Everyone knows that vitamin D is a necessary thing for maintaining proper health. What they don't know is that it is effectively a natural sunscreen. The form that is then sent to the liver for the final stage in being converted to what the body uses for other things comes from the skin, where 7-dehydrocholesterol absorbs UV light to become vitamin D.
This is clearly not the only thing that vitamin D does, though. It is vital in regulating the levels of calcium in the bones and bloodstream. It greatly helps in the production of the autoimmune systems killer cells that seek out and destroy virus-infected cells and bacteria. However, this sunscreen effect does offer a clue as to why people are more apt to get sick in the winter.
Vitamin D Supplement
Since UV light is needed for the body's production of vitamin D, the quality of sunlight one gets directly affects how much of it one has. The reason that winter gets cold is because less sunlight is absorbed by the earth at that time of year, instead bouncing off and out into space. This includes the UV frequencies needed for the body's own production.
Since vitamin D is not produced in nearly as high quantities in the winter, because of the reduction in absorbable sunlight, there is less that comes out the end of the process. That means a reduction in the number of killer cells that would otherwise be busy fighting off infections like the flu virus. Taking a supplement during these months will go a long way toward fending off this disease.