A recent obesity study out of Australia has concluded that when people lose weight, their metabolism automatically slows down and they experience hormonal changes that increase appetite. The researchers theorized that these changes, not portion size, the types of food eaten, or lack of exercise, etc., explain why most dieters so quickly gain back what they lost.
Or to put it in simpler terms; being overweight is not your fault. It's biology.
The study involved healthy people who were either overweight or obese and were put on a highly restricted diet that caused the subjects to lose at least 10 percent of their body weight. (Note well: exercise was not part of this study.) Unfortunately, and in an apparent surprise to the researchers, the diet caused the participants' metabolism to slow down and some key weight regulating hormones to change unfavorably. The participants were then kept on a diet designed to maintain their weight loss. A year later, again in what seemed an apparent surprise to the researchers, the participants' metabolism and hormone levels had not returned to their previous "normal" levels, and they had gained back a substantial portion of what they had previously lost.