Our pediatrician is a big proponent of vitamin D supplementation, and thanks to her, everyone in the family regularly takes vitamin D. She referred us to the GrassrootsHealth.org website for more info about her recommendations. I did some PubMed searching, too, and decided that I agreed that the available evidence suggested that vitamin D supplementation might be a good idea- or at least that it wouldn't be a bad idea. We all know that vitamin D is essential for strong bones (this is because we can't absorb calcium without it), but one of the other things that vitamin D is supposed to help is your immune function.
I submit that "improves immune function" will be irresistable to just about any parent with a child in day care. And indeed, after we started giving Pumpkin vitamin D, I noticed a drop in the number of illnesses in our family (most of our illnesses come home from day care, and Pumpkin is the first one to get sick). However, she had also been at day care for awhile, and presumably built up some immunity to various things, so I considered this suggestive, but nothing more.
This week, I cam across a recent article in Nature Immunology by von Essen, et al, which adds support to the idea that vitamin D is essential for proper immune function. (You can read the abstract of the original paper without a subscription. Here is a write up of the paper, too.)
The paper looksat the function of one type of immune cells, called T cells. T cells are far more responsive to antigen after they have been "primed"- i.e., exposed to antigen once. The current model to explain this involves the fact that T cell activation can occur via two different signaling pathways. The first pathway dominates in "naive" T cells (T cells that have never been exposed to antigen), while the second dominates in T cells that have previously been exposed to antigen. The second pathway is far more efficient than the first, which explains why primed T cells are more responsive than naive ones.
But how do the primed T cells switch to the second pathway? That is where the new paper comes in. The researchers show that the expression of one key protein in the second pathway, phospholipase C gamma1, is increased by the combination of vitamin D and its receptor (cleverly called "vitamin D receptor", or VDR). The expression of VDR, in turn, is increased by the activity of the first pathway. So, the T cells turn on the second, more active pathway using the same pathway that they use to respond to antigen the first time they "see" it- but only if there is sufficient vitamin D around. The researchers even showed that T cells isolated from people with low serum levels of vitamin D were less responsive to antigen than T cells from controls with normal levels of vitamin D.
Maybe it is because my background is in biochemistry and biophysics, but this is the sort of study I like to see when I'm trying to evaluate the benefits of a supplement. Correlative population studies are all well and good, but I'm always a little suspicious of them. Human beings aren't lab animals- it is very hard to control for all of the confounding variables in how we live our lives. A population study indicating a correlation between levels of some vitamin and a certain outcome is a nice story, but it is only the outline of the story. A biochemical mechanism adds some satisfying detail to the story.
And here is the ironic twist to this story- as I write this, my nose is blocked with the second or third consecutive cold I've had this season, which just goes to show that no supplement can guarantee you perfect health.
I am not a medical doctor. This post should not be construed as medical advice. I read the entire paper, but as an interested scientist, not as a careful reviewer. If you're wondering about whether or not you or your children should take vitamin D supplements, all I can really tell you is to talk to your doctor and/or do your own research.