Winter is upon us and so is the risk of vitamin D deficiency and infections. Vitamin D, which is made in our skin following sunlight exposure and also found in oily fish (mackerel, tuna and sardines), mushrooms and fortified dairy and nondairy substitutes, is essential for good health. Humans need vitamin D to keep healthy and to fight infections. The irony is that in winter, when people need vitamin D the most, most of us are not getting enough. So how much should we take? Should we take supplements? How do we get more? And, who needs it most?
I am a medical microbiologist and immunologist who studies the functions of vitamin D in immune cells. My laboratory has been interested in figuring out why the immune system has vitamin D receptors that determine which cells can use vitamin D. In the immune system, vitamin D acts to improve your ability to fight infections and to reduce inflammation.
Where to get your vitamin D
Vitamin D is called the sunshine vitamin since it is made in the skin after exposure to sun. The same UVB rays that cause a sunburn also make vitamin D. Sunscreen, darker skin pigmentation, clothing and reduced daylight in winter diminish the skin’s ability to make vitamin D. The people who experience the biggest seasonal swings in vitamin D levels are fair-skinned individuals living in the northern regions of the U.S. and at higher latitudes around the globe where there is very little daylight in winter.
But those most at risk for low vitamin D levels are people of color and people living at higher latitudes. Dark-skinned individuals are more likely than fair-skinned individuals to be low for vitamin D year-round because the darker skin blocks the UVB rays from producing vitamin D. However, even in dark skinned individuals, vitamin D is lowest in the winter.