Submitted to: International Symposium on Health Impacts of Cadmium Exposure In China
Publication Type: Abstract Only
Publication Acceptance Date: September 12, 2003
Publication Date: November 17, 2003
Citation: Chaney, R.L., Reeves, P.G., Ryan, J.A., Simmons, R.W., Welch, R.M. 2003. An improved understanding of soil Cd risk to humans [abstract]. Proceedings of the International Symposium on Health Impacts of Cadmium Exposure in China. p. 49-51.
It is well established that long-term consumption of rice grown in flooded soils which were contaminated by geogenic Zn+Cd (mining and smelting wastes) causes high prevalence of renal tubular dysfunction. However, consumption of vegetable crops grown on similar aerobic soils, or shellfish which were enriched in Cd from geogenic Zn+Cd has not caused human Cd disease at many locations where soils were 10-fold more Cd contaminated than rice soils which caused Cd disease in Japan and China. Our work has shown that several unusual properties of subsistence rice farming likely cause the greater risk found from Cd in rice: 1) When rice is grown on flooded Zn+Cd contaminated soils, grain Cd can be high but grain Zn is not increased above that on control soils even when soils contain over 5000 mg Zn and 50 mg Cd/kg; 2) Because flooding soils raises soil pH and changes Zn to forms not absorbed by rice, rice does not suffer Zn-phytotoxicity while garden crops and other staple grains grown on aerobic soils both contain Zn and suffer yield loss from Zn phytotoxicity if soil pH is acidic; 3) subsistence rice farmers commonly consume the rice grown on their own land ('home-grown') for their lifetime but few other persons consume such a high fraction of staple foods grown on one contaminated farm; and 4) because polished rice is deficient in Zn and Fe for humans, and these elements strongly counteract Cd absorption in animals, subsistence rice consumers absorb a much higher fraction of dietary Cd than humans exposed to other foods rich in both Zn and Cd and not so deficient in Fe. The Zn and Fe deficiency in these populations is a serious public health problem independent of Cd considerations. Our most recent studies of the effect of consuming rice diets (0.2 mg Cd/kg diet) with marginal vs. adequate Zn-Fe-Ca examined the kinetics of excretion of intestinal Cd. Because of the marginal minerals, a high fraction of dietary Cd is absorbed into enterocytes (80%). The Cd remained in the short-lived enterocytes for up to about 16 days (turnover pool), and part was transferred to tissues. Thus, besides the direct effect of marginal Zn-Fe-Ca diets on absorption of Cd into enterocytes, prolonged duodenal turnover of Cd allows even larger increase in tissue Cd in persons with marginal diets. And only rice allows the combination of such severe malnutrition of Zn and Fe from grains grown on contaminated soils. We believe the implications of this improved understanding of soil Cd risk to humans suggest methods to prevent the adverse effects of soil Cd on rice consumers. Because the rice-induced Zn-Fe malnutrition is so important in absorption of Cd into tissues, provision of Zn-Fe supplements could reduce the net absorption of Cd by as much as 10-fold within days of starting the regular supplementation. This would be much less expensive than exchanging rice, and soil remediation could be undertaken after more deliberate evaluation. An epidemiologic test could be conducted in a short time because blood-Cd would be expected to rapidly drop from such a supplement. Remediation of soil Cd risk has been conducted by removal and replacement of the contaminated soil depth, at great expense. Phytoextraction of soil Cd using newly recognized Cd hyperaccumulator plants appears to offer a 100-fold lower cost method to remove soil Cd without the disruption and expense of soil removal. Plant breeding is needed to improve the wild plants into commercial phytoextraction cultivars before large scale soil remediation could begin. But if risk from the soil Cd is prevented by Fe-Zn-supplements (likely with Ca and Vitamin A, which are often too low in subsistence rice diets), humans will be protected during the soil remediation period.