Vitamin D is well known for its role in helping bones absorb the calcium they need to stay healthy. Recent research has also found that vitamin D levels reflect the health of the processes that maintain immunity, minimize inflammation, and boost tissue repair in the lungs. In particular, vitamin D deficiency may be a cause of asthma and vitamin D supplements may relieve it.
The skin makes vitamin D when it is exposed to ultraviolet light from sunshine. The UV-B wavelengths of sunlight that trigger the production of vitamin D are the same as the wavelengths that cause skin cancer, but the amount of sunshine needed by the skin for the manufacture of vitamin D is less than is likely to cause skin cancer. Just 20 minutes of sunshine exposure each day, preferably in the early morning, before the sun is harsh, is enough for most people to avoid vitamin D deficiency. One of the roles of vitamin D actually is to protect the skin against cancer, although it is best just to get enough sun for making vitamin D without getting so much sun that the skin burns.
Some of the vitamin D produced by the skin circulates to the rest of the body. The protective effects of vitamin D are especially pronounced in the lungs.
Vitamin D was first used as a treatment for tuberculosis in the 1840′s. An English physician known as C. J. B. Williams reported that 206 out of 243 tuberculosis patients “showed marked improvement” after receiving cod liver oil, a natural source of vitamin D. In the nearly 170 years since this discovery, scientists have come to understand that vitamin D protects lung health by regulating important parts of the immune system.
Vitamin D stimulates a group of white blood cells known as the macrophages. These are large immune cells that completely surround and then digest bacteria, dust, and other foreign particles.
Vitamin D deactivates a group of white blood cells known as the B lymphocytes. These are the immune cells that can cause severe reactions to otherwise harmless foreign substances. Increasing vitamin D concentrations in the fluids surrounding these cells keeps them from multiplying and encourages apoptosis, a process also known as “cell suicide.”
Vitamin D increases the numbers of regulatory T cells. These white blood cells fall into two groups. The killer-T cells fight infection with inflammation. The regulatory or helper-T cells limit inflammation to protect healthy tissues. Vitamin D increases the number of the cells that regulate inflammation to keep it from doing more damage than is needed to protect the body from microbial invasion.
Vitamin D decreases the activity of dendritic cells, immune cells with “arms” that can deliver inflammatory chemicals to kill infections in the lining of the lungs.
Vitamin D modifies the way epithelial cells, the cells lining the lungs, produce inflammatory chemicals after a viral infection. Vitamin D decreases inflammation after the infection has passed, without interfering with the ability of the epithelial cells to continue to protect themselves against new infections.
Many researchers believe that the reason asthma has become epidemic is that more and more children (and adults) spent their leisure hours at computers, wii, and video games rather than in outdoor hobbies and play. Additional factors that determine how much vitamin D the body can make include:
How much of the body is exposed to the sun. Women who wear burkas, for example, almost always need vitamin D supplements.
How much body fat lies under the skin. Humans (and all other animals) store vitamin D in fat. People who have more body fat store more vitamin D in their fat and have less vitamin D in circulation.
How dark the skin is. Fair skinned people make more vitamin D with less sun exposure than dark-skinned people. In the United States, African-Americans are at highest risk for vitamin D deficiency.
The most important factor in vitamin D production, however, is geographic location. The farther you go away from the equator, the less sunlight is available for the skin to make vitamin D, especially in the winter months. The relationship between vitamin D levels and asthma was first noticed by epidemiologists who found that:
In the United States, asthma is especially common in the northeastern, New England states, and in Michigan. The lowest rate of asthma in the United States is in Miami, and the highest rate is in Detroit. Asthma is twice as common in Detroit as it is in Miami.
In Australia, asthma is most common in Hobart, Tasmania (which the major metropolitan area farthest from the equator) and least common in Darwin, Northern Territories (which is closest to the equator). Asthma is 2-1/2 times more common in Hobart than in Darwin.
Distance from the equator explains about 95% of the variation in rates of asthma around the world. Air pollution, surprisingly, does not predict asthma rates very well. Cities in southern China (closer to the equator, receiving more sun) that have extremely contaminated air have lower rates of asthma, especially in children, than cities in southern Australia (farther away from the equator, receiving less sun).
Diet is not very closely related to asthma, either. The consumption of omega-3 essential fatty acids in the diet does not seem to be related to asthma rates, at least in population studies. (Changes in diet that increase omega-3 essential fatty acids in the bloodstream may still be helpful for individuals who have asthma.) And consumption of meat, eggs, sugar, and fast foods, surprisingly enough, are not known to increase rates of asthma. The only good predictor of whether many people in an area may be likely to get asthma is sunshine exposure.
It’s clear that getting sunshine reduces the risk of asthma, but does taking supplemental vitamin D to correct vitamin D deficiencies treat asthma?
Here the evidence is not as clear. One study in Poland found that giving children vitamin D supplements reduces the flare-ups in asthma that often occur when children are getting over colds and flu.
Zanolin ME, Pattaro C, Corsico A, Bugiani M, Carrozzi L, et al. The role of climate on the geographic variability of asthma, allergic rhinitis and respiratory symptoms: results from the Italian study of asthma in young adults. Allergy. 2004;59(3):306–14.