The highest arsenic concentrations were found in infants who frequently ate baby rice cereal, with levels more than three times that of babies who didn't eat rice, the study reports.
Babies who ate foods mixed with rice or rice-based snacks had arsenic levels nearly double those of non-rice eaters, according to the report published April 25 in the journal JAMA Pediatrics.
"The arsenic in their urine increased with the number of servings of rice or rice-containing food," said lead researcher Margaret Karagas, chair of epidemiology at Dartmouth University's Geisel School of Medicine in Hanover, N.H.
It's still unclear what health effects these levels of arsenic exposure could have on children, Karagas and other health experts said.
"It certainly sounds concerning," said Dr. Ruth Milanaik, director of the neonatal neurodevelopmental follow-up program at Cohen Children's Medical Center in New Hyde Park, N.Y. "It certainly requires more study."
Milanaik also noted that the findings could have been influenced by other foods the babies were eating. For example, apple juice or drinking water containing arsenic could have been stirred into the babies' rice cereal. "There are so many variables," she said.
The study results come weeks after the U.S. Food and Drug Administration proposed limits on inorganic arsenic levels in infant rice cereals.
The FDA's research found that more than half of infant rice cereals sampled from U.S. retail stores in 2014 failed to meet the agency's proposed action level of 100 parts per billion of inorganic arsenic. By comparison, all samples of non-rice baby foods were found to be well below the FDA's action level for arsenic.
"This is an important step, that they've taken the initiative to propose a limit that applies to infant rice cereals," Karagas said of the FDA.
Arsenic is a known carcinogen and can also contribute to heart disease, according to the FDA.
Some evidence also suggests that arsenic exposure early in life can affect a child's immune system and intellectual development, according to the FDA and background notes from the study authors.
Inorganic arsenic has shown up in infant plant-based foods including apple juice and applesauce due to its use in pesticides, Milanaik said.
"Even though in this country we've outlawed it, we used it for so long that it's leached into our soil and really becomes part of our vegetation," Milanaik said.
Karagas and her colleagues decided to study infants' rice consumption when they learned that rice grains can absorb arsenic from the environment, she said.
The investigators obtained diet information and urine samples from 759 infants born to mothers enrolled in the New Hampshire Birth Cohort Study between 2011 and 2014.
The infants' progress was tracked with phone interviews every four months until they turned 1. At that time, a final interview assessed dietary patterns during the past week, including whether an infant had eaten rice cereal, white or brown rice, or foods either made with rice or sweetened with brown rice syrup.
The researchers found that parents fed rice cereal to four out of five infants during their first year, and that more than three of five started on rice cereal as early as 4 to 6 months old.
At 1 year of age, 43 percent of infants ate some type of rice product within the last week, and one-quarter ate food either made with rice or sweetened rice syrup, the findings showed.
Of the infants who donated urine samples, 55 percent had consumed some type of rice product in the prior two days, the study found.
The American Academy of Pediatrics recommends that infants consume a diverse diet, including a wide variety of grains, Karagas said.
The study authors and the FDA suggest limiting rice consumption in early life. Concerned parents could feed their babies oatmeal or barley, Milanaik said. These, like rice, are iron-fortified.
The FDA also recommends cooking rice in excess water, and draining off that water, which can reduce 40 to 60 percent of the inorganic arsenic it contains.
For more on arsenic in rice cereal, visit the U.S. Food and Drug Administration.
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