A small study finds a higher incidence of Alzheimer’s disease among people exposed to the outlawed pesticide DDT.

A byproduct of the banned pesticide DDT may increase certain people’s risk of developing Alzheimer’s disease, according to a new study in the American Medical Association’s journal Neurology.

Led by Jason R. Richardson, of the Rutgers Robert Wood Johnson Medical School and Environmental and Occupational Health Sciences Institute, a research team has linked residue from the pesticide to the gene variant ApoE-ε4, the greatest known genetic risk factor for late-onset Alzheimer’s disease.

In other research, DDT has been linked to increased rates of diabetes, developmental problems, miscarriages, and certain cancers. Last year, one study linked it to an increase in the likelihood of obesity in third-generation children.

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Dichlorodiphenyltrichloroethane, or DDT, was banned in the U.S. in 1972 but is still used abroad. Researchers concentrated on one of DDT’s metabolite byproducts, dichlorodiphenyldichloroethylene, or DDE.

Using blood samples from 86 Alzheimer’s patients and 79 healthy controls, researchers detected DDE in 70 percent of the controls and in 80 percent of people with Alzheimer’s. The average levels of DDE in the blood were 3.8 times higher in Alzheimer’s patients than in those without cognitive problems, the study showed.

The DDE found in blood samples is likely due to the chemical’s long half-life, continued exposure from food imported from other countries, or from legacy contamination of soil and waterways in the U.S., according to the study authors.

Those with the highest levels of DDE in their blood as well as the ApoE-ε4 gene variant had lower scores on the Mini-Mental State Examination, a cognitive test, compared to those without the high-risk gene.

Researchers concluded that people who carry the ApoE-ε4 gene may be more susceptible to the effects of DDE, but noted that the connection needs to be fleshed out with further research.

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The American Medical Association study, however, is far from definitive proof that DDT exposure increases the risk of Alzheimer’s. Though the study is the largest of its kind to date, it’s still too small to draw concrete conclusions.

Still, researchers concluded that identifying people who have high levels of DDE in their blood and carry the ApoE-ε4 gene can be one way to find cases of Alzheimer’s disease early.

Dr. Heather Snyder, director of Medical and Scientific Relations for the Alzheimer’s Association, told Healthline that she found the study interesting, but with its small sample size and other limitations, it’s difficult to generalize its findings.

But the study does support the theory that a person’s environment can contribute to his or her risk of Alzheimer’s.

“There are so many open questions with Alzheimer’s, and this is an example of important questions being asked,” she said.

Previous research has shown that Alzheimer’s—the most common neurodegenerative disease—is affected by both genes and lifestyle. Regular exercise, a balanced diet, continued social engagement, and regular brain stimulation have been shown to help stave off the progression of Alzheimer’s.

“While we don’t have that prescription for physical activity, per se, we do know the research continues to support the use of physical activity,” Snyder said.

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