In the world of allergy and asthma, vitamin D is now a hot topic. Many allergy specialists are wondering whether vitamin D might be used to help control asthma symptoms. Our attention has turned to vitamin D because of mounting circumstantial evidence that links vitamin D insufficiency with poor asthma control. The International Study of Asthma and Allergies in Childhood (ISAAC) begun in 1991 first noted a possible link between asthma and vitamin D insufficiency in Westernized countries like Ireland, England, Australia, the United States, and New Zealand which have the highest prevalence of asthma. Most citizens in these countries spend the majority of their days indoors and so are commonly vitamin D insufficient, as they don’t spend enough time outside in the sun, which promotes vitamin D production in the skin. In fact, we obtain 80% of our vitamin D from our skin and ingest only a small portion from vitamin D-rich foods like cod liver oil, salmon, cereal, milk, etc. Indeed, vitamin D insufficiency, defined as a blood level less than 30, is rampant in these countries, even in the sun-soaked Australia. One study reports that 48% of the US pediatric population is vitamin D insufficient. But just because lots of Westernized countries have lots of asthma and vitamin D insufficient patients doesn’t mean the two are related.
Furthering the conspiracy theory that vitamin D might play a role in asthma, African Americans and obese patients are more commonly vitamin D insufficient . . . and both groups are also more likely to have asthma. And now we’re learning that vitamin D doesn’t just help control our blood calcium levels, which help keep our bones strong and prevent disease like ricketts. We are beginning to understand that vitamin D does more than just help our intestines and kidneys absorb calcium. Vitamin D receptors have also been found in the lung and in several immune cell lines like B-cells, T-cells, and dendritic cells. What might vitamin D do in our immune system or . . . in our lungs?
One study in pregnant women revealed that vitamin D supplementation reduced the chance their future babies would develop asthma by as much as 40%. The study had been prompted by the view that vitamin D might play a role in the development of the immune system and lungs in utero. This significant finding has not been reliably reproduced however in subsequent studies.
At this point we don’t fully understand how vitamin D might function in our lungs or immune system, but we do think that vitamin D increases production of another type of immune cell called T regulatory cells or Tregs. These cells tamp down immune responses. So we might imagine that a lack of vitamin D and a resultant lack of Tregs might let the immune system run amok, as we see in various autoimmune diseases like diabetes or multiple sclerosis or inflammatory bowel disease or asthma . . .all of which have been associated with vitamin D insufficiency. Again, these associations haven’t been proved to be causal at this point, but the vitamin D story certainly is tantalizing. What more obvious explanation for the increased asthma prevalence in African American patients when compared to Caucasians than color of the skin and its ability to make vitamin D.
In the lung, vitamin D has been shown to increase production of proteins that help fight infections. This might explain why asthma patients experience more severe lung symptoms when they acquire common, upper respiratory, viral infections. A re-analysis of a landmark asthma study called the Childhood Asthma Management Program study (CAMP), indeed supports this view that vitamin D insufficient patients are more likely to experience severe, life-threatening asthma exacerbations. A more rigorous, randomized, controlled study evaluating the effect of supplementing asthma patients with vitamin D has not yet been done so we don’t yet have our definitive answer regarding vitamin D’s effect on asthma. Maybe sicker asthma patients spend more time indoors and so, by force, have lower vitamin D levels and all of the above is just coincidence. We’ll find out soon enough.