Science in Society Archive

Rice in Asia: Too Little Iron, Too Much Arsenic

Asians are getting too little iron and too much arsenic from soil and water. Unfortunately the remedy for one problem may increase the impact of the other. The challenge is to find a remedy that takes care of both problems, says Prof. Joe Cummins.

The problem of too little iron

It has been estimated that 40% of the world's women suffer some degree of iron deficiency. Anaemia is associated with learning difficulties in children, increased susceptibility to disease and reduced work capacity [1]. Pre-menopausal women are most severely affected by iron deficiency, while men tend to retain iron (as indicated below, an iron overload diet may increase risk of cancer in males). Increasing iron in the diet is a desirable goal and rice is the preferred crop for genetic modification (GM) to increase iron in the diet, especially in Asia.

Researchers from the Japanese Electrical Power Research Institute increased the iron content of rice threefold by adding a seed-specific ferritin (an iron storage protein) from soybean under the control of a rice seed storage protein promoter [2]. But although the iron content of the rice grain was increased significantly, there has been concern that the ferritin-bound iron may not be readily available in the digestive tract of mammals.

A Swiss research group transformed rice with a ferritin gene from snap beans under the control of a rice storage protein promoter accompanied by a fungal phytase gene also under the control of the storage protein promoter. The phytase gene produces an enzyme that increased iron availability during digestion. An endogenous rice metallothionein (a ubiquitous metal-binding cellular protein) was over-expressed in the transgenic rice to further aid in iron digestion by providing a form of iron readily taken up in the gut. An antibiotic resistance marker gene for the antibiotic hygromycin was added during the transformations of the rice. The iron content of the rice was doubled while, in contrast to the Japanese study [2], the iron was more readily available during digestion [3]. The Swiss study was supported by the Rockefeller Foundation [4].

However, iron overload is a significant problem in males - it may lead to a condition called hemochromatosis in which the liver and other organs may be damaged, causing liver cancer or colorectal cancer. As much as one person in a hundred may carry a mutation (hereditary hemochromatosis) that makes them sensitive to iron overload at relatively modest iron intake levels [5]. There is an association between increasing iron stores and risk of cancer [6].

In areas of the world where iron deficiency is commonplace, iron-enriched rice may prove beneficial, but the same iron-enriched rice could prove to be a liability in areas where iron intake is at high levels. Iron overload should be considered in the distribution of iron-enhanced rice. The need for labeling of iron rich rice products is evident.

The Arsenic Problem

Asia is facing a growing crisis in the use of arsenic-contaminated ground water for drinking and in irrigation of rice paddies. Arsenic pollution is a severe problem over Bangladesh/West Bengal [7)] and in the Red River Delta of Vietnam [8] but it is also a chronic problem in Taiwan, China and Thailand [7]. Most arsenic pollution is of natural origin, amplified by drawing water from contaminated deep aquifers, but China has arsenic pollution from burning high arsenic-containing coal [9]. Arsenic has been shown (from studies in Taiwan) to cause cancer and circulatory problems at very low levels, the cancers include cancers of liver, lung, bladder and kidney [10]. It has been estimated that the arsenic pollution of drinking water in the United States causes an average of 3000 cancer cases per year [10].

In Asia, the arsenic problem is amplified by the pollution of rice, the primary food source [7,8,11,12]. Arsenic has been accumulating in paddy soil, resulting in the contamination of the rice grain [11]. Rice contributes to an estimated 30 to 60% of the dietary intake of arsenic in polluted regions [11].

There is hope that rice strains can be selected that take in less arsenic than the varieties of rice currently in use. It has been found that arsenic is sequestered on iron-plaques (rust-like deposits) on the surface of roots of rice varieties that accumulate reduced levels of arsenic in grain [11,12]. Rice paddies will continue to be polluted with arsenic in the soil because there is no practical method known to remediate the vast expanses of polluted soil. Breeding rice to reduce grain pollution seems to be an effective first step towards improving the diet in polluted areas and varieties with reduced grain content of arsenic are known.

Iron and arsenic interact in rice

There is a potential conflict in governmental and foundation programmes to develop and disseminate high-iron grain to alleviate iron-deficiency among rice consumers. The high- iron rice varieties currently under development include amplifying the expression of ferritin in grain and solubilising iron for uptake in the gut using a phytase gene from a fungus [3]. Arsenic reduced the concentration of iron in the plant in rice varieties that form iron-plaques on the roots; but in varieties lacking the iron-plaques, iron uptake was not reduced in the presence of arsenic [11,12]. It appears that the iron-plaques sequester both iron and arsenic, so that both iron and arsenic are reduced in the rest of the plant.

The iron-enhanced grains designed to combat iron-deficiency are therefore, very likely to increase grain-arsenic levels in arsenic-polluted areas of Asia because the arsenic will not be sequestered on the root surface in iron plaques but instead will be taken into the shoot and end up in the rice grain. It seems a devil's bargain: either to make high-iron rice available at the cost of elevated arsenic or to make low-arsenic rice available without providing an alternate source of dietary iron.

But this dilemma only exists if one insists on GM rice as the only solution. It disappears instantly when one realizes that iron can be provided through other sources, such as beans and lentils which can easily be grown, and are rich sources of other essential nutrients besides.

Article first published 13/09/04



References

  1. Biotechnology used to fortify rice with iron. Information systems for biotechnology, pp1-2 http://www.nbiap.vt.edu/articles/dec9702.htm
  2. Goto F, Yoshihara T, Shigemoto N, Toki S. and Takaiwa F Iron fortification of rice seed by the soybean ferritin gene. Nature Biotechnology 1999. 17, 282-7.
  3. Lucca P, Hurrell R. and Potrykus I. Fighting iron deficiency anemia with iron-rich rice. Journal of the American College of Nutrition 2002, 21, 184S-190S.
  4. Conway G. Crop Biotechnology: Benefits, Risks and Ownership 2004 http://www.rockfound.org/display.asp?context=1&Collection=4&DocID=141&Preview=0&ARCurrent=1
  5. Haung X. Iron overload and its association with cancer risk in humans:evidence for iron as a carcinogenic metal Mutation Research 2003, 533,153-71.
  6. Toyokuni,S. Iron induced carcinogenesis :the role of redox regulation. Free Radical Biology & Medicine 1996, 20,553-66.
  7. Meharg A. and Rahman M. Arsenic contamination of Bangladesh paddy field soils: implication for rice contribution to arsenic consumption 2003 Environ Sci Technol 2003, 17, 229-34.
  8. Berg M, Tran H, Nguyen T, Pham H, Schertenleib R and Giger W. Arsenic contamination of groundwater and drinking water in Vietnam: a health threat Environ Sci Technol 2001, 35, 2621-7.
  9. Liu J Zheng B, Aposhian H, Zhou Y, Chen M, Zhang A. and Waalkes M. Chronic arsenic poisoning from burning high-arsenic-containing coal in Guizhou, China Environmental Health Perspectives 2002, 110, 119-22.
  10. Morris R. Environmental Health Issues 1995 Environmental Health Perspectives 103 (suppl 8) 1-14
  11. Meharg A. Arsenic in rice-understanding a new disaster for south-east Asia. Trends in Plant Sciences 2004, in press.
  12. Lui W, Zhu Y, Smith E and Smith S. Do phosphorous nutrition and iron plaque alter arsenate (As) uptake by rice seedlings in hydroponic culture? New Phytologist 2004, 162,481-8.

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