[throwaway6041]

Alternative A can have the opposite effect. Unhappy people tend to eat more.

Perhaps it is a "motivator" for people to stop gaining weight by controlling their diet before they are too obese, but once you are obese I think it has mostly negative effects.

Losing significant weight is incredibly hard, and gets harder the more you want to lose. If you have lost 20kg (with great benefits to health) and are still made fun of because you are still above average weight it will have a negative effect and the weight will quickly be gained back