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European researchers followed more than 8,200 middle-aged adults for 25 years -- looking at whether diet habits swayed the odds of being diagnosed with dementia. In the end, people who ate their fruits and vegetables were at no lower risk than those who favored sweets and steaks.
The findings, published March 12 in the Journal of the American Medical Association, stand in stark contrast to many past studies.
Those studies have linked heart-healthy diets to lower odds of mental decline and abnormalities in the brain that can foretell dementia. Currently, groups like the Alzheimer's Association suggest that people adopt those diets as one potential way to stave off dementia.
Most studies, though, have followed people for only a fairly short time -- less than 10 years, said lead researcher Tasnime Akbaraly, from the French national research institute INSERM.
This study is the first to look at diet quality starting in middle age and the long-term risk of dementia, explained Akbaraly.
Her team found that 344 people were diagnosed with Alzheimer's over the quarter-century they were followed. And the rates were similar among the one-third of study participants with the "best" diet quality and the one-third with the "worst."
People in that first group typically had several servings of fruits, vegetables and whole grains every day; at least a couple servings of nuts and legumes each week; regularly had unsaturated fats, like olive oil; and put limits on red meat, sodium and sugary drinks.
No one is advising people to give up on that type of eating, however.
"I would certainly not want anyone to come away from this thinking a healthy diet is futile," said Keith Fargo, director of scientific programs and outreach for the Alzheimer's Association.
"This study has to be viewed within the context of the larger scientific literature on diet and cognition -- which does suggest there's a benefit [from healthy eating]," said Fargo, who was not involved in the study.
According to the Alzheimer's Association, the best evidence is for two heart-healthy diets: the traditional Mediterranean diet and the DASH diet -- which is a standard recommendation for lowering high blood pressure.
The diets differ, but both emphasize a familiar refrain: Get plenty of fruits, vegetables, legumes, fiber-rich grains, "good" fats, and fish and poultry -- and limit red meat, sweets and added salt.
Akbaraly also stressed that her findings do not imply "diet doesn't matter."
For one, diet clearly is vital to overall health -- physical and mental. Akbaraly noted that in an earlier study of this same group, middle-aged adults with the healthiest diets had a lower risk of depression over the next two-plus decades.
And, she said, these findings still leave many questions unanswered -- such as whether diet is more powerful when combined with other lifestyle measures, like regular exercise.
In reality, Fargo said, studies like this one -- which ask people about their usual lifestyle habits -- cannot answer the major question: Will changing my diet -- or any other habit -- lower my risk of dementia?
"You can't rely on observational studies like this to tell you what to do," Fargo said, because they do not prove cause and effect.
More definitive answers, he said, come from clinical trials -- which randomly assign people to adopt a lifestyle change or not.
The Alzheimer's Association is sponsoring an ongoing trial that is testing the effects of diet changes along with other measures -- including exercise and mental-stimulating activities. It is focusing on older adults at increased risk of mental decline.
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