Vitamin D is technically not a vitamin, but rather a hormone. It is a fat-soluble vitamin that is naturally contained in very few foods, added to others, and available as a dietary supplement. It is also created endogenously when UV photons from sunshine impact the skin and initiate vitamin D production.
Vitamin D is a group of fat-soluble secosteroids that increases intestinal absorption of calcium, magnesium, and phosphate, among other things. It improves calcium absorption in the intestines and maintains adequate serum calcium and phosphate concentrations to allow proper bone mineralization and prevent hypocalcemic tetany. In humans, the most important compounds in this group are vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). It is also needed for bone growth and bone remodeling by osteoblasts and osteoclasts.
Humans require two types of vitamin D: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Plants produce vitamin D2, and people produce vitamin D3 when their skin is exposed to ultraviolet B (UVB) radiation from sunshine. Both forms are inactive and must be hydroxylated in the liver and kidneys to enhance calcium absorption in the stomach and maintain proper calcium and phosphate levels in the body.
The production of cholecalciferol in the lower layers of the epidermis of the skin via a photochemical reaction of UVB light from the sun (particularly UVB radiation) or UVB lamps is the primary natural source of vitamin D. Cholecalciferol and ergocalciferol can be obtained by food and supplementation. Only a few foods, such as the flesh of fatty fish, naturally contain significant amounts of vitamin D.
A lack of vitamin D can cause bone density loss, which can contribute to osteoporosis and fractures. Severe vitamin D deficiency can also cause other illnesses, such as rickets in children and osteomalacia in adults. In addition to its traditional role in bone health, vitamin D is thought to play a role in immunological function, cell development regulation, neuromuscular and immune function, and inflammation reduction.
Cow’s milk and plant-derived milk replacements, as well as many breakfast cereals, are fortified with vitamin D in the United States and other countries. Mushrooms exposed to UV radiation provide beneficial quantities of vitamin D2. Because sun exposure in the community varies and suggestions about the amount of sun exposure that is appropriate in light of the skin cancer risk, dietary guidelines normally presume that all of a person’s vitamin D is taken orally.
The recommended dietary allowances (RDAs) for vitamin D are as follows (according to the United States Institute of Medicine):
- Infants 0-12 months: 400 IU (10 mcg) per day
- Children 1-18 years: 600 IU (15 mcg) per day
- Adults 19-70 years: 600 IU (15 mcg) per day
- Adults over 70 years: 800 IU (20 mcg) per day