Vitamin D, also called calciferol, is crucial for strong teeth and bones. It has two main forms: D2 (ergocalciferol) and D3 (cholecalciferol). Unlike other vitamins, the body can produce vitamin D when exposed to sunlight and it acts as a hormone. This fat-soluble vitamin regulates the absorption and utilization of calcium and phosphorus in the body. In addition to maintaining bone density, vitamin D plays a role in various neurological, cellular functions, growth, development, and supports the immune system by aiding in the production of infection-fighting white blood cells. There are different chemical forms of vitamin D, each with varying levels of biological activity.
Sources of Vitamin D
Natural Sources
Exposure to sunlight is the primary way to obtain vitamin D. Around ten minutes of sun exposure per day is enough during clear summer weather, even if only the face is exposed. In winter, it may require up to two hours. Although many people may not get enough winter sunlight, their bodies can still utilize the stored vitamin D from summer exposure. Applying sunscreen after the minimum exposure time has passed is important to reduce the risk of skin cancer. Sunlight converts the chemical 7-dehydrocholesterol in the skin into vitamin D3. Further processing in the liver and kidneys makes D3 more biologically active. Since it is fat-soluble, excess vitamin D can be stored in the liver and fatty tissues for future use. Natural food sources of vitamin D include fish liver oils, butter, eggs, fortified milk, and cereals in the form of vitamin D2. Dairy products, particularly milk, are the primary dietary sources for most people. Other dairy products made from unfortified milk and plant-based foods are poor sources of vitamin D.
Synthetic Sources
Most oral vitamin D supplements come in the form of ergocalciferol. It can also be found in topical (calcitriol or calcipotriene), intravenous (calcitriol), or intramuscular (ergocalciferol) formulations. Non-oral forms of vitamin D require a prescription. Like other supplements, vitamin D should be stored in a cool, dry place, away from direct light, and kept out of children's reach.
Risk Factors for Deficiency
The primary cause of vitamin D deficiency is inadequate sunlight exposure. This can occur in individuals who spend little time outdoors, live in areas with pollution blocking ultraviolet (UV) light, or have weather conditions that limit outdoor activities. Glass and sunscreen also reduce the rays necessary for vitamin D synthesis. People with dark skin may absorb less UV light, affecting vitamin conversion. In northern climates, the angle of the sun during winter may not allow sufficient UV penetration for vitamin D production. Sufficient sun exposure in summer and a good dietary source can supply enough vitamin D for the winter. Vegans, those who don't consume dairy products and have limited sun exposure, and the elderly, who have reduced ability to synthesize vitamin D, are at higher risk of deficiency.
Vitamin D Metabolism
The absorption of vitamin D is enhanced by calcium, choline, fats, phosphorus, and vitamins A and C. Taking supplements with a meal optimizes absorption. Several medications can interfere with vitamin D levels, absorption, and metabolism. Rifampin, H2 blockers, barbiturates, heparin, isoniazid, colestipol, cholestyramine, carbamazepine, phenytoin, fosphenytoin, and phenobarbital reduce serum vitamin D levels and increase its metabolism. Individuals taking medication for epilepsy or a seizure disorder should consult a healthcare provider before taking vitamin D supplements. Overuse of mineral oil, Olestra, and stimulant laxatives can also deplete vitamin D. Long-term use of corticosteroids can lead to osteoporosis and hypocalcemia, which may require calcium and vitamin D supplementation. Combining thiazide diuretics with vitamin D can cause hypercalcemia in individuals with hypoparathyroidism. Concurrent use of digoxin or other cardiac glycosides with vitamin D supplements may result in hypercalcemia and irregular heart rhythms.