Dieters who write down everything they eat each day lose twice as much weight as those who don't, according to one of the largest weight-loss studies ever conducted.

This confirms the importance of keeping a food diary — advice that nutritionists and weight-loss programs have pushed for years.

Scientists at four clinical research centers recruited 1,685 overweight or obese adults who weighed an average 212 pounds. Forty-four percent were African-Americans.

The participants were offered 20 weekly group sessions led by nutritionists and behavior counselors and encouraged to try to lose at least 9 pounds in six months. They were told to consume about 500 fewer calories a day, eat plenty of fruits and vegetables, do about 180 minutes of moderate-intensity physical activity a week, and keep daily food and exercise records.

The findings reported in the August issue of the "American Journal of Preventive Medicine":

Dieters who kept their food diary six or more days a week lost an average of about 18 pounds in six months, compared with about 9 pounds for those who didn't keep food diaries.

"The biggest losers also attended most of the group meetings and did more exercise. Some did 300 minutes or more of physical activity a week, but the average participant in the study did about 117 minutes a week.

About 69 percent of the participants lost 9 pounds or more, which is enough to improve some health measures such as blood pressure, joint pain and pre-diabetes.

About 69 percent of black men and 59 percent of black women lost at least 9 pounds.

"There is a misconception that nobody can lose weight, but in this study, two-thirds of them lost enough weight to make a difference to their health," says Victor Stevens, senior investigator at the Kaiser Permanente Center for Health Research in Portland, Ore.

The National Heart, Lung, and Blood Institute sponsored the study.

These weight-loss techniques work well with everybody, including African-Americans, who are often underrepresented in studies and yet suffer from a high rate of obesity and weight-related illnesses, he says.

Food diaries are a "powerful self-management technique. They help you figure out where the extra calories are coming from," he says. It works best if someone else looks over your food record. "When you put yourself in a position where you are accountable for your behavior, it changes behavior."

Many people are getting lots of extra calories by eating large portions at restaurants, Stevens says.

Dawn Jackson Blatner, a registered dietitian in Chicago, tells people that a food diary "is the best way to monitor every crumb, morsel, nibble, sip, swallow and bite you take."