Perhaps making people keep records of when they go to the gym "for fraud and insurance purposes" would be a good solution. Most gyms require you to swipe a pass, so there would be a record of you being at the gym. The only gym I've been to that doesn't have a system like this was a free city run gym in a low-income area. I know it would be hard to do on a national level, but on the city or even state level, having free gyms in poor areas might also be a solution.