A new study suggests that anyone who has been trying unsuccessfully for years to lose weight must have set unrealistic goals, become frustrated and eventually regained weight.

Doctors at the U.S. National Institute of Diabetes and Digestive and Kidney Diseases recently came up with a more realistic model of how the body responds to changes in caloric intake and expenditure. Dr. Kevin Hall led the study, which was recently published in The Lancet.

The study determined how body weight can slowly rise even when people have not changed their eating and exercise habits, while some can lose weight faster than others. It also explores how even very serious and determined dieters can fail to achieve permanent weight loss.

The study disputes the formula "reduce 3,500 calories (or burn an extra 3,500) to lose one pound of body fat," finding it can ultimately frustrate determined dieters.

The model suggests that more effective weight loss programs might be undertaken in two phases: a temporary, more aggressive change in behavior at first, supported by an important second phase of a more relaxed but permanent behavioral change that can prevent the weight rebound that usually follows a diet phase.

Hall and his team concluded it is easy to inadvertently gain weight from a very small imbalance in the number of calories consumed over calories used. Just 10 extra calories a day is all it takes to raise the body weight of the average person by 20 pounds in 30 years, the study said.

In an interview with The New York Times, Hall said the old assumption that cutting 3,500 calories will produce a one-pound weight loss indefinitely is inaccurate. A more realistic expectation, he said, is that cutting out 250 calories a day - the amount in a small bar of chocolate - would lead to a weight loss of about 25 pounds over three years, with half that loss occurring the first year.

While many people get discouraged when weight loss is slow, Hall said a gradual loss is usually more effective because it allows  new eating and exercise habits to become lasting.

Hall said typical weight-loss programs result in significant losses over six to eight months, followed by gradual regaining in the years that follow. When weight-loss plateaus at six to eight months, many dieters unconsciously start to eat a little more.

Hall noted that physical activity remains important to weight loss and especially to weight maintenance. Studies of more than 5,000 respondents in the National Weight Control Registry have shown that those who lost a significant amount of weight and kept it off for many years practiced two essential things: continuing physical activity and regular checks on body weight.

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