"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."
"...According to the researchers, it is easy to gain weight unwittingly 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 authors wrote.
Furthermore, the same increase in calories will result in more pounds gained by a heavier person than by a lean one — and a greater proportion of the weight gained by the heavier person will be body fat. This happens because lean tissue (muscles, bones and organs) uses more calories than the same weight of fat..."