Past exposure to the banned pesticide DDT is "strikingly" linked to the development of Alzheimer's in later life, a study has shown.
People with the highest levels of DDT residue in their blood were four times more likely than average to succumb to the devastating disease, scientists found.
While experts urged caution over the results, laboratory tests have identified a plausible way that pesticide chemicals might trigger Alzheimer's-related changes in the brain.
Exposure to high concentrations of DDT, or its long-lasting derivative DDE, raises levels of a protein in nerve cells associated with a key hallmark of the disease.
In the patient study, the US team looked for DDE - produced when DDT breaks down in the body - in the blood of 86 Alzheimer's patients and 79 healthy volunteers with an average age of 74.
DDE levels were almost four times higher in the Alzheimer's patients - and those in the highest third of the exposure range assessed in the study quadrupled the chances of developing the disease.
"This is one of the first studies identifying a strong environmental risk factor for Alzheimer's disease," said Dr Allan Levey, director of the Alzheimer's Disease Research Center at Emory University School of Medicine in Atlanta, who co-led the study.
"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."
Alzheimer's Prevention Recipes
The Alzheimer's Prevention Cookbook(01 of10)
Open Image ModalSweet Peach Smoothie(02 of10)
Open Image ModalThe key to this recipe is using a ripe, in-season peach. It’s always good to get to know the produce guys at your local grocery store because they will let you know when peaches are in their prime. Peaches contain numerous nutrients that are good for your body, including niacin, thiamin, potassium, and calcium. They are also high in beta-carotene, which promotes healthy hearts and eyes. The darker the peach’s color, the more vitamin A it has in its pulp. Peaches may also help in maintaining healthy urinary and digestive functions. There’s some evidence that flaxseed oil may help reduce your risk of heart disease, cancer, stroke, and even diabetes. (credit:Photo credit: Caren Alpert)
Sweet Peach Smoothie(03 of10)
Open Image ModalMAKES ABOUT 3 CUPS11/2 cups apple juice1 ripe peach, peeled, pitted, and chopped (about 3/4 cup)3/4 ripe banana, peeled and chopped1 tablespoon vanilla yogurt6 ice cubes2 teaspoons honey2 teaspoons flaxseed oilCombine the apple juice, peach, banana, yogurt, and ice in a blender and puree until smooth. Add the honeyand flaxseed oil and puree briefly to incorporate.Pour into glasses and serve right away.
Breakfast Fried Rice with Scrambled Eggs(04 of10)
Open Image ModalThink outside the (cereal) box: fried rice is a great way to fuel up with carbs in the morning. With brown rice, lots of fresh vegetables, and a minimal amount of fat, this recipe is a healthy take on fried rice and is high in vitamin B6. Lop chong is a dried, cooked Chinese sausage with a slightly sweet and smoky flavor; it will require a trip to the Asian grocery store, but you can choose to leave it out. (credit:Photo credit: Caren Alpert)
Breakfast Fried Rice with Scrambled Eggs(05 of10)
Open Image ModalMAKES 4 SERVINGSFried Rice2 tablespoons chopped lop chong (Chinese sausage; optional)1/4 cup vegetable oil1 tablespoon chopped garlic1 tablespoon peeled, chopped fresh ginger1 green onion, white and green parts, chopped2 tablespoons diced red onion1 or 2 leaves baby bok choy, thinly sliced1/4 cup shredded red cabbage5 sugar snap peas, cut into thin strip on the diagonal2 cups cooked and cooled brown rice4 tablespoons soy sauce4 tablespoons mirinEggs and Garnishes2 large eggs, beaten1 teaspoon sesame seeds2 tablespoons toasted cashews, chopped1 green onion, white and green parts, sliced thin on the diagonal2 tablespoons chopped fresh cilantroTo make the fried rice, in a large wok or large skillet over high heat, fry the lop chong until rendered, less than a minute. Transfer the lop chong to a paper towel–lined plate and discard the fat.Set the wok over high heat and heat until very hot. Add the oil to the wok. Add the garlic, ginger, chopped green onion, red onion, bok choy, cabbage, and snap peas. Cook, stirring, for 1 minute, or until the vegetables have softened and you can smell the ginger.Add the rice and continue to cook, stirring, until everything is coated, 2 to 4 minutes. Add the soy sauce and mirin and toss well. Remove the wok from the heat.To cook the eggs, heat a small nonstick skillet over medium heat. Spray the pan with nonstick cooking spray, and then pour in the beaten eggs. Cook, gently stirring the eggs, until scrambled but still moist.Transfer fried rice to a serving bowl and top with the scrambled eggs. Sprinkle with the sesame seeds, toasted cashews, and diagonally cut green onion and serve right away.
