Vitamin D is produced in the epidermis and dermis of the skin by a small band of radiation from the sun (UV-B radiation).
The primary function of vitamin D is to maintain serum calcium and phosphorus concentrations to support cellular processes, neuromuscular function, and bone ossification. Vitamin D enhances the efficiency of the small intestine in absorbing dietary calcium and phosphorus, and mobilizes calcium and phosphorus stores from the bone. Vitamin D has also been shown to have an effect on the immune system. Leukemic cells have receptors for 1,25(OH)2D3, responding to it by differentiating into macrophages.
Vitamin D is incorporated into chylomicron. Approximately 80% is absorbed into the lymphatic system. Vitamin D is bound to vitamin D-binding protein in the blood and carried to the liver where it undergoes its first hydroxylation into 25-hydroxyvitamin D. This is then hydroxylated in the kidney into 1,25(OH)2D. When there is a calcium deficiency, parathyroid hormone is produced, which increases the tubular reabsorption of calcium and renal production of 1,25(OH)2D. The 1,25(OH)2D travels to the small intestine and increases the efficiency of calcium absorption.
The classic vitamin D deficiency is called rickets, a disease that used to be seen in children. This is a bone-deforming disease characterized by enlargement of the epiphyses of long bones and the rib cage, bowing of the legs, bending of the spine, and weak and toneless muscles. Vitamin D deficiency can result in a secondary hyperparathyroidism which accelerates osteoporosis and a mineralization defect in bones resulting in an adult rickets or osteomalacia. This results in weakened bones and an increased risk of fractures. Chronic intestinal malabsorption syndromes such as liver disease, cystic fibrosis, Crohn's disease, and sprue may also result from a deficiency in vitamin D.
Casual exposure to the sun provides most humans with their vitamin D requirement. The elderly have a diminished capacity to produce vitamin D in the skin. The daily recommendation of adequate intakes (AI) are: 5 mcg for infants and children, 5 mcg for adults up to 50 years of age, 10 mcg for adults 51 to 70 years old, 15 mcg for adults over 70 years old, and 5 mcg for pregnant and lactating women. The unit of measurement often used on labels is international units (IU). One mcg of vitamin D equals 40 IU.
Few foods contain vitamin D naturally. Some are fish liver oil and fatty fish. Milk is fortified with vitamin D and is the primary source of dietary vitamin D. Some cereals, breads, and infant formulas are fortified with vitamin D, too.