By Margherita T. Cantorna

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?