Association between toenail zinc concentrations and incidence of asthma among American young adults: The CARDIA study

Background: As an essential micronutrient, zinc plays an important role in modulating the immune system. However, data on the association between zinc concentrations and asthma incidence are sparse, especially in adults. Methods: We prospectively followed up 3682 individuals aged 20-32 years without history of asthma or current asthma at baseline from 1987-1988 to 2015-2016. Zinc concentrations were measured in toenail clippings collected at Exam Year 2 using inductively-coupled-plasma mass spectroscopy. Asthma was diagnosed by a physician and/or reported medication use for asthma control with verification. Cox regression was used to model the association between toenail zinc concentrations and asthma incidence. Results: Over an average of 22.5 years of follow-up, 508 incident cases of asthma were identified. After adjustment for potential confounders, no statistically significant association was found between zinc concentration and asthma incidence (HR = 0.81; 95% CI: 0.62, 1.07; P-linear trend = 0.26). The observed association was not materially modified by sex, body mass index, smoking or atopic status. When stratifying data by race, a significant inverse linear association was found among African Americans (per 1 standard deviation increment in toenail zinc concentrations: HR = 0.86; 95% CI: 0.75, 0.99; P-linear trend = 0.03), but not in Caucasians (HR = 1.07; 95% CI: 0.91, 1.25; P-linear trend = 0.42), though the test for interaction was not statistically significant (P-interaction = 0.099). Conclusions: Findings from this prospective cohort study do not support a significant longitudinal association between toenail zinc concentrations and incidence of asthma among American young adults. Future studies are needed to confirm our findings.