Healthy eating habits may help preserve cognition in older people, a new study suggests.
The study showed that consuming what researchers are calling a "prudent" diet featuring fruits, vegetables, and whole grains is associated with less cognitive decline, and eating a Western diet rich in red meat, processed foods, and sugar is associated with more cognitive decline, after a 6-year follow-up.
However, the negative effect of the Western diet was counteracted by adding elements of the prudent diet, the researchers found.
"The novelty of our work was the evaluation of the effect of mixed dietary patterns — habitually having a mix of both healthy and less healthy food items on your plate — on cognitive aging," said lead author Behnaz Shakersain, a PhD candidate from the Aging Research Center, Department of Neurobiology, Care Sciences and Society, Karolinska Institute, Stockholm, Sweden.
"We found that people who followed such an eating pattern had only about half the decline in cognitive functioning on average than those individuals with high adherence to the 'Western'-type diet."
Comparing dietary patterns rather than individual nutrients more closely parallels the "real world," said Shakersain.
The study was published in the February issue of Alzheimer's & Dementia.
The analysis included participants in the Swedish National study on Aging and Care-Kungsholmen (SNACK-K), an ongoing longitudinal project focusing on aging. SNACK-K includes a random sample of subjects aged 60 years and older living in the Kungsholmen district of Stockholm. Younger subjects are assessed at 6-year intervals, and older ones at 3-year intervals.
The current analysis included 2223 participants, mean age 70.6 years, who were nondemented and who had a Mini-Mental State Examination (MMSE) score of 27 or more. Of this cohort, 68.2% were from the younger cohort (up to about 78 years), and the rest were from the older age group.
At baseline, researchers collected data on dietary intake from a validated food frequency questionnaire containing 98 food and beverage items. Participants were asked how often on average during the past year they had consumed each item on a nine-level scale ranging from never to four or more times a day.
Researchers converted these 98 items into 35 food groups based on the similarities of the nutrient content. Using a statistical approach called factor analysis, they identified dominant dietary patterns.
The "Western" and the "prudent" dietary patterns were the most dominant.
The Western diet was characterized by more frequent intake of red/processed meat, saturated fat, refined grains, sugar, beer, and spirits. The prudent diet had more vegetables, fruit, cooking/dressing oil, cereals and legumes, whole grains, rice/pasta, fish, low-fat dairy, poultry, and water.
Although the "prudent" diet shares many common characteristics with the Mediterranean diet, it does not include certain Mediterranean diet features such as olive oil and wine, said Shakersain.
According to reported food intakes, each individual had a score indicating adherence to the Western diet and a score indicating adherence to the prudent diet. Investigators categorized factor scores for each dietary pattern into quintiles, representing very low, low, moderate, high, and very high adherence to each of the patterns.
About 68% of the study group had mixed adherence to both the Western and prudent dietary patterns.
Researchers first assessed the association between dietary patterns and cognitive decline separately for each pattern. They then considered the different levels of adherence to both patterns simultaneously "to have the assessment of food intakes closer to real-life scenario," said Shakersain.
The primary outcome was change in the MMSE, a 30-point test used to measure global cognitive function. This test was administered at baseline and at the first and second follow-ups.
The study showed that the highest adherence to the Western dietary pattern was associated with more MMSE decline than the lowest adherence (P < .001), after adjusting for multiple factors (including age, sex, education, total energy intake, civil status, smoking, physical activity, body mass index, vitamin or mineral supplement intakes, vascular disorders, diabetes, cancer, depression, APOE ε4 status, and the other dietary pattern score.)
High adherence to the Western diet was also associated with elevated mortality in older subjects. "In the multi-adjusted flexible parametric survival model, Western dietary pattern scores as a continuous variable ([hazard ratio (HR)] = 1.18; 95% [confidence interval (CI)], 1.04–1.34) and the fifth quintile of the scores of this pattern (HR = 1.79; 95% CI, 1.06–3.02) were significantly associated with all-cause mortality in the older age cohorts," the authors write.
