Vitamin D Status Modifies the Response to Indoor Particulate Matter in Obese Urban Children with Asthma

Background: Indoor fine particulate air pollution (PM_2.5) is linked to asthma morbidity; however, whether vitamin D status influences individual susceptibility to airborne exposures is unclear. Objective: We aimed to determine if vitamin D modifies the effects of indoor PM_2.5 on asthma symptoms in urban children. Methods: A total of 120 children aged 5 to 12 years with physician-diagnosed asthma were evaluated at baseline and every 3 months for 9 months. Indoor PM_2.5, serum 25-hydroxy vitamin D (25-OH D) levels, and asthma symptoms were simultaneously assessed at each time point. Adjusting for confounders, generalized estimating equations assessed the 3-way interaction effects of 25-OH D, obesity, and PM on asthma symptoms. Results: Children were of mean (standard deviation [SD]) age 9.7 (2.2) years, 36% were obese, and 95% self-reported black race. Mean (SD) PM_2.5 indoor exposure was 38.2 (42.9) μg/m³ and 25-OH D was 19.1 (7.5) ng/mL. Three-way interaction models demonstrated significantly greater PM_2.5-associated effects on daytime asthma symptoms only among obese children with low 25-OH D levels (odds ratio [OR]_PM2.5 = 1.26, P = .049 at vitamin D = 15.5 ng/mL, increasingly stronger PM effects at levels <15.5 ng/mL). In homes with increased PM_2.5, higher 25-OH D was associated with decreased symptom odds (eg, OR_{Vitamin D} = 0.87; P = .049 at PM_2.5 = 52.5 μg/m³, increasingly protective effects >52.5 μg/m³) among obese children. Conclusions: Among obese urban children with asthma, low individual 25-OH D enhanced adverse respiratory effects associated with indoor PM_2.5. In high PM_2.5 environments, 25-OH D was protective against asthma symptoms. Optimizing vitamin D status in children may help reduce asthma morbidity driven by indoor air pollution.