Li et al. 2024 — Selenium and cadmium in rice from Chinese black shale exposure areas
This cross-sectional study measured cadmium (Cd) and selenium (Se) in rice, soil, drinking water, blood, and urine from residents of two black shale exposure areas in Hubei Province, China: Enshi County (Permian black shale) and Zhuxi County (Cambrian black shale). The primary research question was whether the putative antagonistic effect of Se on Cd toxicity observable in animal models also protects human liver and kidney function in populations chronically co-exposed to elevated Se and Cd. The study reports Cd concentrations in locally grown rice (n=90 from Enshi, n=78 from Zhuxi including local and ecdemic commercial rice), with mean and range values indicating substantial Cd contamination in locally grown rice from both areas. Sensitive liver and kidney biomarkers in study participants were within healthy reference ranges despite markedly elevated blood Cd (B-Cd) and urine Cd (U-Cd), suggesting a possible Se-Cd antagonism. The authors conclude that USEPA risk assessment methods may underestimate the protective effect of co-occurring Se in geological high-Se areas.
Key numbers
Enshi County local rice (n=90, ICP-MS, Cd in µg/g): Max 2.488, Min 0.003 (Table 2 in paper). Enshi County surface soil Cd: Max 17.29 µg/g, Min 0.84 µg/g. Blood Cd (B-Cd) medians: Enshi 4.821 µg/L, Zhuxi 3.848 µg/L. Urine Cd (U-Cd) medians: Enshi 7.750 µg/L, Zhuxi 7.050 µg/L. These B-Cd and U-Cd values substantially exceed background levels in non-exposed populations (typical adult B-Cd below 0.5 µg/L). Zhuxi County context: local rice production banned from 2022 due to Cd contamination; study captures the transitional period when some residents were still consuming pre-ban local rice.
Analytical methods: ICP-MS for Cd in rice, soil, blood, urine; AFS for Se. LOD for Cd in rice: 0.001 µg/g. Digestion: microwave digestion (MARS6) with 65% HNO3 for biological and food matrices.
Note: the rice Cd data from Enshi represents an extreme high-contamination geological scenario (natural black shale geochemistry, not primarily from industrial or agricultural Cd input). Maximum observed rice Cd of 2.488 µg/g is far above the Chinese standard of 0.2 mg/kg for rice (GB 2762) and above the Codex ML of 0.4 mg/kg for polished rice.
Methods (brief)
Cross-sectional community survey. Study population: long-term residents (>30 years), middle-aged and elderly, no occupational Cd/Se exposure. Rice and soil sampling: 90 rice samples from Enshi arable plots; 78 from Zhuxi (41 local, 37 ecdemic commercial). Blood and urine: venous blood (60 mL) and morning urine from 321 participants. Biochemical liver/kidney biomarkers: automated biochemical analyzer. Statistical analysis: IBM SPSS 20.0; kriging interpolation for geochemical mapping.
Implications
Certification: Documents extreme Cd values in rice grown on black shale soils in Hubei Province China; these values are geological background contamination, not supply-chain management failures. Relevant to any certification program sourcing Chinese rice without provenance to non-black-shale origins.
Courses: Provides a case study on how geological background can drive food Cd contamination at levels orders of magnitude above regulatory limits, independent of agricultural practices.
App: Contributes to geographic_breakdown on rice ingredient page; China (black shale regions, Hubei) represents a distinct extreme-upper-range subpopulation for rice Cd. App should not use these values to characterize typical Chinese rice Cd; they are geological outliers.
Microbiome: Chronic high Cd and Se exposure at these levels would be expected to have significant gut microbiome effects (metal-microbiome mechanisms documented elsewhere in corpus), though this paper focuses on liver/kidney biomarkers rather than gut health.