The role of fingernail selenium in the association between arsenic, lead and mercury and child development in rural Vietnam: a cross-sectional analysis

Background: Arsenic, lead and mercury are common environmental contaminants in low- and middle-income countries. We investigated the association between child toxicant exposure and growth and development and determined if this association was mitigated by selenium concentration. Methods: Toxicant concentrations in fingernail samples, anthropometry, and Bayley's Scales of Infant Development, 3rd edition (BSID) domains were assessed in 36-month-old children whose mothers had been part of a randomised controlled trial in rural Vietnam. Multivariable regression analyses were performed to estimate the effect of toxicant exposure on clinical outcomes with adjustments for potential confounders and interaction with fingernail selenium concentration. Findings: We analysed 658 children who had data for at least one physical or developmental outcome, and at least one toxicant measurement, and each of the covariates. Fingernail arsenic concentration was negatively associated with language (estimate per 10% increase in arsenic: -0.19, 95% CI: [-0.32, -0.05]). Lead was negatively associated with cognition (estimate per 10% increase in lead: -0.08 [-0.15, -0.02]), language (estimate per 10% increase in lead: -0.18 [-0.28, -0.10]), and motor skills (estimate per 10% increase in lead: -0.12 [-0.24, 0.00]). Mercury was negatively associated with cognition (estimate per 10% increase in mercury: -0.48, [-0.72, -0.23]) and language (estimate per 10% increase in mercury -0.51, [-0.88, -0.13]) when selenium concentration was set at zero in the model. As selenium concentration increased, the negative associations between mercury and both cognition and language scores were attenuated. There was no association between toxicant concentration and growth. Interpretation: Arsenic, lead and mercury concentrations in fingernails of 3-year-old children were associated with lower child development scores. The negative association between mercury and neurological development was reduced in magnitude with increasing selenium concentration. Selenium status should be considered when assessing heavy metal toxicants in children and their impact on neurodevelopmental outcomes.