other studies suggest that drinking water might help dieters of all ages, said Brenda Davy, a nutrition researcher at Virginia Tech in Blacksburg. After years of folklore, she added, this may be the first hard evidence that pounding water is viable weight-loss strategy.

"It's this popular idea that, oh yeah, drink more water -- that's what you have to do when you want to lose weight," said Davy, who presented her new findings today at a meeting of the American Chemical Society in Boston. "It seems to be logical, but it had never really been investigated."

Davy and colleagues reported one of their first findings in 2008 in the Journal of the American Dietetic Association. That study found that older adults who drank two cups of water half an hour before breakfast ate about 75 fewer calories -- or 13 percent less -- than a comparable group who hadn't drunk water before the meal. People in both groups were overweight or obese, and all were allowed to eat as much of the food as they wanted.

To see if that behavior would lead to actual weight loss, the researchers started by putting more than 40 overweight and obese adults on a diet. The dieters, all between the ages of 55 and 75, were instructed to eat healthy meals that totaled no more than 1,200 to 1,500 calories a day.

Half of the dieters were randomly assigned to drink a 16-ounce bottle of water before all three meals. The others received water but were not given any instructions about when or how to drink it.

Twelve weeks later, the water drinkers had lost an average of 15.5 pounds, compared to an average 11-pound loss in the other group. That's a 44 percent boost in weight loss, just from drinking water.

Davy's experiments have failed to find the same effect in younger adults, possibly because the gastrointestinal tract empties more slowly as we age, so water might lead to a longer-lasting feeling of fullness in older people.

But water might still work as a diet aid for younger people -- just in different ways. One year-long study, for example, found that younger dieters who reported drinking more than a liter of water a day lost a little more weight than dieters who drank less water.

The reason could be physical. According to some research, water consumption might spark the body to produce more heat, boosting metabolism and burning more calories. Or, drinking more water might simply make people less likely to drink a lot of high-calorie sugar-filled beverages, said Barry Popkin, director of the Interdisciplinary Obesity Program at the University of North Carolina, Chapel Hill.

In hundreds of studies, Popkn said, people eat just as much food no matter how many calories they drink. And Americans are now drinking an average of 235 calories a day -- far more than ever before.

Davy's findings need to be repeated, Popkin added, before doctors can confidently tell dieters that downing water will boost their efforts. But it can't hurt to keep a water bottle nearby, especially if that helps you take in less soda, juice, energy drinks and other caloric beverages.

"Water is by far the healthiest beverage, and if you can't drink water, then drink unsweetened tea, coffee, diet beverages or for kids, low-fat milk," Popkin said. "The fewer calories we get from beverages, the healthier we're going to be."