A new study out in JAMA Neurology suggests that higher levels of the pesticide DDT might be linked to increased risk of Alzheimer’s disease. The keyword here is might, however, since the study was quite small and the connection not clear-cut. Still, there’s certainly a possibility that the long-banned compound could contribute to the development of Alzheimer’s in people who are already at higher genetic risk. In people who are predisposed, DDT might be enough to push vulnerable brain cells over the edge.
DDT has been banned in the U.S.since the 1972, but is still used in other parts of the world. The WHO still recommends it as a key player in the fight against malaria.
The new study actually sprung from previous research on the link between various pesticides and Parkinson’s disease. Researchers noticed unexpectedly high levels of DDE, a metabolite of DDT, in Alzheimer’s patients, who were supposed to be serving as controls. ”In our previous study on Parkinson’s, we used samples from patients with Alzheimer’s as a control,” said study author Jason Richardson. “We found that in the Alzheimer’s samples, beta-HCH and other pesticides were not elevated, but we did find elevated levels of DDE. So we decided to look at DDE more closely.”
In the current study, the team looked at 86 people with Alzheimer’s and 79 without, and measured their blood levels of DDE. People falling in the highest third on the spectrum of DDE levels had a four-fold increase in Alzheimer’s risk. DDE levels were 3.8 fold higher in Alzheimer’s patients than in people without the disease.
But the results weren’t quite so black and white: There were some people with Alzheimer’s disease who had very low DDE levels – and there were some healthy people who had very high DDE levels. And this suggests that there are (of course) others factors at play besides the toxins.
Genetics is one. It may be that people who have a genetic predisposition to Alzheimer’s disease are somehow pushed over the edge with higher exposure to environmental toxins. Indeed, the team found that among people with the highest DDE levels, those who also had the APOE ε4 gene variant, which predisposes a person to Alzheimer’s, had lower scores on cognitive tests than people without the variant. This suggests that there’s an interaction between genetic and environmental triggers, which is exactly what you’d expect.
Still, the connection between the environment and Alzheimer’s risk is striking. “This is one of the first studies identifying a strong environmental risk factor for Alzheimer’s disease,” says study author Allan Levey, the director of Emory’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.”
The research also pointed to a possible mechanism behind the connection. Both DDT and DDE were shown to increase neuronal levels of amyloid precursor protein, which, as the name suggests, is the one that can develop into amyloid-beta, the infamous protein that accumulates between brain cells and clogs the brain with plaques.
More studies are needed, of course, but the current one sheds some much-needed light on the connection between genes and environment in Alzheimer’s disease, which affects so many millions across the globe. In the future, researchers might look at a person’s chemical exposure along with their genetic predisposition to determine earlier who might be at higher risk for Alzheimer’s.
“An important next step will be to extend these studies to additional subjects and replicate the findings in independent laboratories,” Levey says. “The potentially huge public health impact of identifying an avoidable cause of Alzheimer’s disease warrants more study – urgently.”