The new buzz word is autophagy. It sort of means your cells eat themselves -- a sort of cannibalistic behavior of the cells post exercise or fasting, that actually benefits the organism as a whole.
In exercise, the cells are damaged. And this release an activity called autophagy, which is set off to clear up cell debris, and this in turn helps rejuvenate the cells. Interestingly fasting appears to have a similar regenerative impact on the brain.
And it is not just cell regeneration but also helps heal or control -- cancer, neurodegeneration, cardiomyopathy, diabetes, liver disease, autoimmune diseases and infections (according to a study).
Just how much is too much, in terms of exercise? Or how do you modulate your fasting? I find these concerns are pretty well-managed by alternative therapies. As far as yoga is concerned, as a daily practitioner I can vouch for the fact that nothing is too much, and your body progressively gets used to more demand on it. It responds by having less hunger. Unlike after a gym round, after a yoga session your hunger actually gets depressed. It is an interesting phenomenon. Automatically induces less eating, thereby inviting a natural fasting that helps the mind stay fresh too.