In contrast, the highest adherence to the prudent diet was associated with less MMSE decline compared with the least adherence (P = .011).
The cognitive decline associated with more frequent intake of the Western diet was reduced by almost 50% when it was combined with more frequent intake of the prudent diet. This, said the authors, suggests that the detrimental cognitive effects of eating unhealthy foods can be significantly counteracted by more healthy food choices.
However, although even partial substitution of entirely unhealthy food choices with healthier items may help protect against accelerated cognitive aging, "the focus of attempts in the modification of eating behaviors should be towards complete substitution of unhealthy food with healthy choices to have the most benefit," said Shakersain.
The high number of calories, sugar, saturated and trans fats, and in some cases, high intake of alcohol other than wine, might explain the higher cognitive decline in those closely adhering to the Western diet. Animal studies show that these components can lead to increased levels of circulating plasma amyloid β, blood–brain barrier degradation, and hippocampal dysfunction.
As well, this diet contains heat-processed foods that have high levels of advanced glycation end products, which have been associated with increased aggregation of amyloid β, and so may speed up cognitive aging.
In contrast, key elements of the prudent diet (eg, fruits, vegetables, and fish) may enhance cognitive performance by affecting synaptic plasticity or by affecting glucose use and reducing oxidative stress, among other things.
However, Shakersain stressed that when assessing the relationship between diet and disease, it might be better to look at the cumulative effect of overall dietary patterns, rather than focusing on a single food component or nutrient.
"Although this approach may be more informative with regard to potential biological mechanisms, it may fail to disentangle the effects of highly correlated nutrients in a food."
For example, there may be significant intercorrelation and interactions among nutrients, foods, and food groups, she added.
Results of studies such as this one cannot be tested in a randomized clinical trial because it is ethical to assign subjects to a diet with presumed harmful effects. But Shakersain believes the results could be considered in developing or updating dietary recommendations for primary prevention of cognitive impairment and dementia.
Sceptical of Dietary Studies
Medscape Medical News invited David Knopman, MD, professor, Neurology, Mayo Clinic, Rochester, Minnesota, whose research focus is on late-life cognitive disorders, to comment on the study.
Despite the limitation that dietary information was self-reported, Dr Knopman felt that overall, the study "was carefully done and the results are of some interest as a cross-sectional analysis."
However, Dr Knopman admitted to being a "skeptic" when it comes to studies that examine diet and disease risk.
"Over the decades, dietary studies have proved in general to be unreliable and to make claims about causality that are ultimately unjustified," he said.
In this new study, Dr Knopman took issue with calling the diet that was compared to the Western one "prudent." "That's sending a pretty powerful message. Would they ever say a prudent diet is worse?" he asked.
The "big problem" with cross-sectional dietary studies, said Dr Knopman, is that diet is "deeply confounded" by its relationship to other health-related behaviors that are not always measurable.
He pointed out that diet is part of a larger set of behaviors that includes attention to weight, exercise, and not smoking, as well as following recommendations for avoiding diabetes and hypertension.
Another concern for Dr Knopman is that diet is part of a lifelong pattern. "Perhaps it's the case that adherence to a 'prudent' diet on a lifelong basis confers some health benefits," he said. "But with a study like this, how do you know what people ate 29 years ago?"
Telling a 75-year-old patient to change his diet will not have much of an effect on his cognitive health, said Dr Knopman. "I see diet as a lifelong habit that changing in late life is unlikely to have a major impact, unless the diet is really, really abnormal, like eating chocolate bars and beer."
It is also possible, said Dr Knopman, that dietary choices could reflect preclinical disease. "In other words, there could be reverse causality where impending cognitive impairment is shifting people towards eating fast foods."
It is well established for example, he said, that weight loss precedes symptomatic dementia by several years, and that people with dementia have changes in olfaction, so patients on the road to cognitive decline may change their diet.
The authors and commentators have disclosed no relevant financial relationships.
Alzheimers Dement. 2016;12:100-109. Full text
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Cite this: 'Prudent' Diet Linked to Better Cognition - Medscape - Mar 21, 2016.