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 = 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 = 0.03), but not in Caucasians (HR = 1.07; 95% CI: 0.91, 1.25; P = 0.42), though the test for interaction was not statistically significant (P = 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.