DDT was used extensively in agriculture from the 1950s. The synthetic pesticide works by damaging the nervous system of insects. It is known to be toxic to humans.
DDT was banned in the US in 1972, but not until 1984 in the UK. It is still used for agriculture in some countries.
The latest study, which was led by Jason Richardson of Rutgers Robert Wood Johnson Medical School in New Jersey, examined the association between Alzheimer's disease and blood levels of a by-product of DDT called DDE.
The team took blood samples from 86 patients with Alzheimer's and compared them with samples from 79 other people who did not have the disease.
A long half-life
DDE was detected in the blood of 80% of those with Alzheimer's and 70% of those in the control group. The reason why DDE is still found in people's bodies is partly due to its long half-life and partly from historical contamination of soil or from eating food imported from countries where DDT is still in use.
However, average levels of DDE were 3.8 times higher in the blood of Alzheimer's patients compared with those in the control group, the researchers found.
Scores on a test of cognitive function, known as the Mini-Mental State Examination, were lower in the group with the highest levels of DDE who carried a particular gene thought to increase the risk of Alzheimer's compared with those carrying a different gene.
The authors conclude that identifying people who have elevated levels of DDE and who also carry the risk gene may lead to early identification of some cases of Alzheimer's disease.
The study is published in JAMA Neurology.
More research needed
Commenting on the research in a statement, Dr Simon Ridley, head of research at Alzheimer’s Research UK, says: "It’s important to note that this research relates to DDT, a pesticide that has not been used in the UK since the 1980s.
"While this small study suggests a possible connection between DDT exposure and Alzheimer’s, we don’t know whether other factors may account for these results. We can’t conclude from these findings that pesticide exposure causes Alzheimer’s, and much more research would be needed to confirm whether this particular pesticide may contribute to the disease.
"Research to understand the possible environmental risk factors for Alzheimer’s can help us make informed decisions to reduce these risks. Investment in research is crucial if we are to improve our understanding of the different factors that contribute to our risk of the disease.
"In the meantime, we do know that a healthy lifestyle can help reduce the risk of Alzheimer’s, which is why it’s important to eat a healthy, balanced diet, exercise regularly, not smoke, and keep blood pressure and weight in check."
'Elevated Serum Pesticide Levels and Risk for Alzheimer Disease', Jason R. Richardson et al, JAMA Neurology.
Alzheimer's Research UK.
'DDT – A brief history and status', United States Environmental Protection Agency.
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