"If you want to keep lost pounds off, daily weighing is critical," said Dr. Rena R. Wing in a statement accompanying the study appearing in The New England Journal of Medicine this week.

"But stepping on a scale isn't enough. You have to use that information to change your behavior, whether than means eating less or walking more. Paying attention to weight -- and taking quick action if it creeps up -- seems to be the secret to success," noted Wing, who is director of the Weight Control and Diabetes Research Center at The Miriam Hospital and professor of psychiatry and human behavior at Brown Medical School in Providence.

The finding comes from a study in which Wing and colleagues split 314 successful dieters who'd lost at least 10 percent of their body weight -- averaging nearly 20 percent of their body weight or 42 pounds -- within the last two years, into a control group and two intervention groups.

Women in the control group received newsletters in the mail four times per year on the importance of eating right and exercising.

Women in the intervention groups were taught -- either in face-to-face group meetings or via an online program -- techniques known to prevent weight regain such as advice to eat breakfast, get an hour of physical activity each day and weigh themselves daily.

The women reported their weight weekly and were given a goal of maintaining their weight to within five pounds. Women in the intervention groups were also introduced to a color-based weight-monitoring system. Women who remained within three pounds of their starting weight after the weekly check-in fell into the "green zone," and received encouraging phone messages and green rewards, such as mint gum.

Gaining between three and four pounds landed women in the "yellow zone" and prompted advice to tweak their eating and exercise habits, while gaining five pounds or more landed one in the "red zone," prompting advice and encouragement to restart active weight-loss efforts.

The investigators report that significantly fewer women in the intervention groups regained five or more pounds during the 18-month long study; 72 percent of women in the control group regained five or more pounds, compared with 46 percent in the face-to-face intervention group and 55 percent in the Internet group.

"The Internet intervention worked, but the face-to-face format produced the best outcomes," Wing said.

Daily weighing was key to keeping the weight off, the authors say, noting that women in the intervention groups who stepped on the scale each day were 82 percent less likely to regain lost weight compared to those who did not weigh themselves daily.

However, in the control group, daily weighing had little impact on the amount of weight regained. This suggests, Wing said, that women in the intervention groups used the information from the scale to make constructive changes in their eating and exercise habits.