I think it's because of the carbon monoxide. Smoking keeps carbon monoxide really high in a smoker's blood, which basically depresses the entire body, including gut flora like they said, but also every other aspect of metabolism. So it's true, keeping the diet exactly the same people will gain weight. Whether it's muscle or fat depends on different things. In my case it was mostly muscle, but I changed my diet a lot & started bodybuilding when I quit. The figure of 5 lb is way low though, probably because it's not actually possible to keep the diet the same. I know several people who quit and they all gained at least 20 lb.