Researchers at McMaster University in Canada had mice work out on a for about an hour three times a week. A control group of mice Matlock reruns. Well, they may as well have. They did not work out. And the mice that exercised had way less fat in their bone marrow cavities , and a big increase in blood cells in the marrow and in the circulation. I don’t like exercising. But I really don’t like fat in my marrow.