The Lowdown on Vitamin D

Vitamin D comes in several forms; the two most commonly found in foods and supplements are vitamin D2 (ergocalciferol) and D3 (cholecalciferol). Some research suggests that vitamin D3 is more potent than D2, though Michael Holick, PhD, MD, vitamin D expert and professor at Boston University School of Medicine, says, “Vitamin D2 is as effective as vitamin D3 in raising and maintaining serum blood levels of the vitamin.”

Good vitamin D status. Your blood level of 25-hydroxyvitamin D is the primary indicator of vitamin D status. The Institute of Medicine (IOM) says that maintaining a serum level of 20 nanograms/milliliter (ng/ML) is enough for bone health. However, Holick and others recommend higher levels (40–60 ng/ML) for overall health. The level that is considered “normal” may vary among clinical laboratories.

Getting vitamin D in your diet. Milk is fortified with about 100 IU per cup and is the major source of this vitamin in the American diet. The intake of vitamin D dropped 15 percent between 1980 and 2009, mainly due to a drop in milk consumption (Journal of the American Dietetic Association, 2011.) A few foods, including salmon and tuna, are naturally rich in vitamin D, but several products, including some brands of or-ange juice and breakfast cereals, are fortified with the vitamin. Mushrooms are possibly the least known source of vitamin D. When exposed to ultraviolet light, mushrooms can provide as much as 400 IU per three-ounce serving. Check the label to see if your mushrooms are rich in vitamin D. But if you’re not a milk drinker or a mushroom fan, get little sun, or are over 50, you’ll need supplements to meet recommended in-takes.

The sunshine vitamin. Vitamin D is also produced by the body when your skin is exposed to the ultraviolet rays of the sun. These rays trigger the production of a vitamin D precursor found in skin called previtamin D3, which is then activated in the liver and kidneys. Only activated vitamin D can perform in the body. The standard recommendation for getting your D quota from the sun is 5 – 20 minutes of sun exposure between 10 am and 3 pm twice a week on the face, arms, and legs without sunscreen. Vitamin D is fat-soluble, which means your body has the ability to store it for days or months, so it’s okay if you don’t get sun exposure every day. But this varies depending upon the following factors:

• Skin color. Darker skin produces less vitamin D.
• Location. Northern latitudes get fewer ultraviolet rays.
• Age. After age 50 the skin’s ability to produce vitamin D is reduced.
• Medications. Some medications, such as corticosteroids and cholestyramine lower vitamin D levels.
• Chronic kidney disease. This condition lowers the ability to activate vitamin D.
• Cloud cover. Complete cloud cover reduces UV rays by 50 percent.

Future of vitamin D and health. Research into the roles of vitamin D in the body continues. “The effects of vitamin D on the immune system holds a lot of promise,” says Holick. “The importance of maintaining a healthy vitamin D status through life may play an important role in reducing the risk of deadly cancers, heart disease and type 2 diabetes later in life.”

—Densie Webb, PhD, RD