With a rapidly growing world population requiring higher industrial and agricultural products, exposure to elements such as arsenic poses a growing risk to human health. Despite being of growing concern and central scientific interest, the solutions and policies addressing arsenic exposure remain limited.
Arsenic in Rice. Image Credit: gotphotos/Shutterstock.com
Sources and impacts of arsenic exposure
Arsenic is a natural element making up the earth’s crust and is widely distributed throughout the environment in the air, water, and land. It occurs in many minerals, usually in combination with sulfur and metals. For humans, arsenic is highly toxic in its inorganic form.
The most common routes of exposure to arsenic may be via ingestion, inhalation, or skin absorption. Exposure to arsenic can generate several chronic and acute negative health impacts including cancer, skin lesions, cardiovascular diseases, and diabetes.
The risks associated with inorganic arsenic are primarily derived from exposure to contaminated water, food, or air, which most commonly originates from agricultural or industrial sites. Once exposed through accidental consumption, for example, arsenic has a high level of bioaccessibility as documented in a 2019 study published in the Journal of Agricultural Food Chemistry.
A team of Chinese researchers experimented with the level of assimilation of arsenic when ingested, demonstrating that arsenic bioaccessibility ranged from 52.8 to 78.8% in the gastric phase before increasing up to a maximum of 95.8% upon the small intestinal phase of digestion before decreasing consistently. The health-related consequences of arsenic exposure are therefore of concern once individuals are exposed due to the high assimilation rates.
One major source of arsenic exposure is the consumption of rice as rice grains are known to contain higher levels of arsenic concentration which can vary geographically and by rice type. Rice crops require high amounts of hydration and are often supported by fertilizer rich in inorganic elements, including arsenic, or occur in proximity to areas of high human activity, such as more arable land or industrial sites. Due to the high absorption of water and nutrients, rice grains are therefore able to retain high levels of potentially harmful elements such as arsenic.
The issue of arsenic exposure is further confounded because rice is a staple food for nearly two billion people in Asia alone. In South Asian countries such as India, Pakistan, or Bangladesh, where rice consumption rates per capita are highest in the world, arsenic has been reported to be present in underground water and rice.
Indeed, across Asian countries, arsenic has been found in different forms and concentrations, which may be affected by cultivation, cooking, and irrigation methods. Such factors are known to affect the likelihood of these populations contracting diseases and cancers of different parts of the body including the skin, cardiovascular system, and reproductive systems, and even diabetes through epigenetic mechanisms.
Examining the diabetic repercussions of arsenic exposure
Although arsenic exposure is documented to generate a range of health-related impacts, its effects on diabetic mechanisms are particularly well studied. A 2017 review by Iranian scientists was able to explore the relation between rice consumption, arsenic contamination, and diabetes in South Asia. The review collected studies in both animals and humans, collecting evidence demonstrating a strong association between chronic inorganic arsenic exposure to the increased risk or prevalence of especially type 2 diabetes mellitus.
The mechanism underlying this association is multifaceted. Firstly, arsenite, a chemical compound containing arsenic oxoanion, is known to disrupt glucose metabolism. Indeed, arsenite can bind covalently with sulfhydryl groups in insulin molecules and receptors, enzymes such as pyruvate dehydrogenase and alpha keto-glutarate dehydrogenase, and glucose transporters (GLU-T), which may result in insulin resistance. Secondly, arsenic and other metabolites have also been shown to increase peripheral tissues' resistance to insulin, which might lead to hyperglycemia and subsequent diabetes.
The review highlights the multiple pathways that arsenic exposure can induce type 2 diabetes mellitus, demonstrating the complexity in addressing such health risks as single solutions may be insufficient.
The variables associated with arsenic exposure
To prevent the health-related impacts of arsenic, scientists have been able to track some of the reasons underlying the accumulation of arsenic in rice. For instance, studies have reported that arsenic concentration and nutritional status of rice grain depend on the type and nature of the soil that is grown on. Brown rice is also known to contain 80 % more inorganic arsenic than white rice because of the presence of a germ layer in brown rice, which is known to retain a considerable amount of inorganic arsenic.
