Vitamin D does not directly cause the increased absorption, but rather begins a series of events which affect intestinal absorption of calcium. The body produces vitamin D with exposure to ultraviolet rays from the sun. Many dairy products are also fortified with vitamin D. One product of vitamin D exhibits a hormonal effect on the intestinal wall by stimulating the production of a calcium-binding protein. This protein aids in the transport of calcium into the cells of the epithelium and its effect can last up to several weeks. Appropriate sun exposure and dietary intake of vitamin D assist in maximizing calcium absorption by the small intestine.
Many factors affect the supply and demand of calcium in the human body.
The developmental stage of the individual dictates how much calcium is needed to build and maintain strong bones and in pregnant women, support a growing fetus or child. Intestinal function also plays a role in calcium levels. Digestive disorders may reduce the absorption and availability of calcium to the system. In healthy individuals, absorption may be positively or negative affected by how the calcium is ingested. Dairy products provide calcium that is readily absorbed, but available calcium may be negatively affected if the source foods are taken with wheat bran.