Vitamin D and bone

Vitamin D's job in bone runs through calcium. It is needed to keep serum calcium, the calcium in the liquid part of blood left once the cells and the clotting factors are removed, in the normal physiologic range that muscle and bone health require.

The decisive step happens in the kidney, where vitamin D is converted to its active form. That active form stimulates the absorption of calcium and phosphorus in the intestine. It is a key that opens the gut's door to calcium.

The difference shows up when you measure it: without vitamin D the intestine lets through only a small share of the calcium that food delivers, and with the vitamin it lets through considerably more.

When it is lacking, bone suffers, and not the same way at every age. In children the deficiency causes rickets, and in adults osteomalacia: abnormal mineralization of bone, that is, defective deposition of the mineral that hardens it.