The five diets are the alternate Mediterranean diet (aMED), DASH (Dietary Approaches to Stop Hypertension), AHEI-2010 (alternative Healthy Eating Index-2010), PDI (plant-based diet index), and hPDI (healthful plant-based diet index).
Although all five diets were linked with cognitive benefits, aMed came out on top.
According to Professor Koh Woon Puay, professor at Duke-NUS Medical School and lead principal investigator of the Singapore Chinese Health Study: “It is now recognised that health outcomes are related to risk behaviour in midlife. In fact, some evidence suggests that they might even be related to factors in early life.”
The findings were published in The American Journal of Clinical Nutrition.
Types of dietary patterns
The study utilised data from the Singapore Chinese Health Study (SCHS) of 16,948 men and women who were aged 45 to 74 years at baseline (1993–1998).
After 20 years, the participants were re-interviewed at the third follow-up visit (2014–2016) and cognitive function was evaluated when subjects were aged between 61 to 96 years.
Their diet quality was assessed at baseline using a structured semiquantitative FFQ, which was validated using a series of 24-h dietary recalls and selected biomarkers.
Koh explained the five dietary patterns selected have been investigated across different populations in both Western and Asian countries, and can similarly be applied to a typical Singapore diet. A higher score represented better adherence to the diet.
The aMED diet (2005) consist of nine components (total vegetables excluding potatoes; total fruit; nuts; legumes; fish; whole grains; MUFA to SFA ratio; alcohol; and red and processed meat) and was an adaptation of the Mediterranean Diet Score developed from diet from Mediterranean countries.
The DASH diet (2008) contain eight components (total vegetables excluding potatoes; total fruit; nuts, seeds, and legumes; low-fat dairy; whole grains; sodium; sugar-sweetened beverages and fruit juices; and red and processed meat) that were emphasised in the diet designed for hypertension management.
The AHEI-2010 diet includes 11 foods and nutrients [total vegetables excluding potatoes; whole fruit; whole grains; sugar-sweetened beverages and fruit juice; nuts; soy and legumes; trans fatty acids; long-chain (n–3) fatty acids (EPA + DHA), PUFAs; sodium; alcohol; and red and processed meat. It follows the dietary guidelines for Americans, which were revised in 2010 by Harvard.
The PDI diet (2017) scores plant foods positively and animal products negatively (18 food groups).
The hPDI diet (2017) focuses on healthy foods such as whole-grains, vegetables, fruits, nuts, and legumes. Refined grain and sweetened beverages were given negative scores.
Koh told us: “We defined cognitive impairment using a Singapore-modified Mini-Mental State Examination questionnaire/instrument.”
The MMSE questionnaire is the most widely used cognitive screening tool worldwide, and is composed of 30 items assessing orientation, attention, memory, language and visuospatial ability. The MMSE score ranges from 0 to 30, with higher scores indicating better cognitive function.
While MMSE cutoff points of 23/24 was usually used to define cognitive impairment in Western countries, previous studies have showed that MMSE score is significantly affected by education level.
“In our population, the education level was generally low and thus we used education-specific cut-off points, which were 17/18, 20/21, 24/25 for those with no formal education, primary school education, and secondary school or higher education, respectively,” Koh explained.
The study reported that the Odds Ratio (95% CI) for cognitive impairment comparing the highest with the lowest quartile of diet quality scores was 0.67 for aMED, 0.71 for DASH, 0.75 for AHEI-2010, 0.82 for PDI, and 0.78 for hPDI (p<0.001).
Further to that, participants in the highest quartile of the dietary pattern scores had a significant reduction of 18% (for PDI) to 33% (for aMED), in the risk of cognitive impairment.
Researchers said there were several explanations for the protective role of these healthy dietary patterns on cognitive health.
“Healthy dietary patterns could decrease chronic inflammation and oxidative stress, which are related to the pathophysiology of neurodegenerative diseases,
Emerging evidence from animal models and human studies suggests that some components of these dietary patterns (vegetables, fruits, and whole grains) are beneficial for the gastrointestinal microbiome, which can influence the central nervous system,
“Taken together, the healthy dietary patterns and related components could improve cognition via favourable changes in metabolic, inflammatory, and microvascular function.”
Study strengths and limitations
Koh said a strength of the study was in the study design, “The SCHS cohort, being a long-term prospective cohort, has given us the golden opportunity to record lifestyle and dietary patterns in midlife (average age of 53) and follow the participants over more than 20 years to study the associations with their health at older age (average age of 73).”
Other strengths included the multiple diet index score in a single study, “To the best of our knowledge, no study performed in an Asian population has systematically evaluated various a priori dietary indexes with cognitive health simultaneously,” researchers added.
They also acknowledged several limitations in their study.
“First, cognitive function was only evaluated during the third follow-up visit, thus we were not able to capture cognitive decline over time and reverse causation is also possible,
Second, SM-MMSE testing was used for screening purposes rather than clinical diagnosis and may have poor sensitivity for the detection of very severe or very mild cognitive impairment.”
Koh explained that while some of the participants could have changed their behaviour and lifestyle as they aged, studies in other countries, as well as their own data, have indicated that diet tends to be stable after midlife, unless there were catastrophic events or critical illnesses.
“Furthermore, any subsequent, major changes in diet that we are unable to capture in our analysis will likely weaken our findings. Hence, our results are conservative estimates of the true associations between dietary pattern and risk of cognitive impairment,” she said.
She concluded: “Our findings suggest that maintaining a healthy dietary pattern is important for the prevention of onset and delay of cognitive impairment,
“The information provides evidence for public-health policies and education to promote healthy dietary choices in the general population, and defines what healthy dietary pattern means. Our study on cognitive function in the elderly also has important implications for Singapore and other populations that are experiencing rapid ageing.”
Researchers suggested future longitudinal studies with repeated measures of diet and cognition and clinical diagnosis of dementia among diverse populations are needed. This would help identify the critical window for diet intervention to have a major impact on cognition and identify the potential population that would benefit the most.
Source: The American Journal of Clinical Nutrition
“Dietary pattern in midlife and cognitive impairment in late life: a prospective study in Chinese adults”
Authors: Jing Wu, et al.