This New York Times article, The Fat Trap, explores how people lose weight, but almost without exception, gain it right back.
In one study, 50 obese men and women consumed just 500 to 550 calories a day for eight weeks and lost an average of 30 pounds. A year after the study, subjects had regained an average of 11 pounds and reported feeling far more hungry and preoccupied with food than before they lost the weight.
Yeah, I know the diet was too restrictive, but regardless, it is interesting to note what is going on hormonally. A quote from the article: