ISIS Report 13/09/04
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.
A
fully referenced
version of this article is posted on ISIS members’ website.
Details here.
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.
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. 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, the iron was more readily
available during digestion. The Swiss study was supported by the Rockefeller
Foundation.
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. There is an
association between increasing iron stores and risk of cancer.
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 and in the Red River Delta of
Vietnam but it is also a chronic problem in Taiwan, China and Thailand. 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. 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. It has been estimated that the
arsenic pollution of drinking water in the United States causes an average of
3000 cancer cases per year.
In Asia, the arsenic problem is amplified by the pollution of rice, the
primary food source. Arsenic has been accumulating in paddy soil, resulting in
the contamination of the rice grain. Rice contributes to an estimated 30 to 60%
of the dietary intake of arsenic in polluted regions.
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.
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. 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.
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