What does D do in the body?

Vitamin D helps maintain blood levels of calcium, so it increases bone strength. It also works with calcium to prevent falls in older people. But vitamin D is more than just calcium’s sidekick.

Intervention studies have shown that supplementing with vitamin D may provide modest benefits in helping you kick the winter blues and tamp down high blood pressure—and, if you have diabetes—balancing blood sugar. Population studies have uncovered a link between a lack of this vitamin and increased risk of cancer, heart disease, Parkinson’s, osteoarthritis and multiple sclerosis.

D2 or D3—what’s the difference?

There are two types of vitamin D: the kind your skin makes (vitamin D3) and the kind plants make (vitamin D2). Both can also be manufactured commercially: vitamin D3 is made from wool, whereas vitamin D2 is created synthetically. If you have a choice, opt for D3; it’s been shown to be absorbed more than 50% better than D2. For vegetarians and vegans who want to avoid animal products, vitamin D2 is a good option.

How much D do I need?

The Institute of Medicine of the National Academies recently tripled the recommended daily vitamin D intake from 200 IU to 600 IU for all people between 1 and 70. While that may seem like a big increase, some experts feel the new recommendations still fall short. However, researchers from a few universities recommend a daily intake of 800 to 1,000 IU. Why the discrepancy? The current studies on vitamin D vary in design and quality, making it difficult compare their results. Additionally, opinions vary on what an optimal blood level of vitamin D should be. And since vitamin D can be toxic in excess amounts, the IOM chose to stick with conservative recommendations.

What about D-ficiency?

You may think you get enough vitamin D through sun exposure, but the fact is, you may not—particularly if you live in a northern climate, are overweight, or have dark skin. According to a study published in 2009, three out of four Americans have insufficient vitamin D levels, although some investigators have questioned the criteria that were used to define insufficiency. Even people in southern climates may be susceptible, as sun screen blocks the production of this vitamin. So get your levels tested, and if you’re deficient, supplement!