WASHINGTON, January 28, 2014—A study published Tuesday links risk of Alzheimer’s disease (AD) and exposure to the banned pesticide DDT, especially for those over the age of 60. The study also found that individuals with a genetic predisposition to AD were at even higher risk.
While more research is needed, it is important news to the more than five million Americans suffering from AD, the sixth leading cause of death in the U.S.
Published in JAMA Neurology, the study analyzed the blood samples of 86 patients with AD and a control group of 79 patients not suffering from the disease. Researchers looked at levels of DDE, a metabolite of DDT, in patients’ blood, finding DDE in 80 percent of the patients with AD and in 70 percent of those without.
While finding DDE in the blood samples was not significant, the relationship between levels of DDE and AD risk was. The study found DDE levels were 3.8 times higher in individuals with AD than in those without. Additionally, patients in the group with the highest third DDE levels were also over four times more likely to develop AD, pointing to an environmental factor in the risk of developing the disease.
Researchers also found an interaction between genetic predisposition, DDE and AD. In the study, those participants who had both the APOE4 gene variant (one of the 20 identified genes that increases AD risk) and the highest DDE levels in their blood scored lower on cognitive tests than those without the variant. Study authors hypothesize that for individuals with high DDE levels in their blood, APOE4 may be a strong factor in whether the individual will develop AD.
Despite the already-known genetic relationship, researchers were surprised by the strong connection between environmental factors and the risk of AD.
“This is one of the first studies identifying a strong environmental risk factor for Alzheimer’s disease,” said Allan Levey, study author, director of Emory University’s Alzheimer’s Disease Research Center and chair of neurology at Emory University School of Medicine. “The magnitude of the effect is strikingly large – it is comparable in size to the most common genetic risk factor for late-onset Alzheimer’s.”
Other results of the study were not as clear-cut, however. For example, there were individuals with AD that had low levels of DDE, and others without the disease with high levels of DDE in their blood, suggesting that several factors play a part in the risk of developing AD.
Banned in the U.S. since 1972, DDT was used since World War II as a pesticide to control insects in crops and livestock. DDT is still used in many countries and was recommended by the World Health Organization (WHO) in 2006 to control malaria.
Even though DDT has been banned for over 40 years in the U.S., the Centers for Disease Control and Prevention (CDC) reports that 75 to 80 percent of blood samples, collected for a national health and nutrition survey, contain DDE. It is believed that DDE is present in blood samples because DDT persists for decades in the environment. People can also ingest DDT in imported fruits, vegetables and grains from areas where DDT is still used, as well as eating fish from polluted waters.
Researchers point out that the study does not show that DDE causes AD; instead it merely shows an association between the two.
“An important next step will be to extend these studies to additional subjects and replicate the findings in independent laboratories,” said Levey to Forbes. “The potentially huge public health impact of identifying an avoidable cause of Alzheimer’s disease warrants more study – urgently.”