ACAAI 2011: Vitamin D and Asthma, Part 1
NOVEMBER 08, 2011
Michael J. Stillman, PhD
Michael B. Foggs, MD, DFACAAI, FAAAI, FCCP, Chief of Allergy & Immunology, Advocate Medical Group, Advocate Health Care, Chicago, IL, today delivered the prestigious Bela Schick Lecture at the annual meeting of the American College of Allergy, Asthma and Immunology. Dr. Foggs’s lecture, “Here Comes the Sun: Vitamin D and Asthma,” described the complex metabolic relationships and current knowledge gaps regarding vitamin D and its relationship to asthma.
Vitamin D (vit D) is the only vitamin that is not a nutrient; it is a hormone with important immunomodulatory properties, which are not fully understood. Dr. Foggs noted that most studies on vit D are epidemiologic or observational studies, with few randomized controlled clinical trials or in vitro studies.
The research on vit D is problematic. Many studies do not differentiate between vit D insufficiency and vit D deficiency. Most studies are retrospective and lack good data for the true amount of vit D consumed by subjects. Although recent vit D research suggest that higher vit D levels might improve some health outcomes, these data are largely inconclusive and are sometimes contradictory, or find associations but not causality. Dr. Foggs noted that these limitations have made our knowledge on clinical effects on vit D surprisingly limited, based on a lack of valid studies.
The sun is quite inefficient at boosting vit D levels. The Minimal Erythema Dose (MED), which is the degree of skin reddening 24 hours post sun exposure. At one-third MED, the system is overloaded, and one must wait at least a day to get more vit D converted to its active form by the sun’s UVB radiation. Dr. Foggs noted that primary wavelengths provided by tanning beds are primarily UVA band, which does not convert vit D from its precursor to its active form.
Next, Dr. Foggs discussed dietary vit D sources, which are relatively scarce. While cold water fish are popularized for their vit D content, their international unit (IU) values are insufficient requirements. Some people take cod liver oil, but this also contains large amounts of vitamin A. While this may sound advantageous, both vitamins bind to same nuclear retinoic X receptor; therefore, each vitamin may antagonize the other’s effects.
The popular press notes regularly that many people do not get enough vit D. Dr. Foggs defined vit D status values in terms of plasma concentrations. By general international consensus, deficiency levels were <20 ng/mL, insufficiency levels were 20-29 ng/mL, and normal levels were 30-50 ng/mL. (There is some concern with vit D toxicity and hypercalcemia with progressively increasing levels above 50 ng/mL, but this is quite rare.)
Skin types affect vit D biochemistry. Either vit D insufficiency or deficiency may occur in 90% of dark-skinned Americans and up to 80% of white Americans. In winter, darker and lighter skin types can synthesize vit D to the same extent with UVB exposure. The innate ability to synthesize vit D is the same for darker or lighter skin types, but during summer months, darker skin types tan, because they possess more melanin, which blocks vit D synthesis. Vitamin D production in the skin is related to skin type (darker skin requires more sunlight exposure to make sufficient vit D.
To read our article on the rest of Dr. Foggs’ lecture, click here.