Introduction

Vitamin D (VD) has been described as an essential element in the maintenance of mineral homeostasis and bone architecture (X); however, the vision on the physiological functions of said nutrient has changed radically, the discovery of the VD receptor in immune system cells initiated a series of investigations related to the regulatory activity of this vitamin toward inflammatory phenomena and the association between low levels and development of various infectious and degenerative pathologies. Interest in VD has increased considerably because of multiple studies show that between 30 and 50% of the general population have serum VD deficiency¹,². In Mexico, Flores et al. report a prevalence of VD deficiency of 10-24% and 18-30% of insufficiency in children³. Ninety percent of VD requirements are synthesized through proper skin exposure to sunlight; the remaining 10% is obtained from food⁴,⁵.

The main cause of VD deficiency is inadequate sunlight exposure. Sunscreen use reduces its synthesis more than 95%. People with dark skin tone require at least 3-5 times more sun exposure to produce the same amount of VD that a person with white skin tone. There is an inverse association with body mass index (BMI) because more than 30 kg/m² values are associated with deficiency. Other causes include malabsorption syndromes, nephrotic syndrome (a case in which one loses VD bound proteins through urine), anticonvulsants, antiretrovirals, and chronic use of systemic corticosteroids⁶. Endocrinology guidelines for assessment, treatment, and prevention of deficiency suggest that 25 (OH) VD can be measured to evaluate blood levels of VD⁶. An optimal level is defined between 40 and 60 ng/ml, sufficiency between 30 and 39 ng/ml, insufficiency between 21 and 29 ng/ml, and deficiency < 20 ng/ml⁷.

The association between VD levels and respiratory allergic diseases such as asthma and rhinitis has been investigated. Some studies have associated levels < 30 ng/ml with severe asthma, impaired lung function, frequent exacerbations, uncontrollable asthma, and increased use of inhaled steroids and oral steroids cycles in children, such effect implies a wear and tear both economically and in the quality of life⁸. Gupta et al. described VD insufficiency prevalence in 47% and deficiency in 17% of asthmatic patients⁹. Regarding allergic rhinitis, even though it shares similar inflammatory mechanisms with allergic asthma, its association with VD levels is less consistent¹⁰,¹¹.

The objective of this study was to evaluate the prevalence of VD deficiency in patients with respiratory allergies and describe the associated factors.

Material and methods

Results

Discussion

This study shows a VD deficiency of 30.15%, and insufficiency of 47.1%, indicating that only 22.75% of the study population has sufficient VD levels. These data are superior to those showed by the national health and nutrition survey (ENSANUT) 2006 that reported a VD deficiency of 24.1% and insufficient of 30% in children between 2 and 5 years old, and a deficiency of 10% and insufficiency of 18% in children between 6 and 12 years old. This survey was conducted in the general population, unlike our study that included only allergic patients. We found that patients between 12 and 14 years old showed an increased risk of deficiency, unlike the ENSANUT survey, which reported a higher deficiency in the preschool group³. BMI was higher in the group of patients with deficiency, but there was no statistically significant difference between the groups and no association, unlike what is reported in the literature, in which it is stated that patients with a higher BMI have lower VD levels. There are different hypotheses that explain deficiency in these subjects such as low sun exposure, poor supplement intake, accumulation into adipose tissue, or more bone mass needed to support the heavy weight¹².

The season in which VD levels were measures does not show significant differences between groups, and no risk of significant association was found, despite what Webb et al. report, as they observed lower VD levels in the winter months caused by differences in solar radiation during those months. Although, this study was carried out in a Canadian population where solar exposure may be lower in the winter months than in our population¹³.

Significant association was found in types II and V according to Fitzpatrick being greater in the latter. However, we did not explain why skin types being these extremes were associated with the deficiency, different from what Clemens et al. who describe that higher levels of melanin are associated with lower levels of VD due to a decrease in solar absorption in hyper-pigmented skin¹⁴. Regarding the use of sunscreen, a greater association with deficiency was found with its use, but the OR did not show a statistically significant association. Matsuoka et al. described that using a sunscreen with a sun protection factor > 15 decreases VD absorption in the skin up to 99%¹⁵.

Higher levels of IgE and eosinophils were found in patients with VD deficiency, but the OR did not show a significant association. Muehleisen describes the role of VD in the regulation of IgE production, which may explain that low levels of VD produce an imbalance of regulatory T cells and IgE-producing B cells. However, other authors have not been able to find this association¹⁶.

Regarding the association with allergic rhinitis, there are controversial data some authors have documented a strong relationship between levels of VD and allergic process, for example: Vasallo and Camargo demonstrated that VD deficiencies are associated with low tolerance to environmental allergens¹⁷. On the other hand, Mai et al. documented that in adult Norwegian patients there is a significant relationship between deficient levels of VD and the development of allergic rhinitis¹⁸. Rothers et al. found that deficient levels of VD in umbilical cord blood are associated with aeroallergens and predispose to the development of rhinitis and asthma and even Gupta et al. described that low levels of this vitamin are associated with severe asthma, uncontrolled disease, and airway remodeling¹⁹,²⁰. By another hand, others researchers found no significative associations between rhinitis and lower levels of VD⁸. In our case, those patients with persistent disease were associated with the lower VD values without statistical significance, so we cannot establish a proper association between rhinitis severity and VD deficiency. We did not find an association between VD deficiency and asthma severity and in similar way, VD deficiency seems to be associated with higher comorbidities, such as rhinosinusitis²¹. However, we did not find a statistically significant association.

With respect to foods enriched with VD, we found that cod liver supplement consumption is a protective factor for deficiency, which is due to its high content of this vitamin²². Holick et al. described that exposure to sunlight is a protective factor for VD deficiency, but we were unable to demonstrate this association⁶.

This study has some limiting, including a small sample size and that it only included children. We could not find a strong association with some variables considered to be important regarding VD levels, but this could have been more evident with a bigger sample. One of problem in this work is that it only included allergic patients so we have an idea of VD deficiency repercussion in this population but the comparison is not complete without a control group. In the future, it could be convenient to include a control group to establish if this condition is related to the allergic conditions in these patients.

VD deficiency is a global public health problem, and patients with allergic rhinitis and asthma show a high either deficiency or insufficiency - even more than the general population. Different studies have shown an association between VD deficiency and severe manifestations of allergic diseases. However, we could not demonstrate that relationship with statistical significance. It seems that consumption of cod liver supplements is a protective factor for deficiency.

The deficiency of VD is high in allergic patients; the factors associated with the deficiency were little consumption of cod liver, age, skin photo type, and moderate-severe rhinitis. It is necessary more works to confirm our findings and to reveal the mechanisms underlying these observations.

Conclusions

Vitamin D deficiency is frequent in allergic patients due to several factors such as cod liver consumption, age, skin phototype and the presence of moderate-severe allergic rhinitis.

There is a need for similar studies, not only to confirm our findings, but also, those investigating the underlying mechanism by which allergic patients have lower vitamin D levels compared to the rest of the population.