What if, rather than preaching against obesity, we taught that supersized Cokes were a sin, or Quarter Pounders with Cheese? Or pizza or chocolate?
Food and weight aren't easy to preach against. There is no harm in an occasional Coke or Quarter Pounder. They are not addictive (not really... but almost), and by watching intake of other foods we can have one without hurting our overall health.
I was able to sit through a group of classes several years ago where my pastor taught about basic living. He included diet (as in there are five basic food groups, and potato chips and catsup are NOT veggies) as part of the course. Several people later admitted this was the most helpful part of the class for them or their families.
Basic lessons on self control and moderation might also be in order... but those could apply to alcohol and movies and internet and (on and on)... too. That's scary to some people, and that may be why this method is avoided. However, this method would keep people from crash diets (also harmful) as well as giving opportunity to also discuss and speak out against anorexic and bulemic tendencies, which can also be a problem in churches today.