Although DDT was widely used in the USA from the 1940s to 1972 to kill the mosquitoes that cause malaria, it was banned by the Environmental Protection Agency in 1972.

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A new study links exposure to the insecticide DDT with Alzheimer's disease.

Researchers found evidence of DDT exposure in 80% of patients with Alzheimer's disease, as well as 70% of those without the condition.

The body breaks down DDT into a related chemical called DDE. Researchers measured levels of this chemical to assess DDT exposure in 86 people with Alzheimer's disease, which causes memory loss and death, as well in 79 others without the condition. Doctors conducted memory tests on patients while they were alive, then confirmed the Alzheimer's diagnosis after death through an autopsy, says Jason Richardson, an associate professor at the Rutgers Robert Wood Johnson Medical School in New Jersey and lead author of the study, published today in JAMA Neurology.

On average, DDE levels were nearly four times higher in the blood of people with Alzheimer's, compared with adults without the condition, the study says.

Although DDT was widely used in the USA from the 1940s to 1972 to kill the mosquitoes that cause malaria, it was banned by the Environmental Protection Agency in 1972 because of concerns over the risk to wildlife.

DDT persists in the environment and in human blood and tissue, however. Many people today have levels of DDE that are comparable to those found in patients in the study, either because of past exposure or current exposure, which can occur by eating contaminated fish, meat or dairy products, Richardson says. DDT is still used in some countries to control malaria, and its reintroduction was endorsed by the World Health Organization in 2006.

Yet exposure to DDT, by itself, may not be enough to cause Alzheimer's, Richardson says.

In his study, the patients who did the worst on scores of memory and reasoning were those with two risk factors: high levels of DDT exposure, as well as a variation in the APOE gene, which is known to increase the risk of Alzheimer's.

Because of its design, a study like this can't prove that DDT causes Alzheimer's. It can show only an association between the two conditions, says Heather Snyder, the Alzheimer's Association's director of medical and scientific operations.

Alzheimer's is the sixth-leading cause of death in the USA, and affects more than 5.2 million Americans. As the country ages, the number of cases is expected to increase 40% by 2025, according to the Alzheimer's Association.

In an accompanying editorial, scientists Steven DeKosky and Sam Gandy wrote that Richardson's conclusions "should be considered preliminary until there is independent confirmation."

But DeKosky and Gandy note that pesticides have been "linked convincingly" to the risk for Parkinson's disease, another neurological condition. And they add that researchers should spend more time examining environmental links to Alzheimer's, given that scientists may not make many more big breakthroughs on the genetic causes of the disease.

After studying tens of thousands of people, "if there were more genes out there that powerfully affected one's risk," DeKosky says, "we would have found them by now."

Although scientists have found 20 genes that increase the risk of Alzheimer's, only APOE has a large effect on Alzheimer's, increasing the risk by 10 to 12 times in people who inherit two copies of the variation, DeKosky says. Most of the other genes linked to Alzheimer's increase risk by a very small amount.

Snyder says there is good data to suggest that people can reduce their risk of Alzheimer's disease by making certain lifestyle changes -- such as exercising, avoiding tobacco and eating a healthy, balanced diet.

Studying environmental exposures could help scientists better understand Alzheimer's, Richardson says. "That's important," Richardson says. "Because environmental exposures are things you can do something about."

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