• Rice soaked in water accumulates fluoride by absorption and adsorption
• Boiling food in fluoride-containing water highly increases fluoride content in food
• Fluoride interferes with the iodine staining of starch contained in rice
• Infants ingest more fluoride per body weight than do children and adults
• Re-evaluate fluoride guidelines based on the local food and cooking methods
Although drinking water is the main source of fluoride intake, recent studies reported that fluoride intake from foods could also be high, depending on cooking methods. In this study, we quantified the fluoride accumulation in foods soaked or boiled in fluoride-containing water and assessed the fluoride intake in different age groups from food and drinking water. We observed that, in the case of rice soaked in fluoride-containing water, more fluoride was accumulated in the rice than previously estimated. Fluoride interferes with the iodine staining process of rice, indicating fluoride adsorption. Fluoride accumulation in rice and vegetables increased when the soaking temperature was raised to 100 °C due to the gelatinization of rice grains and softening of vegetables. Ingesting foods boiled in fluoride-containing water increased the fluoride intake per body weight of infants more significantly than that in children and adults due to their low body weight. These results indicate that soaking and boiling foods in fluoride-containing water significantly increases fluoride intake compared to previous estimations. Therefore, it is necessary to re-evaluate the fluoride intake from food and drinking water considering the methods used for cooking food in each country and region.
*Original full-text article online at: https://www.sciencedirect.com/science/article/pii/S0304389422019562