Vitamin D

Vitamin D, a prohormone, has 2 main forms: D₂ (ergocalciferol) and D₃ (cholecalciferol). Vitamin D₃ can be obtained from the diet; common sources include milk products fortified with vitamin D, fatty fish oils (eg, in fish oil preparations and salmon), eggs, butter, liver, and fortified cereals. Supplements may contain vitamin D₂ or D₃. Vitamin D₃ is also synthesized in the skin when the precursor 7-dehydrocholesterol absorbs sunlight. Usually, 10 to 15 min of sun exposure without sunscreen at least 2 times/wk to the face, arms, hands, or back provides enough vitamin D; 2000-lux light bulbs used in nursing homes can also increase vitamin D levels. Vitamin D₃ is modified by the liver, forming 25(OH)D₃ (25-hydroxycholecalciferol), which is then converted by the kidneys to 1,25(OH)₂D₃ (1,25-dihydroxycholecalciferol, or calcitriol). Vitamin D₂ is also converted in the kidneys to calcitriol. These metabolic steps are commonly referred to as activation because calcitriol is considered the active form.

With aging, vitamin D levels decrease, but the effects are minimal except in elderly people living in northern latitudes (because biosynthesis of vitamin D via sunlight exposure is reduced), in those who are institutionalized or homebound and thus have little sunlight exposure, and in those who do not ingest enough milk products. Vitamin D deficiency tends to cause osteomalacia, which may result in accelerated osteoporosis in the elderly; severe vitamin D deficiency can result in a painful myopathy.

Many elderly people benefit from taking vitamin D 800 IU daily because vitamin D supplementation may help prevent osteoporosis or slow the progression of osteoporosis, reduce risk of hip fractures, and increase survival. Vitamin D has various effects on immune function; whether vitamin D supplementation enhances immune function in healthy elderly people is unknown.

This topic was last updated March 2006.