Rice and spinach are staples in the diet of babies and young children, but the toxic metals and metalloids found in these foods can have serious health consequences.
In particular, heavy metals such as cadmium, lead, mercury and metalloid arsenic can slow brain development in babies and young children.
From new research published in the scientific journal Environmental geochemistry and health, Scientists at the University of Delaware have found that flooded rice fields tend to contain higher amounts of arsenic and smaller 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 value for adverse health effects.
The findings could help determine an action plan for lowering the levels of these contaminants in foods commonly eaten by infants and children. Earlier this year, the U.S. Food and Drug Administration issued draft guidelines for the amount of lead allowed in baby food. The country is about to set new rules for the threshold levels of arsenic, cadmium and mercury that can be allowed in infant food as part of the Closer to Zero Action Plan.
Flooding rice fields
Crops such as corn, soybeans and wheat are grown in soils that are not very wet. So farmers water them to ensure the plants get the nutrients they need to grow, but never enough to completely flood them.
Rice, on the other hand, is often grown on very wet, flooded soils. Oxygen that is normally contained in small pores in the soil is 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 cling very tightly to those iron oxides,” said Angelia Seyfferth, a soil biogeochemist and professor in UD’s Department of Plant and Soil Sciences, and co-author of the study. “When the iron oxides are used by these organisms to breathe, they go from a solid mineral to a solution phase. You actually dissolve them, and when you dissolve them, the arsenic attached to them goes into the water.”
Seyfferth said once the arsenic is in the water, it can be easily absorbed by the rice roots and transported into the grain.
Seyfferth and research associate Matt Limmer grew rice in 18 small fields at the UD Newark Farm, exposing the rice fields to a variety of flooded and wet conditions.
“We hoped to find optimal irrigation management that would simultaneously minimize both arsenic and cadmium,” Limmer said, “but we found none in this soil.”
After harvesting the grain and analyzing the amount of arsenic and cadmium in it, the researchers found that the more the field was flooded, the more arsenic and less cadmium accumulated in the rice. In 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 to human health,” Seyfferth said.
When the rice fields were flooded and arsenic was absorbed, the researchers noticed methanogenesis taking place, in which organisms in the soil produce the powerful greenhouse gas methane and release it into the atmosphere. Meanwhile, the excess water reduced the sulfate in the soil to sulfide, causing cadmium to precipitate along with the sulfide.
When they dried out the soil, the researchers lowered the levels of arsenic and methane. Sulfide in the soil was oxidized to become sulfate, which is no longer a solid phase, allowing cadmium to easily filter through and escape into the plant.
“By drying out the soil, we kind of put a brake on the microorganisms that breathe with iron oxides and with arsenic,” Seyfferth said. “Then we actually increase the amount of cadmium because we oxidize the sulfide to sulfate. When it becomes sulfate, it is no longer a solid phase with the cadmium and the cadmium can then be free.”
Drying out the soil introduced oxygen into soil pores, Seyfferth said, slowing down the microorganisms that dissolve iron oxides to create methane and changing the chemistry.
“As soon as you introduce oxygen, the dissolved iron oxides are solid again,” Seyfferth said. “They’re kind of like a Brita filter. The arsenic sticks to the iron oxides and isn’t in the water, so the plant roots can’t really get it.”
What they discovered – one metal or metalloid increases while another decreases depending on the moisture level in the soil – presents a bit of a puzzle.
“There is a challenge,” Seyfferth said. “It’s really in what this magic number or the magic water status is in the soil, to try to minimize both of them. There’s really not one that’s universal for all soils.”
Through a research grant from the U.S. Department of Agriculture’s National Institute of Food and Agriculture, the researchers are studying arsenic in rice through fieldwork in Arkansas. They will work directly with farmers to develop tools to help them control the water that floods their rice fields.
Meanwhile, the FDA could release new regulations for arsenic and cadmium in infant foods by the end of this year, as part of the aforementioned Closer to Zero Action Plan. The agency spent some time researching the effects of arsenic and cadmium and two other toxins, mercury and lead, on child development. The FDA has also evaluated new technologies or interventions that could hinder exposure to these toxins.
“Our work can hopefully help shape policy,” Seyfferth said.
Members of the Seyfferth Lab (Matt Limmer, Angelia Seyfferth and graduate students Bekah Hanrahand and Frank Linam) harvest rice at the UD RICE Facility.
Involving farmers
UD researchers also found through a review article in the academic journal GeoHealth that producers are willing to take whatever action is necessary to reduce the levels of metals in their crops, but they need incentives, testing and education to do so.
This was specifically the case for the spinach industries in five states – producers, packers, processors and marketers – that the researchers interviewed. Spinach can contain amounts of cadmium and lead that it absorbs through the soil.
“It’s really important to get feedback from stakeholders to see what is feasible for farmers,” Seyfferth said, “and that it wouldn’t be a big burden on something they’re already doing, or on changing a practice they’re doing , which they have done.” do to meet another standard, such as another food safety standard.”
The researchers examined plant interactions with metals and metalloids, comparing and contrasting how cadmium and lead move through soil and affect leafy greens. They also provide solutions for farmers to reduce the amount of metals and metalloids in the food they grow.
“Farmers often work with very small margins for something like spinach,” says Seyfferth. “If the regulators made it very difficult to reach a certain level of cadmium or lead, maybe they would switch over and grow something else.”
Complicating that further, one challenge with cadmium in spinach is that water is chlorinated to disinfect spinach. But adding chloride actually makes it easier for cadmium to infiltrate a plant’s roots, transferring it to the leafy green tissues.
“We could be exacerbating the cadmium problem by using chlorinated irrigation water for leafy greens,” Seyfferth said. “We may need to think about alternative ways to disinfect irrigation water that don’t involve chlorine.”
Seyfferth said one solution to help lower levels of toxic metals and metalloids in food is to offer subsidies to farmers to implement certain strategies to lower those levels on their own. Cadmium, which can accumulate in spinach leaves, can be reduced by making the soil less acidic and washing the spinach leaves after harvest. Lead is more difficult to remove, but washing spinach leaves with lemon juice extract can remove up to 26 percent of the lead in the leaves, the researchers say.
“The solutions are not a blanket solution,” Seyfferth said. “They’re not for all soils. They really should be site specific.”
Limmer and Seyfferth say more research is needed to find an optimal irrigation strategy that reduces the levels of both arsenic and cadmium in rice.
“Similar experiments should be conducted on different soils,” Limmer said, “ideally under different field conditions.”
As the U.S. awaits draft regulations from the FDA on the allowable threshold levels of arsenic, cadmium and mercury in baby food, Seyfferth said she would like to see the federal government redo a study from the 1980s. The FDA, the U.S. Department of Agriculture and the U.S. Environmental Protection Agency analyzed paired soils and plants in U.S. agricultural fields to get an idea of the concentration of metals and metalloids in those plants and soils.
“Since then, much more spinach is now being grown and grown in areas where it was not grown before,” Seyfferth said. “Some of those soils contain much more cadmium. My goal would be to conduct this study again.”
