Vitamin D, among the many things it does, helps with the absorption of calcium. However, if you do not have enough vitamin K2 (not K1, not the same) you will deposit the calcium all over the place, including soft tissues and arteries(not good). Vitamin K2 helps put the calcium where it belongs, in the bones. We can make K2 out of K1 in our gut, but only a very small amount and way under what we need. Look for Vitamin D and K2 combo or, if you want to go the natural route, eat natto or grass fed beef.