Why is vitamin D important for osteoporosis prevention?

It is commonly known that calcium deficiency is a risk factor for osteoporosis. Calcium supplementation is commonly stressed for osteoporosis prevention however, calcium cannot be absorbed without vitamin D, therefore vitamin D supplementation is as important as calcium. Vitamin D can be converted by our bodies from natural sunlight, however since we live here in Western PA, we do not get enough vitamin D and the daily recommended dose is 400 international units. Taking excess vitamin D is not harmful. So for your bone health, make sure you are getting enough vitamin D as well as calcium.