Associations of Dietary Trace Elements With Muscle Quality Index and Metabolic Syndrome: Evidence from a NHANES Cross-Sectional Study

Objective This study aimed to examine the associations of dietary trace elements with muscle quality index (MQI) and metabolic syndrome (MetS), and to assess potential mediating effects. Methods This cross-sectional study included adults aged ≥ 20 years from the National Health and Nutrition Examination Survey (NHANES) 2011–2018. Dietary intake of trace elements, including iron, copper, selenium, and iodine, was assessed. Muscle quality index (MQI) was defined as the ratio of combined handgrip strength to appendicular skeletal muscle mass. Metabolic syndrome (MetS) was identified according to the National Cholesterol Education Program Adult Treatment Panel III (NCEP-ATP III) criteria. Weighted multivariable linear and logistic regression models were applied to examine the associations between dietary trace element intake and MQI and MetS. Trend analyses and restricted cubic spline (RCS) models were further used to assess dose–response relationships and potential nonlinear associations. Subgroup analyses were conducted according to age, sex, body mass index (BMI), and physical activity level. Additionally, MQI was treated as a mediator to explore its potential mediating role in the associations between dietary trace elements and MetS. Results A total of 1,111 participants were included in the analysis. Weighted multivariable analyses showed that dietary intakes of iron, zinc, and copper were negatively associated with muscle quality index (MQI). In addition, lower intakes of zinc and iodine were associated with a higher prevalence of metabolic syndrome (MetS). In the fully adjusted models, zinc intake remained inversely associated with MetS risk (odds ratio [OR] = 0.68, 95% confidence interval [CI]: 0.50–0.92), suggesting a protective association of higher zinc intake. Iodine intake also demonstrated a stable inverse association with MetS (OR = 0.97, 95% CI: 0.94–1.00), indicating that adequate iodine intake may be associated with a reduced risk of MetS. In contrast, the association between iron intake and MetS was attenuated after multivariable adjustment and did not reach statistical significance. Restricted cubic spline (RCS) analyses revealed significant nonlinear associations between intakes of iron, zinc, and iodine and MetS risk (P for nonlinearity < 0.05). Subgroup analyses further indicated that these associations were more pronounced among older adults, women, and individuals with obesity. Conclusions Higher dietary intakes of zinc were associated with better muscle quality and a lower prevalence of metabolic syndrome. These findings highlight the potential public health relevance of trace elements in supporting musculoskeletal health and reducing metabolic risk.