Ahi Tuna on Rye with Spinach Pesto Yogurt(06 of10)
Open Image ModalThis is not your average tuna sandwich. For one thing, it’s more like tuna tartare on bread. For another, it’s a very brain-healthy meal. Spinach Pesto Yogurt, not mayonnaise, holds the tuna mixture together, which keeps the amount of saturated fat to a minimum. Tuna is an excellent source of omega-3 fatty acids, the pistachios provide vitamin E, and the raisins are a good source of polyphenol antioxidants. Because the tuna is not cooked in this recipe, be sure to purchase sashimi-grade tuna; ask the fishmonger if you aren’t sure about the quality of the tuna on offer at the seafood counter. (credit:Photo credit: Caren Alpert)
Striped Bass with Golden Tomato and Sweet Pepper Stew(07 of10)
Open Image Modal (credit:Photo credit: Caren Alpert)
Spaghetti Squash with Caramelized Onion and Tomato(08 of10)
Open Image ModalSpaghetti squash is an often-overlooked vegetable. But it’s a very powerful ingredient from a brain-health perspective: it’s low in saturated fat, very low in cholesterol, and a good source of niacin, vitamin B6, and pantothenic acid--plus spaghetti squash is a very good source of vitamin C. In this recipe, strands of baked spaghetti squash are the backdrop for sweet caramelized onions that contrast against salty, savory Parmesan cheese. This dish will appeal to adults and kids alike, and it’s a great way to get pasta lovers to eat more vegetables. (credit:Photo credit: Caren Alpert)
Curried Quinoa with Green Onions and Basil(09 of10)
Open Image Modal (credit:Photo credit: Caren Alpert)
Curried Quinoa with Green Onions and Basil(10 of10)
Open Image ModalMAKES 4 SERVINGS11/2 cups Brain-Boosting Broth 3/4 cup quinoa, rinsed1 teaspoon curry powder1 teaspoon minced fresh ginger5 to 7 green onions, white and green parts, chopped1/4 cup chopped fresh basil leaves1/4 cup slivered almonds, toasted1/4 cup dried cherries, chopped Pinch of saltPinch of freshly ground black pepperJuice of 1 lemon2 tablespoons extra-virgin olive oilIn a medium saucepan over high heat, bring the broth to a boil. Add the quinoa, curry powder, and ginger. Turn down the heat to medium-low, cover, and simmer until the quinoa is tender, about 20 minutes. Transfer the quinoa to a rimmed baking sheet, distribute in an even layer, and let cool to room temperature. When cooled, put the quinoa into a medium bowl and add the green onions, basil, almonds, and cherries. Sprinkle with the salt and pepper and toss to combine. Drizzle with the lemon juice and olive oil and toss again. Serve at room temperature or lightly chilled. Brain-Boosting BrothMakes 2 quarts 8 quarts water 3 carrots, coarsely chopped 2 white onions, coarsely chopped2 celery stalks, coarsely chopped2 bulbs fennel, coarsely chopped1 parsnip, coarsely chopped12 cloves garlic, chopped1/4 cup fresh ginger, peeled and choppedStems from 1/2 bunch fresh flat-leaf parsley 1 bunch green onions, green and white parts 1 stalk fresh lemongrass, cut in half lengthwise 1 tablespoon salt 1 teaspoon black peppercorns2 cloves1 teaspoon dried oregano1 teaspoon dried rosemary1 bay leafCombine all of the ingredients in a large stockpot and bring to a boil over high heat. Turn down the heat to maintain a gentle simmer and cook, uncovered, for 2 hours. Strain the broth through a fine-mesh sieve. Use immediately, refrigerate for up to 5 days, or freeze for up to 1 month.
DDT has been banned in the US since 1972 and in the UK since the 1980s as a result of work showing that the pesticide is harmful to wildlife and the environment.
