Vitamin D itself has limited activity in the utilization of calcium, and it is mainly its metabolic product, osteotriol, that plays the most central role. The metabolic process of vitamin D in the body is complicated, firstly, it needs the help of ultraviolet light (that is, more sunlight), and secondly, it has to be transformed in the body by the liver and kidneys before it becomes biologically active. Therefore, people with bad liver and kidneys will have a lower conversion rate of vitamin D and an increased risk of osteoporosis.