This is a myth I can personaly say truly is a myth. I am not saying that no-one who has stopped smoking hasn't gained weight, I am saying that they aren't essentially linked. One does not necessarily cause the other. Gaining weight is usually a consequence of eating more not stopping smoking, although suddenly dropping the stimulant nicotine can reduce your heart rate to a more normal level and cause less of your energy to be used up. This is where you have to start using more energy with your new fitter body. I went running today for the first time since I stopped smoking 20 weeks ago and for the first time I can remember I did not feel like throwing up afterwards.
Most people who gain weight when they stop smoking do so because they are substituting food for cigarettes. Instead of smoking they are eating....bad move. Substitution ultimately will lead to failure. The dependancy has not been removed, it has been transferred and can easily transfer back to smoking, especially when a weight gain is noted. The only way, in my opinion, to successfully stop smoking is to remove the dependancy. This is more of psychological journey than a physical one, one where your attitude to smoking has to fundamentally change. A change of thinking will remove the need to substitute and weight will not be an issue.