Researchers have found that when very overweight people, who tend to have relatively low metabolic rates, lose weight primarily through dieting, their metabolisms generally remain unchanged. They will thus burn significantly fewer calories at the new weight than do people whose weight is normally at that level. Such newly thin persons will, ttherefore, ultimately regain weight until their body size again matches their metabolic rate.
The conclusion of the argument above depends on the assumption that the metabolisms of people who are usually not overweight are much more able to vary than the metabolisms of people who have been very overweight.