People who have immigrated to the US from South Asia have a high risk of developing type 2 diabetes compared with the rest of the population, and public health researchers have been searching for an explanation. Initial results from a study of immigrants from India living in the San Francisco Bay Area suggest a provocative one: exposure to the pesticide DDT (Environ. Sci. Technol. 2019, DOI: 10.1021/acs.est.9b03373).
Type 2 diabetes in South Asia and in the South Asian diaspora is a large and growing problem. An estimated 80 million people in India have the disease. South Asians are a rapidly growing immigrant group in the US, and 23% of them have type 2 diabetes, compared with 18% of African Americans, 13% of Chinese Americans, and 6% of whites. “This high risk of diabetes in South Asian immigrants is not explained by obesity,” a condition commonly associated with diabetes, says Michele La Merrill, an environmental toxicologist at the University of California, Davis. Nor have researchers found a genetic explanation that can fully account for the type 2 diabetes burden in this population. “People have been on the quest to find the missing risk factor,” she says.
La Merrill and other researchers have been exploring a possible connection between exposure to DDT (dichlorodiphenyltrichloroethane) and type 2 diabetes risk. Animals exposed to DDT accumulate high levels of fat in their livers and become resistant to insulin. DDT is a persistent pollutant, both in the environment and in the human body. In the case of immigrants from South Asia, relatively high levels of the pesticide and its more persistent metabolite DDE remain in their bodies even decades after they immigrate, La Merrill says. Her previous work showed that people who have immigrated to West London from India and Sri Lanka have a higher burden of DDE in their bodies than people born in the UK, and that finding correlates with diabetes risk (J. Endocrine Soc. 2018, DOI: 10.1210/js.2017-00480).
Many countries, including the US, have banned DDT due to environmental and human health concerns. Even though the chemical was one of the first prohibited under the Stockholm Convention on Persistent Organic Pollutants, the United Nations allows its use in controlling disease vectors, particularly malaria-carrying mosquitos, when there are no affordable alternatives. India uses far more DDT than any other country (Malaria J. 2017, DOI: 10.1186/s12936-017-2050-2) and is currently the only known producer of the chemical, according to the UN. The country reported making 2,004 metric tons of DDT in 2017.
To examine levels of the pesticide in people who have emigrated from India to the US, La Merrill collaborated with Alka Kanaya, an internist at the University of California, San Francisco’s School of Medicine. Kanaya runs the Mediators of Atherosclerosis in South Asians Living in America (MASALA) study, which aims to find risk factors for heart disease and diabetes in this population. Together, La Merrill and Kanaya worked with a 147-person group within MASALA to test for persistent organic pollutants, including DDT and DDE, in blood samples.
They found that the MASALA participants’ blood contained a median concentration of DDT that is eight times as high as the US median and that higher levels correlated with both type 2 diabetes and risk factors for the disease, including insulin insensitivity and a buildup of fat in the liver.
Chirag Patel, a bioinformatics researcher at Harvard Medical School who has studied diabetes risk in South Asian immigrants in the US, says the new results are “intriguing” but do not yet provide definitive answers. “The sample size is small, and the findings aren’t replicated,” he cautions. But he notes that this kind of research is difficult to do, because health studies focused on South Asian immigrant populations are few and far between.
Kanaya says she’s eager to test the DDT-diabetes connection in a larger study and is expanding the research to the broader, 1,164-person MASALA cohort.
The researchers say this work should, in the meantime, serve as a reminder for immigrants from South Asia and their families to get screened for diabetes. And Kanaya calls for the Indian government to ban the pesticide altogether. “We really hope this research will move health policy,” she says.