Moreover, research also revealed that rice crops can uptake arsenic in various forms and through various mechanisms. For example, arsenate, a common form of arsenic, is taken up by plants via phosphate transporters, while in the soil where rice is grown, the most common form is arsenite and is transported via several mechanisms.
In response to the complexity of addressing arsenic exposure, international policies have been developed in an attempt to mitigate the extent of arsenic contamination. The World Health Organization (WHO) has set advisory levels of As in polished (i.e., white) rice grain at 0.2 mg/kg, but the EU and USA are yet to set legal standards for As in rice and rice-based products. Nonetheless, preliminary standards in the US have been set to at 5 parts per billion (5 ppb), although the regulation of such limits remains challenging.
Emerging solutions and challenges in addressing arsenic exposure.
Despite a broad range of policies, effective strategies addressing arsenic accumulation in crops remain of limited success. In a study from 2019 in the journal Metallomics, researchers discussed the main strategies for lowering arsenic accumulation in rice.
The review suggested that strategies revolve around agronomic and biotechnological approaches including mineral supplementation of soil using iron, phosphorus, sulfur, silicon, water management, soil aeration practices, and the use of biological agents, are designed to lower arsenic concentrations.
Studies have also suggested the use of certain microbes to reduce or prevent the uptake of heavy metals by plants. Microbes have been documented to reduce the bioavailability of metals via alteration of soil pH, release of chelators, reduction and oxidation reactions. Promising candidates include bacterial strains such as Brevibaccilus species, which have been shown to possess genes capable of reducing and oxidizing arsenic, thus reducing arsenic accumulation when the bacteria colonize.
The combination of microbes and leonardite, a soluble mineraloid, was shown to decrease arsenic even further than just microbes or other minerals acting in isolation. This combination was also shown to reduce oxidative stress and reduce the overall accumulation of arsenic in both stalks and roots.
Nevertheless, implementing such measures is challenging due to the context-dependency of solutions, which can vary regionally, by species, and address the multifaceted pathways of arsenic uptake. The presence of different forms of arsenic in rice and its products poses a serious public health concern, especially in Asian countries. These approaches, combined with proper diet management and creating public awareness on potential health risks resulting from chronic exposure to arsenic in rice, could play a key role in risk reduction.
Sources:
- Biswas, J. K., Warke, M., Datta, R., & Sarkar, D. (2020). Is Arsenic in Rice a Major Human Health Concern? Current Pollution Reports, 6(2), 37–42. doi:10.1007/s40726-020-00148-2
- Davis, M. A., Signes-Pastor, A. J., Argos, M., Slaughter, F., Pendergrast, C., Punshon, T., Gossai, A., Ahsan, H., & Karagas, M. R. (2017). Assessment of human dietary exposure to arsenic through rice. Science of The Total Environment, 586, 1237–1244. doi:10.1016/j.scitotenv.2017.02.119
- Dolphen, R., & Thiravetyan, P. (2019). Reducing arsenic in rice grains by leonardite and arsenic–resistant endophytic bacteria. Chemosphere, 223, 448–454. doi:10.1016/j.chemosphere.2019.02.054
- Hassan, FI., Niaz, K., Khan, F., Maqbool, F., Abdollahi, M. (2017).The relation between rice consumption, arsenic contamination, and prevalence of diabetes in South Asia. EXCLI J, 16:1132-1143. doi:10.17179/excli2017-222
- Shri, M., Singh, P. K., Kidwai, M., Gautam, N., Dubey, S., Verma, G., & Chakrabarty, D. (2019). Recent advances in arsenic metabolism in plants: current status, challenges, and highlighted biotechnological intervention to reduce grain arsenic in rice. Metallomics, 11(3), 519–532. doi:10.1039/c8mt00320c
- Yin, N., Wang, P., Li, Y., Du, H., Chen, X., Sun, G., & Cui, Y. (2019). Arsenic in Rice Bran Products: In Vitro Oral Bioaccessibility, Arsenic Transformation by Human Gut Microbiota, and Human Health Risk Assessment. Journal of Agricultural and Food Chemistry, 67(17), 4987–4994. doi:10.1021/acs.jafc.9b02008
Further Reading