Researchers have verified that rice – grown and cooked in water – is a key food source of inorganic arsenic
Kan Shao from Indiana University Bloomington, in the US, says he and colleagues have recently conducted a systematic review and meta-analysis of published papers, and concluded that “rice consumption is a major arsenic exposure pathway in populations with relatively high rice intake”.
His collaborator, Zheng Zhou, recently presented the study’s findings at the Society for Risk Analysis Conference in New Orleans, US. A full paper is currently being finalised.
“Scientists suggest that bioaccessibility of arsenic from drinking water is 100%,” Shao says.
However, he explains, arsenic in rice can bind to other chemicals or proteins and undergoes digestion by the body, so it’s not all necessarily ready to be absorbed.
“That’s why it’s important to understand the bioaccessibility for arsenic in rice,” he adds, “so we can better understand the amount that people are being exposed to.”
Bioaccessible arsenic is the form which can be absorbed following digestion, while the term “bioavailability” refers to the absorption process from the digestive tract to the circulation.
In 2016, the Food and Drug Administration in the US published a risk assessment report for inorganic arsenic in rice, estimating the bioaccessibility as between 70% and 90%.
Shao and Zhou set out to improve the assessment statistically by pooling all available data and applying a “beta distribution”.
“That provides a better probabilistic estimate of the overall bioaccessibility,” explains Zhou.
With the analysis they found that the median bioaccessibility of inorganic arsenic from rice was 90.4% – higher than the FDA’s estimate – with a range from 72.2% to 98.4%.
Major reasons for the variation, say the researchers, include differences in the type of rice, growing conditions and individual digestive processes. For instance, in the US, long grain rice contains higher levels of arsenic than short grain.
Brown rice contains the highest levels because arsenic becomes concentrated in the outer layer of the grain. Whether bioaccessibility is higher is not clear.
Digestive processes include individual differences in people’s background diet and gut microbiome, says Zhou. But this is not yet well understood, and he thinks it is an important avenue for further research.
Another variation could arise from pre-rinsing rice. Research suggests that cooking rice does not decrease arsenic levels. However, pre-washing white, although not brown, rice can lower the arsenic concentration by 17% to 29%.
Arsenic levels are also determined by the degree of local contamination. The element is naturally present in the Earth’s crust, and distributed through air, water and land.
Many countries have high levels of inorganic arsenic naturally present in their groundwater, including China, India, the US, and several South American nations.
Given that India and China are two of the world’s major rice exporters, arsenic exposure poses a global health issue, particularly for cultures that consume rice daily as a staple food.
Human activities increase arsenic levels, including several industrial processes. Pesticides, feed additives, tobacco and pharmaceutical products can also contain arsenic.
Rice products, such as crackers, baby cereal and milk, contain inorganic arsenic. In the UK and US, arsenic levels in rice milk were found to exceed that of water drinking standards.
Inorganic arsenic has been associated with lung and bladder cancer, skin lesions, diabetes, heart disease and impaired cognitive development.