So many people in the industrialized nations are deficient in Vitamin D, it is truly an epidemic. When you consider that it is not just a vitamin, but also a pro-hormone, you can begin to understand the importance of this deficiency. Vitamin D deficiency can lead to weakened bones, yes, but it can also lead to weakened immune systems as well. The risk of cancer, and many other illnesses, increases with a dropping vitamin D level. The best way to get Vitamin D is from the Sun and from your diet. If you live too far north for the Sun, and can't get enough in your diet, then you should supplement. Having a healthy Vitamin D level is very important to your good health and longevity. source: https://www.youtube.com/watch?v=7S8ZEtcpH5I