The health benefits of a Mediterranean-style diet have hit the headlines, with The Daily Telegraph reporting we should, 'Eat oily fish to prevent memory loss,' while the Mail Online focuses on how oily fish can keep the memory 'sharp'...
The health benefits of a Mediterranean-style diet have hit the headlines, with The Daily Telegraph reporting we should, 'Eat oily fish to prevent memory loss,' while the Mail Online focuses on how oily fish can keep the memory 'sharp'.
These somewhat overenthusiastic headlines are based on a large study that looked at how eating habits similar to those seen in Mediterranean countries could affect your chances of developing cognitive impairment.
Researchers found that older adults from the US who followed a Mediterranean-style diet were 13% less likely to have impaired cognitive abilities. This was still the case even after adjustment for other health and lifestyle factors that could also have an influence. However, this association was not seen in people with diabetes.
Cognitive decline and dementia
Cognitive decline is often loosely translated in the media as meaning dementia. This is not the case.
Cognitive decline (such as memory loss) is just one of a range of dementia symptoms. Other signs of dementia can include mood changes and depression.
The Mediterranean diet has been linked to a lower risk of developing several diseases, including cardiovascular conditions such as heart disease and stroke, and forms of dementia (such as Alzheimer's disease).
Drawing conclusions from research into associations between diet and health is difficult, as it is hard to measure the influence of dietary habits properly.
In the case of this study, the diet measurements may be quite specific to the US, so the findings may not apply to British people's diets in the same way.
Overall, however, this large well-conducted study suggests that sticking to a largely Mediterranean-style diet may have benefits for the cognitive functioning of people who do not have diabetes.
Where did the story come from?
The study was carried out by researchers from the University of Athens, the University of Alabama at Birmingham, and other research institutes in the US and the Czech Republic. It was funded by the US National Institutes of Health and was published in the peer-reviewed medical journal Neurology.
The media headlines regarding the importance of eating oily fish instead of red meat do not fully capture the research results. While oily fish is part of the Mediterranean diet, there are many other components that could also have beneficial effects. This study assessed all the components together rather than focusing on oily fish alone, as the media suggests – in fact, the word 'fish' does not appear once in the Neurology article.
Additionally, the 19% reduction in risk quoted by both The Daily Telegraph and the Daily Mail is incorrectly attributed to "people who adhere to a Mediterranean-style diet". This figure actually only applies to non-diabetic people. The risk reduction for the entire study sample was a more moderate 13% reduction in odds. However, both newspapers covered the main methods of the study well.
What kind of research was this?
This was a prospective cohort study that assessed the relationship between how much people adhered to a Mediterranean diet and their odds of developing cognitive impairment over time.
The Mediterranean diet involves eating lots of fruit, vegetables and olive oil, and eating few saturated fats, meat and dairy products. According to the study's authors, the diet has been associated with longer lifespan and a reduced risk of heart conditions, some cancers and Alzheimer's disease. It is the frequent subject of research into the influence of lifestyle on health and longevity.
As a prospective cohort study, this research can tell us how people's diet is linked to new cases of cognitive impairment over time. It also confirms that diet preceded any change in cognitive function, a key factor in assessing causality.
What did the research involve?
As part of the Reasons for Geographic and Racial Difference in Stroke (REGARDS) study, researchers recruited more than 30,000 individuals over the age of 45 from various regions of the United States. At the beginning of the study (baseline), individuals completed a version of the food frequency questionnaire (FFQ) that assessed their dietary habits. It was adapted specifically around foods commonly eaten in the US. Their cognitive functioning was also assessed using the six-item screener (SIS).
Participants were excluded if:
- they had a history of stroke
- there was missing data from the diet questionnaire
- they completed fewer than two cognitive assessments during the study
- the baseline tests revealed impaired cognitive status
The participants were asked to complete the FFQ several times over the first year to validate the results of the baseline diet assessment. The questionnaire was scored on a 10-point scale (0 to 9), with higher scores indicating higher adherence to a Mediterranean diet.
The SIS cognitive assessment was carried out at baseline and on a yearly basis to detect changes in cognitive status and new cases of cognitive impairment. For the statistical analyses, the researchers defined incident (new) cognitive impairment as a change from intact cognitive functions (an SIS score of 5 to 6) to impaired cognitive status during follow-up assessment (an SIS score of 4 or less).
Using data from the FFQ scores, the researchers separated participants into two categories according to their adherence to a typical Mediterranean diet. Scores of 0 to 4 were taken to indicate low adherence, while scores of 5 to 9 indicated high adherence.
The researchers then calculated the odds of new onset cognitive impairment in the group with high diet adherence, and compared this with the odds of new cognitive impairment in the low adherence group.
They adjusted the analyses to control for factors shown to be associated with new-onset cognitive impairment, including:
- demographic factors, such as age, race and sex
- socioeconomic factors, such as region of residence, household income and education
- health status, such as history of heart disease, diabetes, atrial fibrillation, blood pressure, high cholesterol, use of blood pressure medications, symptoms of depression, and perceived general health
- other risk factors, such as body mass index (BMI), waist circumference, smoking status, alcohol use and physical activity level
They also assessed how having diabetes influenced the relationship between diet and cognitive impairment. For this, they carried out two separate analyses similar to those described above: one for individuals with diabetes, and the other for diabetes-free participants only.
What were the basic results?