Non-monotonic relationships between arsenic and selenium excretion and its implication on arsenic methylation pattern in a Bangladeshi population

The toxicity of arsenic differs markedly between individuals and populations, which might be related to the metabolism (methylation) of inorganic arsenic (As), as well as the selenium (Se) nutritional status. Urinary excretion of As (u-As) and Se (u-Se) was examined in an adult population (n=128) living in an As-contaminated area in Bangladesh. Although there was a significant negative correlation between u-Se and u-As (median 137; range 49–927μg/g creatinine), closer examination revealed a non-monotonous relationship between them. A quadratic curve with an axis of As at 155μg/g Cre gave a better fit, and u-As and u-Se were positively or negatively correlated depending on whether the As concentration was lower or higher than 155μg As/g Cre, respectively. Likewise, the relationships between the As methylation pattern and glutathione-S-transferase (GST) polymorphism, body mass index (BMI), and u-Se differed depending on the u-As range; i.e., higher or lower than 155μg/g Cre. Although we did not determine the causal mechanism for these observations, the non-monotonic relationship between As exposure and the variables examined suggested the existence of a threshold at which the handling of As by human body is qualitatively changed. The possible importance of Se nutrition for As toxicity is also discussed. •Bangladeshi population exposed to As through groundwater participated in this work.•Relation between urinary As and Se was non-monotonic, a quadratic curve with upward convex fitted better.•Relation between As methylation and genetic/physiological factors depended on As exposure level.•The non-monotonic relationships may suggest a threshold for change in handling of As.