Vitamin D is a fat soluble vitamin that is required for the body to absorb minerals (calcium, magnesium, iron) from your diet. While some foods contain vitamin D humans get most of their vitamin D from exposure to the sun and UVB. Without enough vitamin D a person can develop Rickett’s, a disease that causes weak, deformed bones. Recently research has suggested that low vitamin D levels in countries with northern latitudes, and therefore low sun exposure in the winter, is responsible for the spike in many autoimmune diseases. One of these autoimmune diseases linked to vitamin D levels is asthma. For several years now epidemiological research has hinted that low vitamin D levels may increase a person’s risk for developing asthma and other atopic diseases. The problem with these studies is that they are at risk for what is known as ‘reverse causation’. This means that the data could mean that asthma causes people to have low vitamin D instead of low vitamin D causing asthma. In addition to this problem, the randomized control trials that have been conducted so far on vitamin D supplementation show only mixed results. Some show that supplementing vitamin D can help can reduce asthma exacerbations while others show no effect at all. Part of the reason there has been mixed results in these clinical trials is due to the low number of people participating in the studies and the short duration of vitamin D supplementation. In order to remedy this discrepancy, a team of researchers from Montreal have performed an elegant study to show that the association between vitamin D levels and asthma is not likely to be causal.
In this study, the team used a powerful epidemiologic technique known as Mendelian Randomization to study people who had genetic mutations that were strongly associated with their vitamin D levels. This included people from three cohorts, SUNLIGHT, GABRIEL and EAGLE. The researchers looked at the mutations in the DNA associated with vitamin D levels in 33,996 people and tested whether the levels were associated with a risk of that person developing asthma, allergies, or eczema. The hypothesis was that if someone had a genetic mutation that made them have vitamin D levels lower than normal then they should be at greater risk of disease than those people who did not have the mutations (and therefore normal vitamin D levels). What they found strongly disagreed with previous evidence:
- There was no association between low vitamin D levels and asthma as an adult or a child.
- There was no association between vitamin D levels are eczema.
- There was no association with allergies (elevated IgE antibody levels).
These results persisted even after controlling for numerous factors including separating by age or gender. There were some limitations to this study, as is the case with all studies. First, the researchers were unable to look for associations with the active form of vitamin D (1,25-dihydoxyvitamin D) which is not always correlated with the levels of total vitamin D they measured in this study. Also, they were underpowered to detect small differences in the risk for childhood asthma (less than 33% increase). Finally, this research was restricted to people of European ancestry so the results may differ across race. Still, the experiment is more robust than previous epidemiologic studies and so provides strong evidence against the association between vitamin D levels and asthma. Additionally, the same authors published a paper recently that, using the same techniques, shows an association between vitamin D levels and multiple sclerosis. So vitamin D levels do affect your risk for some diseases, just not asthma. The recommendation for your overall health is still to get some sun but don’t get burned.
Image Credit: Flickr julochka