The contribution of dietary components to the distribution and bioaccessibility of four unregulated metals in Chinese rice: Implications for health risk assessment and derivation of health risk-based thresholds

The absence of risk thresholds for unregulated metals allows contaminated rice to enter the market, posing potential health risks. This study collected 117 Chinese rice samples to evaluate how dietary components affect the distribution and bioaccessibility of four unregulated metals in rice grains, with implications for health risk assessment and derivation of health risk-based threshold. Sequential extraction analysis showed that 75.09–84.47% of unregulated metals were associated with protein-bound fractions. RF models identified protein-associated components as the dominant contributors to both metal accumulation (35.7–39.2%) and bioaccessibility (43.1–49.4%) prediction models, while other dietary components exerted metal-specific effects. Protein is associated with 73.36-80.24% of total health risks, with phytate and Ca is linked to 9.19–12.14% for Co and Ni, and dietary fiber contributing 12.14–12.64% for Mo and V. A significant negative correlation ( p < 0.01, r = −0.53 to −0.62) was observed between health risk-based thresholds and protein content, whereas a positive correlation was found with Ca content (r = 0.27–0.45), suggesting that rice with higher protein and lower Ca contents may require more stringent thresholds. These findings highlight the need to incorporate multiple dietary components into future health risk assessments and regulatory decision-making for unregulated metals.