Arsenic speciation varies with type of rice

New studies suggest that U.S. rice contains more methylated arsenic, a less toxic form of the metal, than rice from Europe and Asia does.

Rice can be grouped into two types, depending on the form of arsenic in the grain, says Yamily Zavala, a research associate in John Duxbury’s laboratory at Cornell University. In two new papers published in ES&T DOI 10.1021/es702747y; 10.1021/es702748q), the researchers report that as arsenic levels rise, U.S. rice contains more methylated arsenic, the less toxic form, whereas rice grown in Europe and Asia contains the more toxic, inorganic arsenic.

Zavala noticed a trend in her data from a market-basket study of U.S. rice. When rice contained low levels of arsenic, the dominant form was inorganic arsenite. As arsenic concentrations increased, the dominant form became dimethyl arsinic acid (DMA).

Intrigued, Zavala examined the literature and saw the same pattern in other studies of U.S. rice. Indeed, rice researcher Andrew Meharg of the University of Aberdeen (U.K.) and collaborators noted previously that the amount of DMA is dependent on the rice cultivar and that DMA is the predominant arsenic species in U.S. rice (Environ. Sci. Technol. 2005, 39 [15], 5531–5540).

However, when Zavala and Duxbury examined worldwide speciation data, they discovered a second rice population—one dominated by inorganic arsenic, even in rice with high arsenic levels. They grouped all rice into two types: inorganic arsenic-type and DMA-type.

The researchers believe it likely that DMA-type rice transforms arsenic by methylation, as do bent grass, humans, and microbes. When inorganic arsenic is present in the soil solution, the roots take it up preferentially, says Duxbury, who thinks it unlikely that DMA-type plants take up substantial quantities of DMA or MMA (monomethylarsonic acid) from soil.

The topic of arsenic in U.S.-grown rice has caused discord between Meharg and the USA Rice Federation. New studies may provoke more controversy as Meharg reports on levels of inorganic arsenic in rice milk and baby-food rice purchased in the U.K.

Previously, Meharg and co-workers reported higher levels of arsenic in rice from the south central U.S. than in rice from California (Environ. Sci. Technol. 2007, 41 [7], 2075–2076; 2178–2183). They speculated that arsenic in south central U.S. rice may have originated from pesticides in soil previously used to grow cotton. Zavala and Duxbury confirmed these results, finding especially high arsenic levels in rice from one Texas supplier.

Zavala and Duxbury analyzed rice obtained from several different countries and combined their data with literature values to yield what they label a global “normal” range of 0.08–0.20 milligrams per kilogram (mg/kg) arsenic for rice. Because of the higher arsenic levels in rice from Texas, the mean for U.S. rice was 0.198 mg/kg, identical to the mean for European rice and substantially higher than that for Asian rice (0.07 mg/kg).

USA Rice Federation spokesperson David Coia says that the papers by Zavala et al. “bring much-needed balance to the discussion by considering speciation. That U.S. rice may be safer than rice from Asia and Europe is a message we hope resonates clearly from these publications.”

“It’s important to remember that arsenic is a ubiquitous element in soils and is found in all grains worldwide,” emphasizes Coia. “U.S. rice remains a safe and wholesome commodity and a highly valued product in markets worldwide,” he adds.

Rice breeder Steven Linscombe of Louisiana State University agrees with Coia. “This research has verified that arsenic levels vary in rice, depending upon environmental conditions such as soil, irrigation water, growing conditions, and specific variety. Most importantly, this research has confirmed that while arsenic is detectable at very low levels in U.S.-produced rice, it is of a form and at such low levels that it presents no health risks.”

If some rice can methylate arsenic, can that ability be transferred to other rice varieties through conventional plant breeding or genetic techniques, asks Duxbury. He suggests that the best way to answer this question is by collaborating with rice producers and breeders.

“The DMA rice type could have come from breeding programs where rice has been selected based on its resistance to straighthead disorder, which causes yield reduction due to blank florets. It could be possible that the new resistant cultivars were able to metabolize arsenic as a detoxification pathway and accumulate it in the grain without affecting grain filling,” explains Zavala. U.S. rice breeders have been breeding for resistance to straighthead for more than 30 years, says Duxbury. —BARBARA BOOTH