- Co-occurrence of As and F was found in water and urine samples.
- Interactions between the exposure of As and F on arsenic metabolism in humans were found.
- Interaction terms show increased MAs% and decreased DMAs% and As methylation indices.
- F exposure may modify the As metabolism profile, in low-moderate As exposure levels.
Arsenic (As) and fluoride (F) are two common groundwater toxicants. The toxicity of As is closely related to As metabolism, and several biological and environmental factors have been associated with As modification. However, limited information about the effect of F exposure on the modification of the As metabolism profile has been described. The aim of this study was to assess the interaction effect of AsF coexposure on the As metabolism profile in an adult population environmentally exposed to low-moderate As levels. A cross-sectional study was conducted in 236 adults from three Mexican communities. F and As concentrations were quantified in water samples. The concentrations of urinary F and As species [inorganic arsenic (iAs), monomethylated arsenic (MAs) and dimethylated arsenic (DMAs)] were also determined and used as exposure biomarkers. As species percentages and methylation indices were estimated to evaluate the As methylation profile. Our results showed a relationship between the water and urine concentrations of both contaminants and, a significant correlation between the As and F concentrations in water and urine samples. A statistically significant interaction of F and As exposure on the increase in MAs% (??=?0.16, p?=?0.018) and the decrease in DMAs% (??=?-0.3, p?=?0.034), PMI (??=?-0.07, p?=?0.052) and SMI (??=?-0.13, p?=?0.097) was observed. These findings indicate that drinking water is the main source of AsF coexposure and suggest that F exposure decreases As methylation capacity. However, additional large and prospective studies are required to confirm our findings, and to elucidate the involved mechanisms of interaction and their implications in adverse health effects.