Vitamin D takes part in over 600 chemical processes in your body. One of its key roles is in balancing the immune system: it enhances the part of the immune system that helps you fight off infections and lowers the parts that are involved in autoimmunity and allergic reactions. If there is one piece of advice to prevent catching colds and flu I would recommend increasing your vitamin D to optimal levels. Current thinking is that optimal means 70 mg/dL. In my experience most people need 5000 – 10,000 IU per day to get there. I have yet to see a patient with adequate blood levels who is not supplementing.
Vitamin D levels can be checked by a routine blood test that most primary care doctors will order for you.
If you supplement with Vitamin D it is crucial that you balance that with enough Vitamin K2. Vitamin D will, among other things, boost absorption of calcium from food into your bloodstream. Vitamin K2 is necessary to move the calcium out of the bloodstream and into tissues, like bone and brain cells. Otherwise it is left to accumulate outside cells creating stone formation, bone spurs, and calcification of arteries. If we were all eating free range and wild animals with healthy guts, we could get these vitamins naturally.
What is left out of these studies is an appreciation of the importance of genetic differences (SNPs), particularly in the vitamin D receptor (VDR). There are some individuals who will not achieve optimal blood vitamin D levels despite adequate supplementation or sun exposure. Many of these seem to have a genetic defect in using vitamin D. If you have a specific medical condition or have not been able to get your 25(OH) Vitamin D levels up check with a practitioner, you should have both 25(OH) Vitamin D and 1,25(OH) Vitamin D tested. SNPs in the VDR (Vitamin D Receptor) gene can be found through genetic testing.