The Trouble With Rice
Nicky Loh/ReutersAs a plant, rice is particularly prone to absorbing certain toxic metals from the soil.
For the past few years, Mary Lou Guerinot has
been keeping watch over experimental fields in southeast Texas,
monitoring rice plants as they suck metals and other troublesome
elements from the soil.
If the fields are flooded in the traditional
paddy method, she has found, the rice handily takes up arsenic. But if
the water is reduced in an effort to limit arsenic, the plant instead
absorbs cadmium — also a dangerous element.
“It’s almost either-or, day-and-night as to
whether we see arsenic or cadmium in the rice,” said Dr. Guerinot, a
molecular geneticist and professor of biology at Dartmouth College.
The levels of arsenic and cadmium at the
study site are not high enough to provoke alarm, she emphasized. Still,
it is dawning on scientists like her that rice, one of the most widely
consumed foods in the world, is also one of nature’s great scavengers of
metallic compounds.
Consumers have already become alarmed over
reports of rice-borne arsenic in everything from cereal bars to baby
food. Some food manufacturers have stepped up screening for arsenic in their products, and agencies such as the Food and Drug Administration now recommend that people eat a variety of grains to “minimize potential adverse health consequences from eating an excess of any one food.”
But it’s not just arsenic and cadmium, which
are present in soil both as naturally occurring elements and as
industrial byproducts. Recent studies have shown that rice is
custom-built to pull a number of metals from the soil, among them mercury
and even tungsten. The findings have led to a new push by scientists
and growers to make the grain less susceptible to metal contamination.
The highest levels often occur in brown rice,
because elements like arsenic accumulate in bran and husk, which are
polished off in the processing of white rice. The Department of
Agriculture estimates that on average arsenic levels are 10 times as
high in rice bran as in polished rice.
Although these are mostly tiny amounts — in the part per billion range — chronic exposure to arsenic, even at very low levels, can affect health. The F.D.A. is now considering whether a safety level should be set for arsenic in rice.
“Rice is a problem because it’s such a widely
consumed grain,” said Rufus Chaney, a senior research agronomist with
the U.S.D.A.’s Agricultural Research Service, who is leading a
investigation of metal uptake by food crops. “But it’s also a
fascinating plant.”
Like people, plants have systems for taking
up and absorbing necessary nutrients. In plants, these “transporter”
systems work to pull minerals such as iron, calcium, zinc and manganese
from the soil.
The rice plant has a well-designed system for
taking up silicon compounds, or silicate, which help strengthen the
plant and give stiffness and shape to its stems. Tissues generally
referred to as phloem move such water-soluble nutrients throughout the plant.
But that delivery system also inclines the
plant to vacuum up arsenic compounds, which are unfortunately similar in
structure to silicate. And the traditional methods of growing rice,
which often involve flooding a field, encourage formation of a soluble
arsenic compound, arsenite, that is readily transported by the rice plant.
“The issue with the rice plant is that it
tends to store the arsenic in the grain, rather than in the leaves or
elsewhere,” said Jody Banks, a plant biologist at Purdue University, who
studies arsenic uptake in plants. “It moves there quite easily.”
The highest concentrations of arsenic in
rice-growing regions are mostly found in parts of Asia — including
Bangladesh and India — where the underlying arsenic-rich bedrock contaminates groundwater used for both drinking and irrigation of rice fields.
But arsenic at lower levels is found in all
soils, including American fields. The fertile soils fanning out across
the Mississippi River floodplain are up to five times as high in arsenic as other parts of Louisiana, Mississippi and Arkansas, according to studies done by the United States Geological Survey.
It’s for that reason, as well as for water
conservation, that scientists have experimented with reducing the amount
of water used for rice fields. But as Dr. Guerinot has found, that
makes cadmium more available to the plant instead.
Other plants also take up cadmium, Dr. Chaney
noted, usually by the channels normally used to acquire zinc from the
soil. But the rice plant, curiously, absorbs nearly all of its cadmium
through a manganese transport system. And this route — discovered by a
determined group of Japanese researchers — brings a new set of
complications.
While zinc is relatively common in soil,
soluble manganese is less readily found. So cadmium has little
competition in the rice plant’s transport system — meaning that it is
accumulated with apparent enthusiasm.
The association between cadmium in rice and
human disease goes back decades. Most scientists cite the
identification of itai-itai (ouch-ouch) disease in Japan during the
1960s as the first recognition of this problem. The name comes from the painful effects of bone fractures, one of many health problems related to cadmium exposure.
Researchers eventually discovered that
cadmium pollution from mines and other industry had spread into rice
farming areas in Japan, causing the grain to be loaded with the toxic
metal. A host of similar problems have occurred in China, setting off an uproar over tainted rice last year.
Scientists say that the cadmium occurring
naturally in American soil is not high enough to cause acute disease.
Still, because rice is such an important food crop, scientists are
searching for ways to block its metal-acquiring tendencies.
There are efforts to breed rice plants that transfer more zinc and iron into the grain,
which would both increase nutritional quality and reduce toxicity.
There are also programs, including the experiment in Texas, that try to
breed improved rice cultivars less prone to absorb toxic minerals.
And researchers have explored the idea of
genetic engineering to make the plant’s transport systems more precise
so that cadmium or arsenic is filtered out.
Finally, they are looking into using other plants to reduce the toxic elements in the soils themselves, a process called phytoextraction. Dr. Banks, for instance, is studying a fern that deftly pulls arsenic from the soil and stores it in the fronds.
The plant, known as a Chinese brake or ladder fern, is so talented in this regard
that the Chinese have approached American scientists about the
feasibility of using it to clean up contaminated soils. Of course the
ferns eventually have to be incinerated or taken to a toxic disposal
site.
“You definitely wouldn’t want to eat them,” said Dr. Banks.
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