Fish was part of healthy eating while red meat was among the unhealthy foods.
The American Academy of Neurology has published the results in the journal Neurology. The healthy eating linked to the stronger cognitive health was a diet with not much red meat, moderate alcohol and lots of fruits and vegetable, nuts and fish.
The 27,860 over-55s included for the analysis, from across 40 countries, were studied over an average of around 5 years.
Certain health conditions were excluded at the start of the study of people at high risk of cardiovascular disease. None of the participants had diabetes or a history of heart disease, stroke or peripheral artery disease; nor had any recently experienced serious disease outcomes such as a stroke or congestive heart failure.
Participants who experienced heart disease or stroke during the study were no longer followed for diet and mental power.
To take a baseline measure of cognitive health and monitor any decline, thinking and memory skills were tested at the start of the study, then 2 years and about 5 years later.
A maximum of 30 points was possible against these thinking and memory tests and cognitive decline was noted when scores dropped by 3 points or more, which happened for 17% overall - a total of 4,699 participants.
Cognitive decline lowest among those who reported healthiest diets
The proportion registering a decline was lower for people reporting the healthiest diets - 14% of these showed a drop in thinking and memory, compared with 18% of the people eating the least healthy diets.
This new study linking brain power and diet involved a test of cognitive health that is used during dementia diagnosis. The mini mental state examination (MMSE) measures:
- Orientation to time and place
- Word recall
- Language abilities
- Attention and calculation
- Visuospatial skills.
For the measure of diet, the participants were asked at the start of the study to say how often they ate certain foods, including vegetables, nuts and soy proteins, whole grains and deep-fried foods. They also reported levels of alcohol intake and gave data to produce a ratio of fish to meat and eggs in their diets.
The measure of diet quality was a modified version of the healthy eating index used by the US government.
Among the 5,687 people with the healthiest diet, 782 made up the 13.8% having cognitive decline, while of the 5,459 people with the least healthy diets, 987 accounted for the 18.1%.
The relative difference from these figures produces a 24% lower likelihood of a drop in thinking and memory for people eating well.
Study author Dr. Andrew Smyth, of McMaster University in Hamilton, Ontario, Canada, and the National University of Ireland in Galway, says diet in later life is only part of the picture:
"Adoption of a healthy diet probably begins early in life, and a healthy diet might also go along with adoption of other healthy behaviors."
For their data, the authors examined participants from randomized drug trials in cardiovascular disease supported by pharmaceutical company Boehringer Ingelheim.
In background to their work, the authors cite previous brain health links to healthy diet but point out that using the large multinational prospective cohort study allows observation of "more precise associations between diet (assessed using standardized methodology) and cognitive outcomes."
Explaining what biological explanations may lie behind the emerging evidence, the authors say: "Dietary intake may modify the risk of cognitive decline through multiple mechanisms, including increased risk of stroke (both overt and covert) and through deficiency of nutrients required for neuronal regeneration (for example, group B vitamins, and vitamin C)."
The risk factors for dementia listed by the US National Institute of Neurological Disorders and Stroke includes a number that can be modified by dietary and lifestyle measures.
The new study ends by stating:
"In conclusion, we report that higher diet quality is associated with a reduced risk of cognitive decline. Improved diet quality represents an important potential target for reducing the global burden of cognitive decline."