Every cell of your body has receptor sites, like key holes, that are waiting to be opened by the appropriate key.  The keys can be vitamins, minerals, hormones and neurotransmitters.  Each cell will have receptor sites for many different things, depending on the organ they make up.  For example, your thyroid cells require iodine, magnesium and Vitamin D to function (among other things).

Every single cell of your body requires Magnesium and Vitamin D to function.  In the event of a deficiency (which is the common state of nutrition in America) cell dysfunction occurs. 

Vitamin D is a fat soluble vitamin. Vitamin D is unique in that your body can produce it from being exposed to sunlight. It is the only vitamin our bodies can produce. As with any fat soluble vitamin, it is possible to get too much.

We have had blood work done to determine what level of Vitamin D we should be consuming at different times of the year (decreased when sun exposure increases)  In the summer time in Maine, my family all takes 2000 IU of Vitamin D (one drop). In the winter time in Maine we all  take 6000-8000 IU  (3-4 drops).

Vitamin D is vital to for your body to absorb calcium and phosphorus.  It is integral to a healthy immune system, which is why viruses increase in the winter when the body has less exposure to sunlight to synthesize cholesterol to vitamin D.  It has many other functions in the body and is essential to health as it helps the cells throughout your body to communicate and function properly.