Comment by prostoalex

The number of fat cells is set in childhood and stays constant throughout adulthood, according to research conveniently titled "Fat cell number is set in childhood and stays constant in adulthood" http://scienceblogs.com/notrocketscience/2008/05/04/fat-cell...

The size of those fat cells can be changed through diet and lifestyle, but since the number of fat cells cannot be lowered naturally, sticking to a healthier lifestyle is extremely hard for a [formerly] obese person, as there's just one thing the fat cells are programmed to do, and that is to grow in size.