Heavy metal exposures and constipation among United States adults: A cross-sectional study

Constipation is a common gastrointestinal disorder that can substantially impair quality of life. Environmental exposure to heavy metals has been implicated in gastrointestinal dysfunction; however, population-based evidence regarding their associations with constipation remains limited. This study aimed to evaluate the associations of blood lead, mercury, and cadmium levels with constipation among US adults. This cross-sectional study used data from the National Health and Nutrition Examination Survey (NHANES) 2005-2010 and included 11,014 adults aged ≥20 years. Survey-weighted multivariable logistic regression models were constructed, with metal levels analyzed as log-transformed continuous variables and quartile categories based on the original concentrations. Restricted cubic spline (RCS) analyses were performed to assess dose-response relationships. Subgroup analyses, interaction tests, and sensitivity analyses were also conducted. In the fully adjusted model, blood lead and mercury levels were each significantly inversely associated with constipation. Each 1-unit increase in log-transformed blood lead was associated with 50.4% lower odds of constipation (OR = 0.496, 95% confidence interval [CI]: 0.331-0.745), whereas each 1-unit increase in log-transformed blood mercury was associated with 29.9% lower odds (OR = 0.701, 95% CI: 0.542-0.905). Blood cadmium showed no significant association. RCS analyses further confirmed linear inverse associations of both lead and mercury with constipation, whereas no significant association was identified for cadmium. A significant interaction was identified between educational level and the association between blood lead and constipation (P for interaction = .04). The sensitivity analyses yielded generally consistent results. Blood lead and mercury levels were significantly and linearly inversely associated with constipation among US adults, whereas blood cadmium showed no significant association. Educational attainment may modify the association between blood lead exposure and constipation. Further prospective and mechanistic studies are needed to clarify the nature and underlying mechanisms of these associations.