Dose-response relationships between urinary cobalt concentrations and obesity, insulin resistance, and metabolic-related disorders in the general population.

Excessive cobalt exposure has been shown to induce various adverse health effects in animal and human toxicity studies. However, the relationships between cobalt exposure and obesity, insulin resistance, and metabolic-related disorders are rarely studied in epidemiological studies. This study aimed to explore the dose-response relationships between urinary cobalt concentrations and obesity, insulin resistance, and metabolic-related disorders. Adult participants (≥20 years) from the 2005 to 2018 National Health and Nutrition Examination Survey were included. Restricted cubic splines and the log-binomial regression were adopted. Multivariate adjusted prevalence ratios (95% confidence intervals) comparing extreme quartiles of urinary cobalt concentrations were 1.43 (1.29-1.57) for obesity, 1.43 (1.10-1.86) for insulin resistance, 1.21 (1.09-1.34) for metabolic syndrome, 1.16 (1.10-1.23) for elevated waist circumference, 1.20 (1.09-1.33) for elevated triglycerides, 1.14 (1.01-1.29) for reduced high-density lipoprotein cholesterol (HDL-C), 1.06 (0.98-1.15) for elevated blood pressure, and 0.91 (0.79-1.06) for elevated fasting glucose, respectively. Results from the restricted cubic splines showed that an initial steep increase in risk was followed by a weaker increase in risk or a platform beyond 1.0 µg/L for obesity (prevalence ratio (95% confidence interval): 1.41 (1.29-1.55)), insulin resistance (1.33 (1.07-1.65)), metabolic syndrome (1.18 (1.11-1.31)), elevated waist circumference (1.18 (1.11-1.25)), elevated triglycerides (1.21 (1.11-1.33)), and reduced HDL-C (1.15 (1.01-1.30)), respectively. In conclusion, the prevalence of obesity, insulin resistance, metabolic syndrome, elevated waist circumference, elevated triglycerides, and reduced HDL-C progressively increased with increasing urinary cobalt concentrations. The above associations are non-linear and there is not a "safe threshold" below which there are no toxic effects of cobalt.