As I've read, there are like five main factors in weight gain after quitting smoking.
- Nicotine, being a stimulant, makes the body burn more calories.
- Increased taste sensation, leads to food bein' flat out yummy.
- Smoking makes digestion less efficient, meaning that less of the food is absorbed in the gut.
- Smoking has a satiating effect, making one less hungry.
- Ex smokers tend to eat more to make up for the oral gratification they miss.
It'd be interesting to see a study comparing weight gain for different quit smoking techniques.