警惕大米重金屬污染
健康2014年05月05日
稻米特別易於從土壤中吸收有毒害的金屬。
Nicky Loh/Reuters
在過去這幾年裡，瑪麗·洛伊·格里諾特(Mary Lou Guerinot)一直在監測得克薩斯東南部的試驗田，了解大米作物從土壤里吸收金屬和其他讓人擔心的元素的情況。
她發現，如果稻田使用的是傳統的灌溉法，稻米會快速吸附砷。可如果為了控制砷而減少灌溉水源，稻米則會吸附鎘——這同樣是一種危險的元素。
格里諾特博士是達特茅斯學院(Dartmouth College)分子基因學家、生物學教授，她說：「對於我們在大米中是會測出砷還是鎘，這簡直成了道非此即彼的選擇題。」
她強調指出，在試驗田裡檢出的砷或鎘濃度，還不足以拉響警報。不過，這已經足以令像這樣的科學家驚覺：作為全球最廣泛消費的食物，大米同時還是自然界中最主要的金屬化合物吸附體。
已經有很多報告指出，從麥片棒到嬰兒食品的各類大米製品中檢出了砷，這令消費者已經產生了警惕。一些食品加工企業已開始主動篩查產品砷含量，而像美國食品藥品監督局(Food and Drug Administration)這類機構現在建議人們要確保所吃的穀物品種多樣化，好「將過量食用單一食品導致的潛在負面健康影響最小化」。
但令人憂心的還不僅僅是砷和鎘，這兩種物質既可能作為天然產生的元素出現在土壤中，也可能是工業副產品。近期的一些研究顯示，稻米擁有從土壤中積蓄所有金屬的能力，當中包括了汞甚至鎢。這些發現促使科學家和種植者們採取新一輪行動，努力培育對金屬污染不那麼敏感的穀物。
糙米所含的重金屬含量往往最高，這是因為像砷這類元素是在米糠和稻殼中積蓄，在白米的精加工過程中會被碾壓脫除。美國農業部預測，平均來說米糠中所含的砷含量可高達大米的10倍。
「大米會帶來問題，因為這是一種廣泛食用的穀物，」美國農業部農業研究服務方面的資深農學家魯弗斯·查尼(Rufus Chaney)是一個農業作物金屬積蓄調查項目的領導人。他說：「但它同時又是種十分吸引人的作物。」
跟人類一樣，植物也擁有佔據並吸收必需營養物質的系統。在植物中，這類「運輸」系統可用來從土壤中吸收鐵、鈣、鋅和錳這類礦物質。
稻穀作物有一套巧妙的系統，可以吸收硅化合物，或稱硅酸鹽，以此來幫助植株健壯，莖桿強韌。莖皮部將水溶營養物質推運到植株各部分，用來保證各個組織營養供應。
糟糕的是，砷化合物的結構與硅鹽相似，因此這種傳輸系統也使得水稻易於吸附砷化合物。同時，以傳統方法種植水稻，往往需要灌溉整片農田，這種方式更容易產生水解砷化合物亞砷酸鹽，並在水稻植株中傳輸。
「水稻作物的問題在於，它往往會在稻米，而非葉片或其他部位積聚砷，」普度大學(Purdue University)植物生物學家喬迪·班克斯(Jody Banks)研究的領域是植物中的砷積聚，她說：「而且吸附的速度極快。」
在產大米區，檢出砷濃度最高的區域為亞洲部分地區——包括孟加拉和印度——這些地區陸殼下層的基岩富含砷，會污染地下水，而這些水源既用於飲用，又用於稻田灌溉。
不過，較低濃度的砷已在世界各國，包括美國的土壤中檢出。據美國地質調查局(United States Geological Survey)的研究，密西西比河泛濫平原沖刷的肥沃土壤，其砷含量可高達路易斯安那、密西西比和阿肯色州其他地區的五倍。
正是基於此，同時考慮到保護水資源，科學家們曾嘗試過減少稻田灌溉水量。但正如格里諾特博士所發現的那樣，這種作法又會使得水稻吸附過量的鎘。
查尼博士指出，其他植物也會吸收鎘，通常採用的是從土壤中吸收鋅的那些管道。但有趣的是，稻米作物則是經由吸收錳的管道來吸附幾乎所有鎘。而這條管道——它是由一群堅韌的日本研究人員發現的——則會帶來一系列新問題。
鋅在土壤中相對常見，可溶性的錳則較少被人發現。因此在稻米作物的輸送系統中，鎘幾乎碰不到任何競爭對手——這意味着，它會十分帶勁地儲存這種金屬。
大米中的鎘和人類疾病之間的聯繫，可追溯到幾十年前。大部分科學家援引20世紀60年代日本出現的「痛痛病」(itai-itai disease)，相信這是可確認的最早病例。人體接觸鎘會出現大量病徵，其中一項是骨折，該病名就源於骨折帶來的痛苦。
研究人員後來發現，是礦井和其他工業帶來的鎘污染擴散到了稻田中，導致大米吸附了大量有毒金屬。類似的問題也曾在中國出現，引發了去年民眾對毒大米的抗議。
科學家表示，在美國土壤中自然存在的鎘濃度不足以引發急性疾病。但是，由於大米作為糧食作物的意義實在太重要了，科研人員一直在想辦法阻斷它吸附金屬的趨向。
研究人員已經嘗試培育將更多鋅和鐵輸送到大米中的稻米品種，這既能提高作物營養水平，又可降低毒性。此外目前還有其他項目正在進行，包括德克薩斯州的一項試驗，目標是培育不易吸收有毒礦物質的稻米品系。
同時，研究人員還在嘗試轉基因技術，通過精確設計稻米作物的傳輸系統來阻斷鎘或砷。
最終，他們在設法通過其他植物來減少土壤中的有毒成份，這個過程稱為植物提取。比方說，班克班博士正在研究用一種蕨類植物巧妙地吸附土壤中的砷，並儲存在其葉片中。
「你絕對不能吃它，」班克斯說。
本文最初發表於2014年4月19日。翻譯：學清
The Trouble With Rice
May 05, 2014
As a plant, rice is particularly prone to absorbing certain toxic metals from the soil.
Nicky Loh/Reuters
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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