Yet there are fears that its long-lasting effects live on in people exposed to the chemical when it was widely used to keep food crops free of insect pests.
DDT has a very long "half life" - the length of time it takes to reduce to half its original level - in the body of between eight to 10 years.
Levels of its DDE metabolite accumulate in body tissues as people age, the researchers point out. They believe this could be one reason why age is by far the largest risk factor for Alzheimer's disease.
In addition, DDT may continue to contaminate food products imported from countries where its use is still permitted to control malaria-spreading mosquitoes.
"We are still being exposed to these chemicals.. both because we get food products from other countries and because DDE persists in the environment for a long time," said Dr Jason Richardson, from Rutgers University in New Jersey, who led the research published in the journal JAMA Neurology.
Studies of cultured brain cells have shown that exposure to DDT and DDE raises levels of a precursor to beta-amyloid, a protein that forms sticky clumps in the brains of Alzheimer's patients.
Beta-amyloid plaques increase in the brain as the disease progresses and are thought to play a key role in Alzheimer's.
The new research suggests that exposure to DDT may directly contribute to beta-amyloid plaque development, according to Dr Richardson.
However, the scientists stress that not everyone exposed to DDT will develop Alzheimer's. Some Alzheimer's patients in the study had no trace of the chemical in their blood, and some healthy participants showed evidence of high exposure.
One deciding factor could be the presence or absence of a mutant gene called ApoE, known to be strongly linked to Alzheimer's.
Genetic risk factors such as ApoE may combine with environmental factors such as exposure to pesticides to drive the disease forward, say the scientists.
Dr Simon Ridley, from the charity Alzheimer's Research UK, said: "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."
How To Talk About Memory Loss
Step 1(01 of05)
Open Image ModalPlan a conversation in a familiar, non-threatening environment (credit:Alamy)
Step 2(02 of05)
Open Image ModalExplain why talking is important - you’re worried because you care (credit:Alamy)
Step 3(03 of05)
Open Image ModalUse examples to make things clearer (credit:Alamy)
Step 4(04 of05)
Open Image ModalHave an open conversation - ask how they’re feeling about their memory? (credit:Alamy)
Step 5(05 of05)
Open Image ModalMake a positive plan of action together (credit:Alamy)
Carol Brayne, professor of public health medicine at Cambridge University, said it was too early to come to any conclusions about a possible link between DDT exposure and Alzheimer's.
"I would be very, very cautious indeed about over-interpreting the results at this stage without confirmation from research based on stronger designs," he added.
But he said the research had produced a "very interesting result" that should be followed up by further studies.
DDT, or dichlorodiphenyltrichloroethane, was developed during the Second World War and used extensively in agriculture from the 1940s onwards.
Its agricultural use was banned worldwide in 2001 under the Stockholm Convention on Persistent Organic Pollutants. The Convention included an exemption that permitted its continued use in combating malaria.
Prof Alan Boobis, an expert in biochemical pharmacology at Imperial College London, said: "The authors of the paper have looked for associations between DDT/DDE levels and Alzheimer's disease occurrence, but from the evidence they present we still don't know whether DDT/DDE might be associated with Alzheimer's disease through the much higher exposures that occurred several decades ago, or through the lower levels of exposure occurring now from residual environmental levels.
"This distinction is important because some confounding factors have not been accounted for in this study, for example co-exposures in the past, many of which would no longer be detectable in biological samples."
David Coggon, professor of occupational and environmental medicine at the University of Southampton, said: "The findings should be a stimulus to further research using more rigorous epidemiological methods, but of themselves, they do not provide strong evidence of a hazard.
"As DDT has been banned in the UK for many years, there is no scope for additional regulatory controls on a precautionary basis."
Dr Doug Brown, director of research at the Alzheimer's Society, said: "Many of us recall the controversial pesticide DDT, which was banned in the UK following concerns about its affect on wildlife and humans. This small study suggests that increased levels of DDT in the blood might be linked to an increased risk of Alzheimer's disease.
'Whilst this isn't the first time DDT has been linked to health problems, it is hard to draw any firm conclusions from this small sample of people from the United States. Alzheimer's disease is a complex condition and we know that genetics, the environment and lifestyle factors can all contribute to its development. More research is needed to unravel its causes and determine whether testing for pesticides in the blood could be useful to predict who is at greater risk."