One the one hand, this would be good for people who overeat (more calories than they usually expend). On the other hand, in absolute terms, it is just turning agriculture into a source of power very inefficiently (a methane digester would be much better).
Efficiency is not the goal, it's more symbiosis. Humans benefit from "wasting" energy through exercise. Instead of lifting weight and riding bikes, why not try to leverage it.
