Vitamin D actions, side effects and interactions
Vitamin D (also referred to as “calciferol”) is a vitamin that is naturally present in some foods. It is added to some processed foods and is also available as a dietary supplement. Vitamin D is produced in the body when ultraviolet (UV) rays from sunlight strike the skin and trigger vitamin D synthesis.
Vitamin D is necessary for many important body functions, the most important being the building and maintaining healthy bones, nerve function and immune system.
Research on vitamin D use for specific conditions shows:
- Cancer. Vitamin D, especially when taken with calcium, might help prevent certain cancers.
- Cognitive health. Vitamin D might play a role in cognitive health.
- Inherited disorders. Vitamin D supplements can be used to help treat inherited disorders resulting from an inability to absorb or process vitamin D, such as familial hypophosphatemia.
- Multiple sclerosis. Long-term vitamin D supplementation reduces the risk of multiple sclerosis.
- Osteomalacia. Vitamin D supplements are used to treat adults with severe vitamin D deficiency, resulting in loss of bone mineral content, bone pain, muscle weakness and soft bones (osteomalacia).
- Osteoporosis. Enough vitamin D and calcium in the diet can slow bone mineral loss, help prevent osteoporosis and reduce bone fractures.
- Psoriasis. Applying vitamin D or a topical preparation that contains a vitamin D compound called calcipotriene to the skin can treat plaque-type psoriasis in some people.
- Rickets. This rare condition develops in children with vitamin D deficiency. Supplementing with vitamin D can prevent and treat the problem.
Safety and side effects
Taken in appropriate doses, vitamin D is generally considered safe.
However, taking too much vitamin D can be harmful. Children age 9 years and older, adults, and pregnant and breast-feeding women who take more than 4,000 IU a day of vitamin D might experience:
- Poor appetite
- Weight loss
- Heart rhythm problems
- Kidney damage
Possible interactions include:
- Aluminum. Taking vitamin D and aluminum-containing phosphate binders long term might cause harmful levels of aluminum in people with kidney failure.
- Anticonvulsants. The anticonvulsants phenobarbital and phenytoin (Dilantin, Phenytek) increase the breakdown of vitamin D and reduce calcium absorption.
- Atorvastatin (Lipitor). Taking vitamin D might affect the way your body processes this cholesterol drug.
- Calcipotriene (Dovonex). Don’t take vitamin D with this psoriasis drug. The combination might increase the risk of too much calcium in the blood (hypercalcemia).
- Cholestyramine (Prevalite). Taking this weight-loss drug can reduce your absorption of vitamin D.
- Cytochrome P450 3A4 (CYP3A4) substrates. Use vitamin D cautiously if you’re taking drugs processed by these enzymes.
- Digoxin (Lanoxin). Avoid taking high doses of vitamin D with this heart medication. High doses of vitamin D can cause hypercalcemia, which increases the risk of fatal heart problems with digoxin.
- Diltiazem (Cardizem, Tiazac). Avoid taking high doses of vitamin D with this blood pressure drug. High doses of vitamin D can cause hypercalcemia, which might reduce the drug’s effectiveness.
- Orlistat (Xenical, Alli). Taking this weight-loss drug can reduce your absorption of vitamin D.
- Thiazide diuretics. These blood pressure drugs might decrease urinary calcium excretion. This could lead to hypercalcemia if you are taking vitamin D.
- Steroids. Taking steroid medications such as prednisone can reduce calcium absorption and impair your body’s processing of vitamin D.
- Stimulant laxatives. Long-term use of high doses of stimulant laxatives can reduce vitamin D and calcium absorption.
- Verapamil (Verelan, Calan). Avoid taking high doses of vitamin D with this blood pressure drug. High doses of vitamin D can cause hypercalcemia, which might reduce the drug’s effectiveness.
Vitamin D obtained from sun exposure, foods, and supplements is biologically inert and must undergo two hydroxylations in the body for activation. The first hydroxylation, which occurs in the liver, converts vitamin D to 25-hydroxyvitamin D [25(OH)D], also known as “calcidiol.” The second hydroxylation occurs primarily in the kidney and forms the physiologically active 1,25-dihydroxyvitamin D [1,25(OH)2D], also known as “calcitriol” .