"When a T cell is exposed to a foreign pathogen, it extends a signaling device of 'antenna' known as a vitamin D receptor, with which it searches for vitamin D," Geisler said in a prepared statement. If there is an inadequate vitamin D level, he noted, "they won't even begin to mobilize."
In fact, a 2009 Archives of Internal Medicine study found that 77 percent of U.S. adults and teenagers surveyed did not have the estimated minimum healthful level of 30 nanograms per milliliter in their blood. And just three percent of blacks in the survey were getting enough of the vitamin, the 2009 report found.
We already know that some people with absorption problems in their gut (like wheat gluten allergy) don't absorb vitamin D very well, even if it is in their diet, and either need high oral doses or sub-lingual Vitamin D supplements.