L-shaped Association between Dietary Copper Intake and Chronic Obstructive Pulmonary Disease among American Adults: Findings from the National Health and Nutrition Examination Survey 1999-2018.

Background: Copper plays a vital role in human physiological functions. However, limited epidemiological evidence exists for an association between dietary copper intake and chronic obstructive pulmonary disease (COPD).

Objectives: This study aimed to assess the association between dietary copper intake and COPD in American adults.

Design: This cross-sectional study utilized data from the U.S. National Health and Nutrition Examination Survey (NHANES) from 1999 to 2018.

Participants/setting: Following inclusion criteria, data from a total of 39,644 adults aged 20 and above were included in this study, out of which 3,159 individuals had been diagnosed with COPD by a healthcare provider.

Main outcome measures: Dietary data were obtained through 24-hour dietary recall, while COPD diagnosis information was derived from a standardized medical condition questionnaire.

Statistical analyses performed: Multivariable logistic regression analysis was conducted to explore the association between dietary copper intake and COPD. To assess the dose-response relationship, a three-piecewise restricted cubic spline analysis was employed. Potential thresholds were identified with a two-piecewise logistic regression model. Subgroup analysis was performed to assess heterogeneity and explore potential interactions.

Results: Compared to individuals in the lowest tertile of dietary copper intake (T1, < 0.89 mg/day), those in the highest tertile (T3, > 1.35 mg/day) had an adjusted odds ratio (OR) of 0.79 for COPD (95% CI: 0.68-0.92, p = 0.002). The relationship between dietary copper intake and COPD followed a non-linear L-shaped curve (p < 0.001). Threshold analysis found a statistically significant inverse association between dietary copper intake and the odds of COPD for intakes < 1.5 mg/day but no significant association for intakes ≥ 1.5 mg/day. Subgroup analysis indicated an interaction effect with age.

Conclusions: An L-shaped inverse association was observed between dietary copper intake and COPD in US adults, at a threshold of approximately 1.5 mg/day, with an observed age interaction.