Lower Cadmium Bioavailability and Toxicity in Japonica Rice than in Indica Rice: Mechanisms and Health Implications

Cadmium (Cd) is efficiently transferred from soil to food crops, notably rice. Research indicates that indica rice grains may accumulate more Cd than japonica cultivars; however, differences in Cd bioavailability (the fraction of ingested rice Cd absorbed into the systemic circulation) and toxicity remain unexplored, thus hindering a comprehensive understanding of exposure and health risks. To address this, a mouse bioassay was conducted to evaluate the relative bioavailability (RBA) of Cd in 35 samples each of japonica and indica rice, determining which type exhibits lower Cd bioavailability. The results revealed a significantly lower mean Cd-RBA in japonica rice (49.6 ± 7.8%) compared to indica rice (65.6 ± 12.2%). This disparity is attributed primarily to the 1.25- and 1.37-fold higher concentrations of calcium (Ca) and iron (Fe) in japonica rice, which enhanced Ca and Fe absorption from the intestine and reduced duodenal expression of Ca and Fe transporters by 1.8-5.9 times in mice consuming japonica rice, thereby decreasing Cd transcellular transport. Additionally, japonica rice consumption promoted beneficial gut probiotics (Bifidobacterium pseudolongum and Lactobacillus reuteri) and metabolites, particularly short-chain fatty acids and peptides, potentially increasing mineral absorption and reducing Cd uptake. Moreover, mice fed japonica rice exhibited 1.35-1.47 times higher gene expression of intestinal tight junctions, enhancing intestinal barrier function and reducing extracellular Cd transport. Consequently, consuming Cd-containing japonica rice was associated with lower oxidative stress, inflammation, and cancer risks in mice compared to indica rice consumption. This study significantly enhances our understanding on the health risks associated with Cd in different rice subspecies.