Rice Milling Enhances Cadmium Bioavailability via Nutrient Loss and Transporter Activation

Rice polishing enhances sensory appeal but removes essential nutrients, potentially influencing the bioavailability of toxic elements like cadmium (Cd). This study investigated how rice milling alters Cd accumulation and intestinal transport mechanisms. Twelve rice samples from a Cd-contaminated region were milled into paired brown and polished forms and fed to mice. Despite only a 13.1% average reduction in total Cd, polished rice resulted in 1.16-4.90-fold greater Cd accumulation in kidneys and livers. A gradient milling experiment confirmed that increased milling intensity led to 18.5-96.2% and 16.7-141.6% higher Cd accumulation in kidneys and livers, respectively. Supplementation with Ca, Fe, Zn, or low-Cd bran reduced Cd accumulation by up to 46.1% and downregulated key intestinal transporters (CaT1, DMT1, ZnT1) by 36.9-96.5%. A ¹⁰⁸Cd isotope tracing experiment demonstrated that transporter upregulation, induced by long-term mineral deficiency, was the primary driver of increased Cd bioavailability, exceeding the effects of acute mineral intake differences. These findings provide mechanistic evidence that rice polishing enhances Cd absorption by depleting micronutrients and stimulating metal transport pathways. Considering the global reliance on polished rice, this work highlights the potential nutritional and toxicological trade-offs associated with rice processing.