However, whether this theory is true or not is in question. One study found no evidence that set point weight is the reason some people gain weight after losing it.
Weinsier, R; Nagy, T; Hunter, G; et al. Do adaptive changes in metabolic rate favor weight regain in weight-reduced individuals? An examination of the set-point theory. Am J Clin Nutr, 2000 Nov;72(5):1088-1094.