The Mediterranean diet is tasty and healthy
Despite the media focus on the Mediterranean diet, this was only a small part of a review which aimed to discover whether some modifiable risk factors (such as high cholesterol or high blood pressure) were linked to the risk of developing dementia in people with existing mild cognitive problems.
The review found collated relevant studies, covering a wide variety of potential risk factors, finding the most evidence around diabetes, high blood pressure and mental health.
Researchers found that the evidence suggested diabetes increased the risk of “conversion” from mild cognitive impairment to dementia. However, this increased risk was not adjusted for other potential confounding factors such as physical activity or smoking – which could influence the results.
The researchers found a single study assessing the Mediterranean diet in people with one type of mild cognitive impairment (particular problems remembering specific events). It found that the Mediterranean diet was associated with a reduced risk of developing Alzheimer’s among people with this type of mild cognitive impairment. However, it does not provide strong enough evidence to suggest that following the Mediterranean diet will definitely reduce the risk of a person with mild cognitive impairment developing dementia.
While this review is helpful, there is still a lot to be learned about risk factors for dementia and how to reduce risk.
Where did the story come from?The study was carried out by researchers from University College London and Johns Hopkins Bayview Medical Center, Baltimore, US. One of the authors reported receiving financial support from various sources including the National Institute on Aging and National Institute of Mental Health, as well as various pharmaceutical companies. The other authors reported no financial relationships with commercial bodies.
The study was published in the peer-reviewed medical journal the American Journal of Psychiatry.
The main body of The Independent’s article is quite representative of this study, focusing on links with diabetes and mental health symptoms and risk of dementia. However, the choice to focus the headline on the Mediterranean diet is quite confusing and misleading. The Mediterranean diet was not the main focus of the review or its findings, and the evidence on it in the review comes from only one study. The Express' coverage was similarly skewed in focusing on diet.
What kind of research was this?This was a systematic review that looked at which modifiable behaviours are associated with the development of dementia in people who have mild cognitive impairment (MCI).
The researchers say that recent public health campaigns have increased the recognition and diagnosis of MCI – described as a state between normal ageing and dementia. MCI is where someone (or a relative or doctor) has concerns about their cognitive symptoms, but they have normal functional activities and do not meet a diagnosis of dementia.
Almost half of people with MCI are reported to develop dementia within the following three years. However, ways to prevent the onset of dementia are unclear. The researchers had conducted a previous review of randomised controlled trials investigating treatments for MCI, but they found no consistent evidence that any of the treatments reduced the person’s cognitive decline, or the risk of the person developing dementia.
In the absence of trial evidence, they aimed next to look at observational studies to get an idea of which modifiable risk factors (such as different lifestyle choices) are associated with an increased or reduced risk of dementia. This may help to identify ways people might be able to reduce the risk of dementia developing.
A systematic review is the best way of gathering all the available evidence on a particular question. However, such reviews are always going to be inherently limited by the quality of the underlying studies identified.
What did the research involve?The researchers searched two literature databases (PubMed and Web of Knowledge) using relevant search terms to identify published longitudinal studies of “potentially modifiable risk factors” in people with MCI that looked at dementia outcomes.
The researchers defined MCI as cognitive impairment identified from objective neuropsychological tests, in the absence of dementia or significant functional impairment. Dementia outcomes were dementia of any cause, or Alzheimer’s dementia specifically.
They assessed the quality of identified studies, specifically looking for studies which recruited a representative sample of the general older population, had followed at least 70% of the included participants for at least one year, and used objective assessment methods for diagnosing MCI and dementia.
They also graded the quality of evidence supporting each of their conclusions as follows:
- grade 1 evidence: consistent evidence from higher-quality studies
- grade 2 evidence: evidence from a single higher-quality study or consistent evidence from other studies
- inconsistent evidence: described as “troublingly inconsistent”
What were the basic results?The search identified 62 relevant studies, nine of which were considered high-quality. 30 of these studies were pooled in meta-analysis. The studies looked at the following risk factors in people with MCI, as follows:
DiabetesThe pooled results of seven of 10 studies (grade 2 evidence) found people with MCI who had diabetes were at increased risk of developing dementia during follow-up compared to those without diabetes (unadjusted odds ratio (OR) 1.65, with a 95% confidence interval (CI) of 1.12 to 2.43).
