As the winter closes in, the issue of vitamin D takes on extra importance, especially for people in more northern climes. Vitamin D acts as a prohormone in the body to regulate many normal body functions and tissues. We think of it mostly for its role in maintaining good bone and cardiovascular health.
Vitamin D is a fat-soluble vitamin that can be made by your own body through exposure of your skin to sunlight. The fat underneath the skin forms vitamin D3 (cholecalciferol), which is further processed to become the active form (calcitriol) by the liver and kidney. If there is not enough exposure to UVB light, or if there are impairments in liver, kidney, or hormonal functions, vitamin D levels and function can become imbalanced. You can have your vitamin D levels tested. A serum level of 25-hydroxy vitamin D'25(OH)D'reflects both dietary and body synthesis of the vitamin.
Other conditions that interfere with effective vitamin D functions include lack of physical activity, smoking, heavy drinking, and some medications. Even inflammatory conditions, such as chronic HIV infection, may change how well vitamin D works in the body.
Food sources of vitamin D tend to be somewhat limited, so some foods (like milk) are fortified with vitamin D. There have been several reports of benefit for moderate vitamin D supplementation in its active form (calcitriol): 1,25 dihydroxy vitamin D3'also called 1,25-dihydroxycholecalciferol or 1,25(OH)2D3. These benefits include improved bone health and better survival with cardiovascular disease. Supplemental levels are best individualized according to each person's risk factors, but care should be taken to make sure that nutritional balance is maintained and that a person doesn't consume too much. The adequate intake of vitamin D is set at around 200 IU (five micrograms) per day. However, general recommendations for vitamin D supplementation for people at risk for bone loss might hover around 1,000 IU per day.
There is a lot of debate about vitamin D, and the Institute of Medicine is now revisiting the recommendations for vitamin D; a new report of their review is expected next year.
Fields-Gardner is the director of services for the HIV nutrition company Cutting Edge and is a member of the International AIDS Society and the American Dietetic Association's Dietetic Practice Group on HIV and AIDS. She is the author of Living Well With HIV and AIDS: A Guide to Nutrition and a coauthor of HIV Medications: Food Interactions and A Clinician's Guide to Nutrition in HIV and AIDS.