Stress, leading on to High blood pressure, and many, many more. Bruce Krahn’s Lean Belly Breakthrough
Your breathing will also be affected, meaning that you won't be able to join in with as many physical activities as you would like, and your health in general will suffer.
I've heard that nicotine is more addictive than Heroin so one should also consider the fact that once you are hooked, smoking will be an extremely hard habit to give up. It won't be a simple fact of "oh I'll just smoke until I've lost some weight, then I'll give it up" it will be a lot harder than that. Even if you are able to successfully stop smoking, guess what happens then....you will somehow find the weight you lost, so just don't even go there in the first place.