Science 11 July 2008: Vol. 321. no. 5886, pp. 184 - 185
DOI: 10.1126/science. 321.5886. 184
News of the Week
BEIJING–Rice is the staff of life for 3 billion people, predominantly in Asia. But does the food that sustains half of humanity also increase the risk of cancer for some? That question arises from three sets of findings including data now in press that report elevated arsenic levels in rice and products such as rice bran and rice crackers.
Much of the arsenic found in these studies is in an inorganic form the oxides arsenate and arsenite known to sicken people exposed via drinking water. Cancer runs high in this population. “The problem is big,” says Steve McGrath, a biogeochemist at Rothamsted Research in Harpenden, U.K., who studies contaminants in crops and is familiar with the new findings. Because rice accumulates arsenic, he says, even the background level “is a problem for people who eat much rice in their diet.”
Experts caution that there are no data linking rice and cancer. Although there’s “a definite need to reduce arsenic levels in rice,” says Richard Loeppert, a crop scientist at Texas A&M University in College Station, “it’s not an immediate hazard.” A lead researcher, environmental biologist Zhu Yong-Guan of the Research Center for Eco environmental Sciences in Beijing, acknowledges that “we still don’t have all the answers.” “But arsenic is arsenic,” he says.
China agrees: It’s one of a handful of countries that regulate arsenic levels in food. In 2005, the government lowered the acceptable limit in rice from 700 to 150 micrograms (ìg) of inorganic arsenic per kilogram. Fish and other seafood contain an organic compound, arsenobetaine, that’s largely benign at dietary levels. In a guidance document issued for shellfish consumption in 1993, the U.S. Food and Drug Administration recommended a “tolerable daily intake” of inorganic arsenic of 130 ìg. But most governments, including the United States and the European Union, have not set legal limits on inorganic arsenic in food. The recent findings could provide an impetus for regulators to move faster.
Zhu and others are not waiting; they’re already exploring ways to defang rice, which contains at least 10-fold higher arsenic concentrations than wheat and other cereals. Possibilities include altering farm practices–growing paddy rice in raised beds, for instance–and engineering rice plants to shed arsenic. The task is urgent, some say, because the global food crisis is increasing rice cultivation near mines or smelters, or on land formerly used to grow cotton or other crops that are often heavily treated with arsenic-
based pesticides. Paul Williams, a postdoctoral researcher working with Zhu and Andrew Meharg, an environmental chemist at the University of Aberdeen, U.K., says, “We fear that more and more marginal land contaminated with arsenic will be used for growing rice.”
Inorganic arsenic in a single dose of about 100 milligrams can kill by shutting down energy metabolism. Its chronic, low-dose effects are more insidious and first came to light in the early 1980s in India and Bangladesh, where many people who relied on arsenic tainted wells developed arsenicosis, an ailment marked by rough skin that is often a prelude to serious diseases such as skin or bladder cancer. Tainted wells typically contain hundreds of micrograms of arsenic per liter, well above the maximum contaminant level of 10 ìg per liter set by the World Health Organization (WHO) and adopted by most countries. Regions with high natural arsenic levels have been trying to develop alternative water supplies (Science, 23 March 2007, p. 1659). “Billions of dollars are spent to decrease arsenic levels in water,” says Zhu. “Even if we solve that problem,” he says, “it still gets into rice.”
Paddy rice takes up arsenite readily from waterlogged soil, from which the element is liberated by anaerobic microbes, McGrath and colleagues reported online last month in Environmental Science & Technology. (Other crops grown in watery environments such as lotus, water chestnut, and water spinach also tend to have high arsenic levels.) WHO’s limit for arsenic in water equates to a daily intake of 10 ìg in food, Zhu, Williams, and Meharg note in an article this month in Environmental Pollution. Assuming an average daily rice consumption of 200 grams–a lowball estimate in Asia–the researchers calculate that arsenic levels would have to be as low as 50 ìg per kilogram to remain below the WHO limit for water. However, Zhu and his colleagues report, surveys around the world have found that arsenic levels in rice “commonly exceed” the 50-ìg threshold and can reach 400 ìg per kilogram.
Last April, Meharg’s group caused a sensation when it reported disturbing levels of arsenic in rice porridge sold in U.K. supermarkets for weaning infants. According to their findings in Environmental Pollution, 35% of baby rice samples they tested had arsenic levels exceeding China’s permissible level (sciencenow. sciencemag. org/cgi/content/ full/2008/ 430/1).
Two upcoming reports from the team could cause another stir. They found what Zhu calls “extremely high” levels of inorganic arsenic in rice bran, a common item in health food stores and a popular supplement for malnourished children in international aid programs. The samples of rice bran products they tested came from Japan and the United States. “Their research has shown the size of the problem and its international dimension,” says McGrath, who calls their analyses “state of the art.”
The food industry has sought to allay concerns. For example, after news reports last autumn about arsenic in U.S. rice, a top official at NutraCea, a company based in Phoenix, Arizona, that sells rice bran and bran extracts, in a letter to customers wrote that “the levels found in U.S. rice are well within food tolerances established by the Food and Drug Administration. U.S. rice has been consumed for over a hundred years with no reported human health problems.” Williams argues that “there are no standards” in the United States for permissible maximum amounts of inorganic arsenic in food.
Experts have floated several mitigation strategies. Arsenic levels are lower in rice from certain regions, including California and parts of India; rice from these sources could be blended with higher arsenic rice before sale. But blending “may be difficult in poor areas with little infrastructure and subsistence diets,” McGrath says. Another tack would be to tilt production toward upland rice, which is grown on dry land and absorbs far less arsenic than paddy rice. A third approach–growing paddy rice aerobically in raised beds–reduces the mobilization of soil arsenite and “can dramatically decrease arsenic transfer from soil tograin,” McGrath says. But that would require a fundamental change in farming practices in Asia.
One attractive possibility is to tweak rice’s metabolism. “Arsenic that accumulates in grain is effectively under genetic control,” say Zhu. A decade ago, a team led by Barry Rosen, a molecular biologist at Wayne State University in Detroit, Michigan, showed that a family of proteins called aquaglyceroporins transports arsenic and other metalloids across cell membranes. Building on that work, Thomas Jahn’s group at the University of Copenhagen in Denmark last month unveiled in BMC Biology an aquaglyceroporin subfamily, nodulin26 like intrinsic proteins (NIPs), in plants, including rice. It may be possible, says team member Gerd Bienert, to engineer plants to express NIPs that resist taking up arsenic–although that will be tricky, as NIPs facilitate the uptake of vital nutrients such as boron or silicon. Rosen’s lab hopes to target arsenic by engineering aquaglyceroporins to discriminate between metalloids.
Another approach pursued by Rosen and Zhu is to create transgenic rice equipped with a bacterialenzyme arsenite S adenosyl methyltransferase- -that converts inorganic arsenic to methylated species, including a volatile compound. “We propose that rice expressing the enzyme will volatilize arsenic, producing rice grains with reduced arsenic content,” Rosen says. Field testing will begin in China soon. Rosen, Zhu, and colleagues are also using conventional breeding techniques to select for cultivars that accumulate little arsenic. To be a hit on the farm, any new varieties will have to have decent yields: A hypothetical cancer risk pales in comparison with an empty stomach.
Monday, July 14, 2008
Science 11 July 2008: Vol. 321. no. 5886, pp. 184 - 185