[aruanan]

Body weight and size are individually determined not only by eating habit choices but by genetics and metabolism.

Fat mass is determined only by how much the energy intake is in excess of the energy expenditure. Now, you may feel more hungry or hungry more often and that may have something to do with genetics, but the gain or loss in fat mass is entirely a matter of increase or decrease of energy intake relative to energy expenditure. There is no other way short of surgery.

[brytlea]

While this is certainly what we are taught (I have a degree in exercise physiology) it doesn’t appear to be the entire story in practice.