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Vitamin D levels modify pollution-driven asthma symptoms in pediatric obesity

Circulating vitamin D levels affect the risk for respiratory symptoms related to indoor air pollution among children with obesity and asthma, findings from a recent study suggest.

Asthma, obesity and air pollution disproportionately affect urban minority populations, and rates of vitamin D deficiency are highest among the black pediatric population, according to the researchers. Therefore, in this study, they sought to determine whether personal vitamin D status in a predominantly black urban cohort of children with asthma mitigates the effects of indoor air particulate exposure and whether any differences exist according to obesity status.

The study included 120 children aged 5 to 12 years with asthma (mean age, 9.7 years; 55% boys; 95% black) from Baltimore who were enrolled from the Domestic Indoor Particulate Matter and Childhood Asthma Morbidity (DISCOVER) study from 2009 to 2015. Serum 25-hydroxyvitamin D (25-[OH]D), asthma symptoms and fine particulate matter (PM_2.5) exposure during a 7-day period were evaluated at baseline and every 3 months for 9 months. The average BMI across all children was in the 71st percentile and 36% were considered obese. The mean PM_2.5 indoor exposure was 38.2 µg/m³ and the mean 25-(OH)D level was 19.1 ng/mL.

Effects of vitamin D levels

Results showed that lower serum 25-(OH)D levels strengthened the adverse association between PM_2.5 and limited activity (P for interaction = .003), trouble breathing (P for interaction = .054), feeling bothered by asthma (P for interaction = .03), having any daytime symptoms (P for interaction = .006), nighttime symptoms (P for interaction = .034) and needing rescue medication (P for interaction = .032) in children with obesity.

According to the data, low 25-(OH)D levels increased the adverse effect of PM_2.5 on daytime asthma symptoms (OR_PM2.5 = 1.26; P = .049 for a vitamin D level of 15.5 ng/mL), and the effect of PM_2.5 on daytime symptoms became stronger with decreasing levels of vitamin D less than 15.5 ng/mL. A similar result was seen for the effects of PM_2.5 on nighttime asthma symptoms in children with obesity with 25-(OH)D levels less than 16.4 ng/mL.

Conversely, at extreme levels of indoor air particulate pollution, higher levels of vitamin D protected against an increased likelihood for daytime asthma symptoms driven by PM_2.5 among children with obesity (OR_{vitamin D} = 0.87; P = .049 at a PM_2.5 concentration of 52.5 µg/m³, with increasingly stronger effects at higher PM_2.5 concentrations).

However, there were no associations between vitamin D levels and asthma symptoms associated with fine particulate pollution in children who were not obese.

“The protective effects of increasing vitamin D toward asthma symptoms within this population were observed in highly polluted urban homes. These results offer hope for potential future strategies to combat the hazards of environmental pollutants faced by vulnerable populations,” the researchers wrote.

Strengths and limitations

Although the study had strengths, including the prospective collection of longitudinal data, consideration of confounders and objective measurement of indoor air particulate pollution, it was limited by its small sample size and lack of correction for multiple testing, according to Erick Forno, MD, MPH, from the division of pulmonary medicine and the department of pediatrics at the University of Pittsburgh School of Medicine and UPMC Children’s Hospital of Pittsburgh, and Augusto A. Litonjua, MD, MPH, from the division of pulmonology and the department of pediatrics at the University of Rochester and Golisano Children’s Hospital in Rochester, New York.

“Moreover, it is imperative to keep in mind that this is an observational study when interpreting the results. What is presented as ‘protective’ is really an inverse association, whereby children with lower vitamin D levels have increased odds of reporting asthma symptoms. Rather than a therapeutic target, low vitamin D could be a biomarker (or a consequence) of PM_2.5 exposure, obesity and/or poor asthma control. This observational study was not designed to evaluate whether normalization of vitamin D levels (eg, via supplementation) could improve or reverse the findings,” they wrote in an accompanying editorial.

Forno and Litonjua also noted that the data on vitamin D supplementation for childhood asthma are conflicting.

“In this setting, studies like the one by Bose et al become particularly important, allowing us to identify subgroups that are more likely to benefit from an intervention. Thus, it will be critical to replicate these findings, especially given the complex three-way interactions described, which altogether apply to a small number of study participants,” they wrote. “If validated in similar but larger cohorts, the next step might be to tailor interventional studies for this specific group of children. Air pollution, obesity and low vitamin D may all play important roles in asthma risk and morbidity, but of these three factors vitamin D deficiency would certainly be the easiest to remediate.” – by Melissa Foster

Disclosures: The study was funded by the U.S. Environmental Protection Agency and the NIH. The authors report no relevant financial disclosures. Forno reports he has received funding from the NIH. Litonjua reports he receives author royalties from UpToDate and funding from the NIH.