On a basic level, our bodies need vitamin D to absorb and regulate calcium and phosphorus and to use the former to build strong bones. It's also essential for ensuring that our muscles, nerves, and immune system function properly. And, some research suggests vitamin D deficiency may be associated with an increased risk of depression, heart disease, and cancer.
While vitamin D is naturally present in a wide variety of foods, including most types of fish and dairy products, the body is also capable of creating its own supply. When we're exposed to direct sunlight (i.e., unobstructed by windows or clouds), our skin reacts with the UVB rays by producing vitamin D. In the winter, the angle of the sun in most parts of the country results in UVB levels that are drastically lower than usual, resulting in most of us making much less vitamin D than we normally would. Those with darker complexions also produce less vitamin D from sunlight and are thus at a greater risk of a deficiency.
Of course, now that summer is tantalizingly close, most of us will be able to get our daily dose of D the old-fashioned way. But, what about sunscreen? Does coating your body with SPF sabotage its ability to make enough of the stuff? While some studies have suggested that using sunblock negatively impacts vitamin D creation, most of the accepted research has found little effect of chronic sunscreen use on vitamin D production. However, I'd like to point out that most of these studies looked at products with SPFs between 15 and 20; that industrial-grade SPF 90 cream could have a serious impact on your body's ability to make its own vitamin D.
For those who are worried about sun damage (or are fish-averse), there are countless supplements that will ensure your vitamin D levels never drop too low. But, considering that it can take as little as 15 minutes for your body to make a full day's supply of the nutrient (depending on your skin type), consider taking a walk in the sunshine to get your D rather than popping a pill. It's better for your sanity, anyway.