If you’ve been trying for years to lose unwanted pounds and keep them off, unrealistic goals may be the reason you’ve failed. It turns out that a long-used rule of weight loss — reduce 3,500 calories (or burn an extra 3,500) to lose one pound of body fat — is incorrect and can ultimately doom determined dieters.

That is the conclusion reached by Dr. Kevin D. Hall and his colleagues at the National Institute of Diabetes and Digestive and Kidney Diseases. Recently they created a more realistic model of how the body responds to changes in caloric intake and expenditure, basing their calculations on how people of different weights responded to caloric changes in a controlled setting like a metabolic unit.

Their work, spelled out in a new study published in The Lancet, explains how body weight can slowly rise even when people have not changed their eating and exercise habits.

Their research also helps to explain why some people can lose weight faster than others, even when all are eating the same foods and doing the same exercise, and why achieving permanent weight loss is so challenging for so many.