How Does Contamination of Rice Soils with Cd and Zn Cause High Incidence of Human Cd Disease in Subsistence Rice Farmers
2015
Chaney, Rufus L.
Rice (Oryza sativa L.) grown on Zn mine waste contaminated soils has caused unequivocal Cd effects on kidney and occasional bone disease (itai-itai) in subsistence rice farmers, but high intake of Cd from other foods has not caused similar effects. Research has clarified two important topics about how Cd from mine waste contaminated rice soils has caused Cd disease: (1) bioaccumulation of soil Cd into rice grain without corresponding increase in Zn, and (2) subsistence rice diets potentiate Cd absorption/bioavailability and risk to farm families. Absorption of Cd by rice roots occurs on the NRAMP5 Mn²⁺ transporter. Although other transporters can influence Cd uptake-transport to shoots and grain, making NRAMP5 null greatly reduces grain Cd. Zn²⁺ has little ability to inhibit Cd²⁺ transfer in rice but clearly inhibits Cd uptake in other plant species. The bioavailability of dietary Cd is increased for subsistence rice diets. Research has identified that low levels and bioavailability of Zn and Fe in polished rice grain cause upregulation of Cd absorption on the Fe²⁺ transporter of duodenum cells (DMT1). Added dietary Zn can also inhibit intestinal Cd absorption somewhat. Nutritional stress (Fe, Zn deficiency) in humans consuming subsistence rice diets thus promotes Cd accumulation and adverse effects. No other dietary (crop) Cd exposure has caused unequivocal Cd-induced renal proximal tubular dysfunction (the first adverse Cd effect) in humans. Recognition of the very unusual nature of Cd risk from rice compared to other crops should be taken into account in setting international limits of Cd in rice and other foods.
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