A recent investigation into how mechanical loading can turn MSC's toward bone even if they are given chemical signals to become fat, has broad implications for exercise programs. While we fret about calories or eating too much fat, their study suggests that MSC's can become fat cells simply by lack of physical activity. Clinically, we likely see this in many AVN patients who may have part of their disease pathogenesis in their low activity lifestyle. I've always thought that in many of these patients their MSC's would prefer to become fat over bone (hence the lesion-where weak fatty marrow tissue replaces strong bone). This would also have big implications for a host of disease states that are likely activity related and involve poor bone formation such as osteoporosis. We've all known for sometime that being more active is good for our health and our bones, but this new research helps us understand that at the earliest stages of adult stem cell development, activity gives our cells clues to either strengthen our bodies or make them weaker. For example, most animal studies of orthopedic healing would suggest that even in bone or cartilage injuries, more activity promotes better repair. While this can be a two edged sword (too much activity on an injured part can cause it to fail while not enough will reduce the strength of the repair), many cell based treatments now recognize that promoting activity as quickly after the procedure is key to a good recovery. In other words, keeping an animal off a cartilage lesion "to let it heal", in general promotes new cartilage that is not as strong or chemically complete as when an animal is allowed to do what it wants (meaning some limping while it hurts and then full activity when it feels able).
The take home message? We Americans have gotten used to low activity levels that not only promote obesity by reducing our caloric burn, but also increase the tendency of our stem cells to turn into fat when they should be bone (and likely muscle, cartilage, tendon, ligament, etc... )