The study relied on self-reported food surveys to score the relative healthiness of participants' diets. The authors looked at two scales: the Alternate Healthy Eating Index and Mediterranean diet scores, basing their primary conclusions on the former.
Isaacson, who was not involved in the new study, said the way it measured healthy eating harkens back to how doctors used to think about what was healthy ̵1; as with the now-dated food pyramid. It "lumps together" foods that are known to benefit brain health, such as leafy green vegetables, with foods such as beans, with an impact that's unknown, he added.
Study author Tasnime Akbaraly wrote in an email that the primary scale "is based on a set of specific and limited food groups and might not cover all aspects of a 'healthy' diet … and may not be adapted to the dietary habits of all populations." However, the study showed consistent results over a long period of time, following people for a median of nearly 25 years. Most observational studies on diet and cognitive health, she added, cover much shorter windows.
But in the new study, it's not like what people ate did not matter. The authors found that healthier diets were associated with lower mortality risks, in line with previous research.
"Not evidencing an association between midlife diet and dementia does not mean that diet does not matter on cognitive health," said Akbaraly, a researcher at Inserm, the French National Institute of Health and Medical Research, at the University of Montpellier in France.
The study also found a "slight decrease in diet quality in the years preceding dementia diagnosis [which] suggests that unhealthy diet could be part of the cascade of changes occurring in the preclinical phase of dementia," said Akbaraly, also an honorary researcher at University College London.
Isaacson said the research is strong, in a sense, for how long it followed participants, but it's always important to look at who's being studied for commonalities in diet, stressors, education level and other factors that could skew the data one way or the other.
In this case, the new study included more than 8,200 London-based civil servants, over two-thirds male and predominantly white. Among them, there were 344 recorded cases of dementia. The study used electronic medical records to identify cases of dementia, which the authors said could overlook milder cases that have not been diagnosed yet. But it also allowed them to evaluate everyone they recruited for the study, rather than just those who showed up for follow-up visits.
"It is already well known that within this cohort there were marked differences in health according to their social position, with the higher grades of civil servants having better health and the lowest grades having the worst health, "Tom Sanders, professor emeritus of nutrition and dietetics at King's College London, said in an emailed statement.
" What we don 't know is anything about the nutritional quality of the diets and the linkage of dietary choice with other aspects of lifestyle such as smoking and alcohol intake, "Sanders added," both factors known to increase risk of dementia. "
When it comes to dementia risk, experts say that what you eat is only part of a very complex equation.
"It would be wrong to suggest that diet in middle-age is not important," Sanders said.
"You can not just eat a magic blueberry or follow a certain diet and think that your likelihood of Alzheimer's is going to go away, "Isaacson said. "People take different roads to Alzheimer's disease and dementia."
CNN's Sandee LaMotte contributed to this report.