What is Vitamin D?
Most scientists classify vitamin D as a vitamin. However, in the presence of sunlight, skin cells can synthesize a sufficient supply of vitamin D from a derivate of cholesterol. Because a dietary source is not required if synthesis is adequate to meet the needs, the vitamin is more correctly classified as a “conditional” vitamin, or prohormone (a precursor or an active hormone). In the absence of UV light exposure, an adequate dietary intake of vitamin D is essential to prevent the deficiency diseases rickets and osteomalacia and to provide for cellular needs.
After exposure to the sun, humans produce vitamin D3 (cholecalciferol) from a derivate of cholesterol. The liver and kidneys each add a hydroxyl group (-OH) to this to yield the active form of vitamin D.
It is recommended that people should expose their hands, face, and arms to UV light at least 2 or 3 times a week for 10 to 15 minutes. People with darker skin may need sun exposure of 30 minutes or more.
Vitamin D in Food
The best sources of vitamin D are:
- Fatty fish (e.g., sardines, mackerel, and salmon)
- Cod liver oil
- Fortified milk
- Some fortified breakfast cereals
Although eggs, butter, liver, and a few brands of margarine contain some vitamin D, large servings must be eaten to obtain an appreciable amount of vitamin. Thus, these foods are not considered a significant source.
Most fortified foods and supplements containing vitamin D are in the form of ergocalciferol, or vitamin D2, the same form found naturally in foods.
Vitamin D Needs
The Food and Nutrition Board has set an Adequate Intake, rather than a RDA, for vitamin D. A more precise RDA level could not be set because the amount of vitamin D produces by sun exposure varies considerably among individuals.
The Adequate Intake for vitamin D is 5 μg/day (200 IU/day) for people under age 51, 10 μg/day (400 IU/day) for people between 51 and 70, and 15 μg/day (600 IU/day) for older adults. Older adults as well as others who have limited sun exposure may need 20 to 25 μg/day (800 to 1000 IU) from a combination of vitamin D-fortified foods and a supplement to decrease the risk of bone loss and other chronic diseases.
Although full-term infants are born with a supply of vitamin D, the American Academy of Pediatrics recommends that breastfed infants be given a vitamin D supplement of 5 μg/day (200 IU) until they are weaned to infant foods fortified, or rich in vitamin D.
Vitamin D Functions
- Hormonelike functions, which help regulate the body’s concentration of calcium and phosphorus
- Immune function
- Cellular metabolism
- May decrease the risk of certain types of infections and autoimmune diseases, such as multiple sclerosis
- Offers protection against diabetes, hypertension, and certain cancers.
Although the exact role of vitamin D in many tissues is not fully known, studies suggest that it may be involved in cell cycle regulation. Much of this evidence has com from observational studies; thus, intervention studies of vitamin D supplementation are needed to determine its role in the diseases.
Vitamin D Deficiency Diseases
Vitamin D maintains calcium and phosphorus homeostasis and bone health. When there is inadequate calcium and phosphorus in the blood available for deposition in the bone, the skeleton fails to mineralize normally. This causes the bones to weaken and bow under pressure. When these effects occur in the growing bones of a child, the disease is called rickets. Signs of rickets include enlarged head, joints, and rib cage.
In developed countries, rickets is most commonly associated with fat malabsorption, such as is seen in children with cystic fibrosis.
Vitamin D deficiency in adults is called osteomalacia, which means “soft bones”. It is characterized by poor calcification of newly synthesized bone, resulting in fractures in the hip, spine, and other bones. Osteomalacia is most likely to occur in adults with kidney, liver, gallbladder, or intestinal disease because these diseases affect both vitamin D metabolism and calcium absorption. Other individuals at risk include people with darker skin and with limited UV exposure.
Vitamin D Toxicity
Vitamin D toxicity can occur from oxcessive vitamin D supplementation. It does not result from excess sun exposure or from natural sources in the diet. However, because of the serious consequences of vitamin D toxicity, an Upper Level of intake for vitamin D (50 μg/day [2000 IU/day]) has been set.
This UL was set because high intakes of vitamin D can cause an over absorption of calcium and hypercalcemia (increased calcium in the blood).