Vitamin D
 

A person might consider Vitamin D to be the partner to Calcium in biological function. It helps build strong bones and teeth, as well as improving the strength and durability of both red and white blood cells. There are many foods, most notably milk, that are fortified with Vitamin D. Usually, acceptable levels of Vitamin D can be maintained naturally in the body. This is because our bodies can produce plenty of it with some exposure to sunlight. Our modern society however has made daily exposure to the sun somewhat inconvenient or even impossible for some people, making it necessary to consume the vitamin in supplemental form.