A chapter of The New Evolution Diet shows why and how muscle is medicine. I used this phrase in the book as a short hand way to describe the important hormones and metabolic changes that using muscle and having muscle bring. I cover many of the hormones and cytokines that are produced by muscle and muscle activity in that chapter and the ability of muscle to produce fuel for the heart and brain.
A "new" (evolutionarily old, but recently discovered) has now been discovered. It was there all the time doing its duties, but the wide-ranging effects of it are impressive. It is called irisin and the discovery is described in this POST on the American Council on Exercise.