Studies outline toxic metals found in rice and spinach, researchers call for further investigation

10 Sep 2024 --- Toxic metals and metalloids found in rice and spinach can cause severe health impacts to babies and young children. In particular, heavy metals such as cadmium, lead, mercury and metalloid arsenic could delay brain development in babies and young children.

In new research published in the academic journal Environmental Geochemistry and Health, University of Delaware (UD) scientists have found that flooded rice fields tend to contain higher amounts of arsenic and lower amounts of cadmium. The drier those rice fields are, the lower the amounts of arsenic and the higher the amounts of cadmium. However, the higher cadmium is lower than the existing threshold for adverse health effects.

The findings could help establish a course of action for decreasing the levels of these contaminants in foods typically eaten by infants and children. Earlier this year, the US Food and Drug Administration issued draft guidance for the amount of lead allowed in baby foods. 

The organization is on the verge of setting new regulations for the threshold of arsenic, cadmium and mercury that can be allowed in infant food as part of its Closer to Zero Action Plan.

Flooding rice fields

Crops such as corn, soybeans and wheat are grown in soils that are not very wet.

Angelia Seyfferth, a UD soil biogeochemist and professor in the Department of Plant and Soil Sciences, harvests rice in the University of Delaware Rice Facility.In contrast, rice is often grown in very wet, flooded soils. The oxygen that would normally reside in tiny pores in the soil gets lost very quickly and is replaced by water. The limited oxygen shifts the microorganisms in the soil, and those microorganisms start breathing with iron oxide minerals that give the soil a rusty orange color.

“Arsenic likes to stick really tightly onto those iron oxides,” says Angelia Seyfferth, a UD soil biogeochemist and professor in the Department of Plant and Soil Sciences and a co-author of the research. 

“When the iron oxides are used by these organisms to breathe, they go from a solid mineral to a solution phase. You essentially dissolve them, and when you dissolve them, the arsenic that’s stuck onto them goes into the water.” 

Seyfferth affirms that once the arsenic is in the water, it can easily be absorbed by the rice roots and transported into the grain. 

Seyfferth and research associate Matt Limmer grew rice in 18 small fields on the UD Newark Farm, exposing the rice paddies to different flooded and wet conditions. 

“We were hoping to find an optimal irrigation management that minimized both arsenic and cadmium simultaneously,” Limmer says, “but we didn’t find one in this soil.”

Once they harvested the grain and analyzed the amount of arsenic and cadmium in it, the researchers instead found that the more flooded the field, the more arsenic and less cadmium accumulated in the rice. By contrast, the drier the field, the more cadmium and less arsenic accumulated.   

“But, even under those drier conditions when there was more cadmium, the concentrations of cadmium in the grain were not of concern for human health,” Seyfferth notes.

Through a US Department of Agriculture National Institute of Food and Agriculture research grant, the researchers are studying arsenic in rice through some field work in Arkansas. They will work directly with farmers to develop tools to help them manage water flooding their rice paddies.

Meanwhile, the FDA could release new regulations for arsenic and cadmium in infant food by the end of this year, as part of its aforementioned Closer to Zero Action Plan. The agency has researched the effects of arsenic and cadmium and two other toxins, mercury and lead, on child development and has also been evaluating new technologies or interventions that could stymie exposure to these toxins.

“Our work could hopefully help shape policy,” Seyfferth explains.

Involving farmers

Seyfferth said one solution to help decrease the levels of toxic metals and metalloids in foods is offering subsidies to farmers to do certain strategies to reduce those levels on their own. Cadmium, which can build up in spinach leaves, could be reduced by making soils less acidic and washing spinach leaves after harvest. Lead is harder to remove, but washing spinach leaves with lemon juice extract could remove up to 26% of the lead in the leaves, the researchers say.  

“The solutions are not a blanket solution,” Seyfferth comments. “They’re not for all soils. They would have to be really site-specific.” 

Limmer and Seyfferth believe more research is needed to find “an optimal irrigation strategy that lowers levels of both arsenic and cadmium in rice.”

“Similar experiments need to be done in a variety of soils,” Limmer notes, “ideally under different field conditions.”  

As the US waits for the FDA’s draft regulations on the allowable threshold of arsenic, cadmium and mercury in baby food, Seyfferth says she would like to see the federal government review a study last done in the 1980s. 

The FDA, US Department of Agriculture and the US Environmental Protection Agency analyzed paired soils and plants in agricultural fields across the US to get an idea of the concentration of metals and metalloids in those plants and soils. 

“Since then, there’s a lot more spinach being grown now and being grown in areas where it wasn’t grown before,” Seyfferth assers. “Some of those soils are much higher in cadmium.”