Just like all the noisy birds you hear when you wake up in the morning, we are increasingly learning that all the cells in your body seem to be talking to each other. So muscle cells also make several hormones that have been found to talk to bone cells and fat cells. One of these muscle hormones is irisin, named after the Greek goddess, Iris, who ran fast and carried messages between the Olympian gods. The production and release of irisin into the blood is increased when mice and men exercise. It was first discovered and named in 2012. Scientist do get some liberal arts education.
Since exercise increases bone strength but does not make much difference in reducing fat, together with all the other wonderful things exercise does – improve quality of life, reduce hypertension, heart attacks, stroke, etc., etc. – we would expect that irisin would increase bone production and not be telling fat cells much of anything.
But recent investigations of the mechanisms of action of irisin on those two cell types is complicated. (1)
In the case of bone, irisin administration actually seems to promote bone loss. In the case of fat cells irisin causes white fat to turn “brown.” Brown fat is the specialized fat cell that can waste calories by generating heat.
So trying to figure out why God made irisin is hard. In the case of bone it must be part of a bigger conversation. In the case of fat maybe increased exercise would be helped by lightening up on fat-energy stores, or, maybe muscles work better at higher body temperature. But I’m just spitballing here.
Maybe some day we will be able to skip the pain and time consumption of exercise and just take pills to get all those benefits. In the meantime we have to exrecise more.
1. Farmer SR. Boning up on irisin. N Engly J Med 2019; 380(15):1480-2.