Notes:
As I mentioned, many animals are capable of obtaining vitamin D through cutaneous production as well as through their diets. There are many reasons to believe that polar bears, however, do not produce much if any vitamin D through cutaneous pathways. Certainly in their natural habitat at higher latitudes of the Arctic there is far less ultraviolet penetration which, as you will recall, is necessary for the cutaneous conversion of 7-dehydrocholesterol. Futhermore, a study by Dr. Kenny out of Denver Zoo (Zoo Biology 1998) determined that polar bears have only minimal quantities of 7-dehydrocholesterol in their skin. Additionally, polar bears have black skin and a very thick coat. Melanin, or pigment, is an effective absorber of ultraviolet radiation and would therefore compete with 7-dehydrocholesterol for any photons that were able to penetrate the polar bear’s thick coat. As you can imagine, even if you went to the Bahamas, if you wore a turtleneck, sweatpants and a face mask, you would not get much of a tan or make much vitamin D.
Thus, it seems safe to assume that polar bears are largely dependent upon their diet to meet their vitamin D requirements.
The combination of decreased vitamin D intake or availability and lack of exercise (captivity) may lead to decreased bone density and strength, which would predispose to fracture