A role of vitamin D in regulating immune function was first proposed after the identification of vitamin D receptors in lymphocytes. It has since been recognized that the active form of vitamin D, 1α,25(OH)2D3, has direct effects on naive and activated helper T cells, regulatory T cells, activated B cells, and dendritic cells. There is a growing body of literature linking vitamin D (serum 25(OH)D, oral intake, and surrogate indicators such as latitude) to various immune-related conditions, including allergy. However, what is the role of vitamin D in allergy?
What is vitamin D?
Vitamin D is a secosteroid obtained by endogenous production in the skin with exposure to UVB radiation or from food sources. Within the body, vitamin D acts as a hormone and is well recognized for its role in calcium and phosphorus homeostasis and skeletal health. In addition, all tissues in the body have receptors for the active form of vitamin D, 1,25-dihydroxyvitamin D (1α,25(OH)2D3) and there are extrarenal tissues capable of converting 25(OH)D to the active metabolite, including certain immune cells.
In addition to skeletal health, vitamin D has been linked to several types of cancer and autoimmune disorders with evidence of a role in the development and maintenance of lung structure and function. It is this association with immune and airway function that provides the basis for the hypothesis that vitamin D may have direct links with asthma and allergic diseases.
Vitamin D in allergy
The epidemic increase in allergic diseases is now a major public health crisis, affecting more than 40% of the population in developed countries, with a huge impact on individuals, societies and health systems. The greatest burden of these diseases occurs during childhood, when the increasing rates of disease are most evident.
As these younger generations reach maturity, it is possible that the impact of allergic diseases will increase further, especially as the same trends are emerging in developing countries in lifestyle transition. Moreover, while the incidence of asthma appears to have plateaued in some developed countries, many of these regions are facing an increase in food allergy in what appears to be a “second wave” of the allergy epidemic.
The increase in allergic diseases is unequivocally linked to environmental and lifestyle factors associated with industrialization and progressive “Westernization.” Several causal factors have been proposed for this, including significant changes in lifestyle patterns, nutrient intake, exposure to microbial organisms, and air pollution.
However, most of these notions cannot individually explain the increase in disease. More recently, variations in vitamin D status and intake have been implicated in the development of allergies and considered as one of several explanations for epidemiological and immunological associations.
Vitamin D in childhood
Evidence suggests that adequate or higher concentrations of 25(OH)D in childhood are generally protective of allergic diseases. So far, the main focus of these studies is respiratory allergic outcomes such as allergic rhinitis, asthma, and sensitization to inhalant allergens.
Contrary epidemiological observations between serum status (usually protective) and oral exposure (supplementation increases allergy risk in many studies) highlight the uncertainty surrounding vitamin D in allergy development and impede population recommendations.
Most of these observational studies did not have direct serological measurements of 25(OH)D levels to accurately confirm vitamin D status, and cross-sectional study designs involving older children and adults fail to provide evidence on etiological factors. Although the significance is unclear, the results show that time and route of exposure can play a critical role in immune function.
Clinical practice
It is known that a high percentage of pregnant women worldwide have concentrations of 25(OH)D in the insufficient and deficient ranges. There is a lot of evidence showing that vitamin D affects the differentiation and proliferation of immune cells and that genetic polymorphisms can contribute to disease susceptibility.
Because the early stage of life is relevant to the programming of future immune function, it is crucial to understand the effects of vitamin D and identify optimal status levels. Therefore, vitamin D supplementation in pregnancy and childhood plays an important role in the regulation of the immune system, being relevant for allergic conditions.
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Vitamin D and allergy
– Jones AP, Tulic MK, Rueter K, Prescott SL. Vitamin D and Allergic Disease: Sunlight at the End of the Tunnel? Nutrients. 2012; 4(1):13-28. https://doi.org/10.3390/nu4010013
