What you need to know about Vitamin D:

1. We’ve heard a lot about the “power of Vitamin D,” but what is vitamin D anyway and why all the hype? Simply put, vitamin D is an essential fat-soluble vitamin—“essential” because our bodies need it for maintenance and repair and “fat-soluble” because we store excess amounts of it in our fat tissue.

2. Vitamin D has become a hot topic for its apparent protective effects against diabetes, heart disease and cancer, as well as its potential to enhance immune function. Recent studies have also indicated that if you’re overweight or obese, you are more likely to be vitamin D deficient. Now, vitamin D deficiency doesn’t cause people to be overweight. Quite the contrary: your fat tissue just likes to hold onto the vitamin D, making it unavailable when your body needs it.

3. Our greatest source of vitamin D is not from our diets, but from the sun. Sun exposure for as little as 10 to 15 minutes a day provides about 80 to 100 percent of our daily vitamin D needs. In colder climates north of 42° latitude (like in Boston), though, we synthesize absolutely no vitamin D from sun exposure between the months of November and March. The angle of the sun is simply too low to convert the vitamin D precursor that is found in your skin to its active form. Plus, as you get older, your skin becomes less efficient at synthesizing vitamin D, so even if you do get some sun exposure (say, with a midwinter trip to Florida or Los Angeles), you could still be low.

4. Not only is vitamin D sometimes tough to get from the sun in winter, there aren’t many great food sources either. Fatty fish, fortified milk, cereals, and orange juice have some vitamin D, but not enough to meet the latest dietary recommendation of 600 IU/d per day (or 800 IU/d for those over 70).

5. All of this means that for most of us, particularly those of us who live at northern latitudes, our vitamin D levels start to decline in October and reach their lowest levels in March. That makes March the perfect time to get your vitamin D levels tested to see if you are deficient.

6. So, should you take a supplement? This is one of those rare cases where I would suggest considering it. First, ask your doctor to check your vitamin D levels—do this today if you think you might be. If you’re deficient, he or she will recommend a supplement. If you can’t get to the doctor right away, a safe daily dose would be 1,000 IU/d (you can find this at your local grocery store). Just don’t take more unless it’s under the guidance of a health care professional.