Info I just learned from Thin for Life book. Everyone has a RMR (Resting Metabolic Rate) of energy that our body needs for bare minimum things such as breathing, heart pumping and to regulate body temperature. Even if we aren't active this takes about 20 to 30 percent of our calories.
There was a research done with 5 women whose individual RMR was 1263, 152, 1778, 1979 and 2152 calories a day. They said if these women ate just 1200 calories a day for 3 months, they would lose 11, 20, 28, 35 and 40 pounds, respectively. This RMR explains why some people lose more quickly than others. One doctor says that if you are a slow loser that you need to learn to have the attitude when losing a small amount that "at least I did lose something and I didn't gain". The book says that even if you have a low RMR which you have no influence over, you still can control over the amount of calories that you expend in exercise. They also bring out the fact that as you cut back calories that your RMR slows down and when you reach your goal that you won't be able to eat as many calories you did before dieting. Example. If it took 2400 calories to maintain when you weighed 200 lbs, then you will never be able to go back to eating 2400 calories when you reach 150 lbs. Also, unfortunately, RMR drops as most of us age. The good news is that the exercise we do may offset at least some of this change.
This explains a lot for why we have difficulty losing and we lose so slowly when we do.