High blood pressureThe pooled results of seven of 11 studies (grade 2 evidence) did not find that this was associated with significantly increased odds of dementia (OR 1.19, 95% CI 0.81 to 1.73).
High cholesterolTwo studies (grade 2 evidence) found high cholesterol did not predict development of dementia (OR 0.92, 95% CI 0.50 to 1.68).
SmokingThree studies (grade 1 evidence) found that smoking seemed to be associated with a decreased risk of dementia, but the relationship was no longer significant in the individual studies after adjusting for age. This suggested that the reason for the association could be due to smokers being more likely to die before they developed dementia.
AlcoholThree higher-quality studies did not find any clear association between moderate alcohol consumption and development of dementia (grade 2 evidence).
Metabolic syndromeOne identified study (grade 2 evidence) found that metabolic syndrome predicted any-cause dementia in a specific type of MCI, termed "amnestic MCI". This was defined to be, “MCI with progressive symptoms and particular impairment of episodic memory”.
Mental health factorsThe pooled results of four studies (grade 2 evidence) found an association between neuropsychiatric symptoms and dementia (OR 3.11, 95% CI 1.38 to 7.02). Pooled results of 13 studies did not find a significant association between depression symptoms and dementia (OR 1.35, 95% CI 0.89 to 2.06).
However, there was variability across studies. The studies following samples from the population that reported conversion from any type of MCI consistently found depressive symptoms were associated with increased risk of development of all-cause dementia (grade 1 evidence). However, the findings of studies in people with amnestic MCI and in groups of people with MCI identified through medical clinics were less consistent.
There was inconsistent evidence on any association with anxiety or apathy.
Dietary factorsThe Mediterranean diet was the media focus, but only one study was identified, including 482 people with MCI. It was of high quality, and reported that a Mediterranean diet (low in meat and dairy products; high in fruits, vegetables, legumes, cereals and fish) was associated with a lower risk of conversion from amnestic MCI to Alzheimer’s dementia (grade 2 evidence). Of other dietary factors, three studies found suggestions that lower folate level is associated with an increased risk of conversion to dementia (grade 2 evidence).
EducationSeven studies (grade 1 evidence) found that number of years in education for people with amnestic dementia did not predict dementia.
How did the researchers interpret the results?The researchers conclude that diabetes increases the risk of conversion from mild cognitive impairment to dementia.
They report that other potentially modifiable risk factors include the metabolic syndrome, neuropsychiatric symptoms, and low dietary folate. They suggest that dietary interventions and interventions to reduce neuropsychiatric symptoms may decrease risk of onset of new cases of dementia.
ConclusionThis was a systematic review that identified longitudinal studies that have looked at the association between modifiable risk factors in people with mild cognitive impairment and the development of dementia.
Despite the media headlines highlighting the Mediterranean diet, this was only a small part of this review – just one study of around 400 people that found the Mediterranean diet reduced risk of conversion from one type of MCI (amnestic MCI) to Alzheimer’s dementia.
So, despite the often-studied potential health benefits of the Mediterranean diet, this single study does not provide firm and conclusive evidence that following the Mediterranean diet will reduce the risk of a person with MCI developing dementia. Ideally, these findings need to be confirmed in other studies.
Such a systematic review is always going to be inherently limited by the underlying quality and methods of the included studies. This research found the largest body of evidence for diabetes, high blood pressure and mental health factors.
The largest body of evidence in this review suggested that diabetes increases the risk of conversion from MCI to dementia. However, even then this increased risk was in analysis that was not adjusted for other potential confounders. For example, with a condition such as diabetes, it’s possible that other cardiovascular risk factors may be involved in any association with the development of dementia, particularly vascular dementia.
It is also important to recognise that while this review did not find association with other factors such as high cholesterol, smoking or alcohol and dementia development, this is not to say these modifiable risk factors are definitely “safe”. The review only found few and variable quality studies addressing these factors.
It is important to note that the researchers’ previous systematic review of randomised controlled trials did not find that any interventions reduced the risk of conversion from MCI to dementia. In the absence of such evidence, observational studies can provide an idea of which factors look like they could be increasing risk. However, we can’t say for sure that changing them will definitely reduce risk.
Overall, this systemic review provides a summary of the currently available evidence on MCI and the risk factors for dementia in people with MCI. However, there is still much to be learned about risk factors for dementia and how to reduce risk.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.