With summer drawing to a close, many people will head from the beach back to their office, where they will spend the coming nine months under dim artificial lighting. Aside from being a sign of the cold and dark winter days ahead, this could have a negative impact on individuals' vitamin D levels.

While the nutrient can be found in some foods, the primary source of vitamin D is sunlight. The skin produces it following exposure to ultraviolet radiation. For many reasons, people tend to get less of this during the winter.

First of all, people tend to spend less time outdoors in the fall and winter. Even when they do venture outside, the cold temperatures necessitate bundling up and covering nearly every inch of exposed skin.

Furthermore, the northern hemisphere is actually farther away from the sun during the winter months. Because of this, the ultraviolet radiation reaching the surface is less strong. That means it would take even more time in the sun to produce adequate vitamin D levels.

This is why it is so important for individuals to either eat a vitamin D-rich diet or take nutritional supplements during the winter.