American tax dollars are not spent on general medical care. What you are thinking of is an insurance rebate which could cause problems for poorer Americans who need the money and may neglect their health until it becomes life threatening if you used doctor visits as the metric. If you mean under a single payer system then yes it would work but only if the obese population was charged a higher premium. It should also be noted that many insurance companies will give you discounts or even pay for gym memberships if you use them regularly.