Vitamin D supplements come in two forms: vitamin D2 and vitamin D3, which are generally listed on the ingredient list of the supplement label. The natural form the body uses is vitamin D3, which is produced in the skin from a form of cholesterol combined with sun exposure. Historically, the type of vitamin D used for supplements has been vitamin D2, which is prepared from yeast. However, many supplement manufacturers have started reformulating their products with vitamin D3 as well. According to the National Institutes of Health, most experts recommend vitamin D3 to meet vitamin D needs. When shopping for vitamin D supplements, look for vitamin D3 on the label.
Improving Vitamin D Status
Low vitamin D status can be corrected with vitamin D supplements. Vitamin D3 appears to be more effective in raising vitamin D in the body than vitamin D2. In a 1998 study published in the "American Journal of Clinical Nutrition," the increase in vitamin D levels was 1.7 times greater in the subjects who took 4,000 international units of vitamin D3 than those taking the same dose of vitamin D2. Another study published in the May 2006 issue of "European Journal of Clinical Nutrition" found that postmenopausal women with low bone density need at least six times more vitamin D2 than vitamin D3 to improve the body vitamin D status.
Vitamin D needs to be converted to the functional form in the liver. This conversion process is five times faster with vitamin D3 than vitamin D2, according to a study published in the "Scandinavian Journal of Clinical and Laboratory Investigation" in 1986. In other words, with the same dose of the vitamin, you can expect to get five times as much functional vitamin with vitamin D3.
Excess vitamin D can lead to high blood calcium, which increases the risk of kidney stones and hardening of blood vessels, according to the National Institutes of Health. To prevent the toxicity, the body uses a protein that partners with vitamin D and prevents its activation. Vitamin D2 does not bind to this protein as well as vitamin D3 and thus has greater potential for toxicity, according to a study that appeared on the February 2001 issue of the "American Journal of Clinical Nutrition."
Vitamin D2 supplement is less reliable that vitamin D3, especially for the elderly. A study in the "Journal of the College of American Nutrition" in 1999 reported an age difference in the effect of vitamin D2 supplement, producing a two-fold higher vitamin D level in the younger men than in the older. However, when the researchers used vitamin D3 supplements, they could reliably increase vitamin D in both young and old adults.