Adding a higher proportion of fat costs more, whereas adding a higher proportion of lean tissue (like muscle) costs less, because the energy content of fat is more than twice that of protein. One way to see that difference is to look at the energy burned when we lose weight—the mirror image of growth. The energy we expend to lose weight has to be equal to the energy content of the tissue that’s been lost. Since the tissue we lose during weight loss is mostly fat, the general rule is that it takes about 3,500 kcal to burn off one pound.
