Smokers, especially heavy smokers, have shown to have a genetic variant that is associate with increased body mass (BMI). The gene variant CHRNA5-A3-B4 is associated with smokers who have a low BMI, but the BMI in non smokers that have this gene to be higher. This gene that is linked to those who have never smoked is still not understood. There are assumptions that this gene variant reacting to nicotinic acetylcholine could be the reason for the low BMI.
http://abcnews.go.com/blogs/headlines/2013/09/florida-county-bans-smokers-from-new-jobs/http://www.sciencedaily.com/releases/2014/12/141204143124.htm
