Vitamin D is converted in the body to a hormone which is responsible for improving the absorption of phosphorous and calcium from your intestines, and this is important because those minerals nourish your bones.
Deficiencies of vitamin D can lead to softer, thin, brittle bones, which may result in catastrophic hip or knee problems.
Without adequate vitamin D, your body absorbs only 10 - 15% of dietary calcium from your meals. With sufficient D, your intestinal calcium absorption increases to 30 - 40%
Imagine how that feels to your bones! Really, really good!