Vitamin D describes a group of fat-soluble steroids. Vitamins D2 and D3 can be converted to the active steroid hormone in the human body, although vitamin D3 is more efficiently converted than D2. The active form of vitamin D is hydroxylated in two places. The most accurate measurement of vitamin D levels in the body is a blood test that detects the levels of circulating 25-hydroxylated vitamin D.
Vitamin D Sources
Vitamin D3 is synthesized in the skin from a cholesterol precursor and the synthesis requires exposure to ultraviolet light. The Office of Dietary Supplements of the National Institutes of Health, or ODS, states that skin production of vitamin D3 is adequate to maintain bodily functions in most people; however sun exposure is inadequate for maintaining vitamin D3 levels during the winter months in the northern latitudes. The active form of vitamin D can be synthesized from dietary sources of vitamin D2 and D3 and vitamin D supplements. Foods that contain the most vitamin D3 are salmon, tuna and mackerel. Beef liver, cheese and eggs contain lesser levels of vitamin D3. In the United States many milk products are fortified with vitamin D3. Mushrooms that have been exposed to ultraviolet light are a good source of vitamin D2. Because vitamin D3 is more efficiently converted to the active hormone, supplements containing vitamin D3 are more effective than those containing vitamin D2.
Vitamin D Metabolism
Vitamins D2 and D3 obtained from skin synthesis, dietary sources or supplements are converted in the liver to the pre-hormone, 25-hydroxylated vitamin D3, also known as calcidiol. 25-hydroxylated vitamin D3 is transported in the blood to the kidneys where it is converted to the active hormone, 1,25-dihydroxyvitamin D3, or calcitriol. Some cells of the immune system can also convert 25-hydroxylated vitamin D3 to the active hormone.
Vitamin D3 Functions
The primary function of the activated hormone of vitamin D3 is to maintain calcium absorption from the gut. It is also involved in bone growth and remodeling, maintaining phosphate balance and immune system and neuromuscular function. The actions of the vitamin D3-derived hormone have been demonstrated to affect the growth and death rate of certain types of cells. The ODS reports that several studies have demonstrated that vitamin D3 protects against colon, prostate, pancreatic and breast cancers.
Vitamin D Deficiency
The ODS reports that vitamin D deficiency is indicated by blood concentrations of 25-hydroxylated vitamin D less than 15 nanograms per milliliter. Vitamin D deficiency has been most commonly associated with bone disorders. In children severe vitamin D deficiency can cause rickets, which leads to progressive softening of the bone causing fractures and deformities. In adults, vitamin D deficiency can cause osteomalacia and osteoporosis. A review in “Experimental Biology and Medicine” states that evidence is accumulating to support that vitamin D deficiency plays a role in the development and progression of autoimmune diseases, such as multiple sclerosis, type 1 diabetes, rheumatoid arthritis and inflammatory bowel disease.
Excessive vitamin D intake from supplements may cause nonspecific systems like nausea, constipation and weakness. Vitamin D in excess can also cause high levels of calcium in the blood leading to symptoms of confusion and heart rhythm abnormalities. Hypervitaminosis D is rare and extremely high supplementation is required to cause toxicity. Currently, the upper recommended limit for vitamin D supplementation is between 200 and 400 international units per day; however actions are now being taken to readdress this limit and the ODS notes that many nutritionists and researchers advocate doses greater than 1700 IU per day. A study in the “The American Journal of Clinical Nutrition” purports that doses of vitamin D3 well exceeding 10,000 IUs per day for several months are required to reach toxic blood levels of 25-hydroxylated vitamin D3.