Stem cells in bone marrow can become bone or fat, depending on activity.
Choose bone over fat! The more fat one has, the less bone… perhaps making ankle and hip fractures more likely as we age. Women are especially prone to osteoporosis.
From the New York Times:
Still, one lesson is indisputable. Don’t sit still more than you need to, Dr. Rubin said, and don’t let your children loll about either. “One of the concerns raised” by these experiments, she said, “is that if you make fat cells when you’re young, then you’ve lost any opportunity to have that particular cell be bone,” and the fat cell will remain just that, for life.
In reading the work, I’m not sure if resistance training is to be preferred to aerobic exercise. I try to work out 4 times a week, splitting sessions between the two types of exercise. I know I can’t add as much muscle at 53 as I could have 20 or 30 years ago. But people my age (and much older) can still add muscle by strength training.