Eating a Mediterranean diet “can reduce the risk of developing conditions such as diabetes, obesity and high blood pressure”, The Daily Telegraph has today reported. Mediterranean diets are typically high in fruit and vegetables, low in meat and use olive oil in place of dairy fats.
The news comes from a new review of research on the Mediterranean diet that combined and analysed the results of 50 studies in more than 500,000 people. Among the most notable findings were that those eating the diet had lower blood pressure, lower blood sugar and higher levels of ‘good’ cholesterol. The study also found an overall reduction in symptoms of metabolic syndrome, which is a combination of risk factors that increase the likelihood of cardiovascular disease.
This new review did not assess the development of heart disease and diabetes, but it has demonstrated that the Mediterranean diet reduces the development of metabolic syndrome and its components, which are often precursors to the development of these conditions. There were some differences between the gathered studies that suggest that the results should be interpreted somewhat cautiously, although the trends seen do support other research about this dietary pattern.
Where did the story come from?
The study was carried out by researchers from universities in Athens and Ioannina in Greece and in Naples, Italy. The authors do not specify whether they received external funding. The study was published in the peer-reviewed Journal of the American College of Cardiology.
The press has covered this study well, although headlines stating that the Mediterranean diet “cuts risk of heart disease” may incorrectly imply that the study directly measured outcomes of heart disease. The study was concerned with a range of risk factors that are likely to precede heart disease, such as high blood pressure.
What kind of research was this?
This is certainly not the first time that research on the Mediterranean diet has made headlines, with numerous individual studies on the diet having received press coverage. However, this systematic review and meta-analysis provides the most up-to-date assessment of the evidence for the diet as a way to reduce the risk of developing cardiovascular disease in adults.
Specifically, the authors combined the results from 50 previous studies that had measured the effects of the diet on metabolic syndrome, a cluster of risk factors in adults that together can considerably raise the likelihood that a person will develop diabetes or heart disease. Metabolic syndrome is defined formally as the presence of any three of the following:
- high blood pressure (greater than 130/85mmHg or active treatment for hypertension)
- high blood sugar (fasting plasma glucose >5.6mmol/L or active treatment for hyperglycaemia)
- high blood fat (triglycerides ≥1.7mmol/L)
- low levels of ‘good cholesterol’ (<1.03mmol/L for men or <1.29mmol/L for women)
- a large waist circumference (≥102cm in men and ≥88cm in women or ≥90cm in Asian men and ≥80cm in Asian women)
Lifestyle interventions, particularly changes in diet and increases in physical activity, are established ways to prevent metabolic syndrome and, consequently, to reduce the likelihood of cardiovascular disease and diabetes. The Mediterranean diet is generally considered to consist of a high concentration of good oils (monosaturated fatty acids), usually from olives and olive oil; daily consumption of fruit, vegetables, wholegrains and low-fat dairy; weekly fish, poultry, nuts and legumes; low red meat consumption and moderate alcohol consumption. It has been linked to reduced risk of cardiovascular disease, cancer and diabetes.
This study takes a new angle by specifically looking at the effects diet has on the risk factors that often precede the development of cardiovascular diseases.
What did the research involve?
The researchers set out to identify all English language research studies published up to April 30 2010, that assessed the effects of a Mediterranean diet on the development of metabolic syndrome or its components. They searched well known medical databases including PubMed, Embase and the Cochrane Central Register of Controlled Trials. They did not exclude studies on the basis of study design at this stage.
Their search initially identified 474 studies, but after excluding those that did not meet specific inclusion criteria (such as those that failed to be randomised if they were trials, those that failed to compare the Mediterranean diet against another diet or those that missed some of the key components of the Mediterranean diet) they were left with 50 studies that were eligible for analysis. There were 2 cohort studies, 35 randomised controlled trials and 13 cross-sectional studies. They provided a total study population of 534,906 individuals.
The researchers extracted the results data from each study, specifically the reports of changes in or progression of metabolic syndrome or any of the main components (waist circumference, blood pressure, blood cholesterol, blood fat or blood glucose). The results were then pooled using the statistical techniques of meta-analysis. The researchers differed the techniques according to whether they were combining the results from the randomised controlled trials, the cohort studies or the cross-sectional studies. The researchers also rated the quality of each study to help provide a measure of their confidence in the results obtained from the pooling.
From their analyses, the researchers then reported how a Mediterranean diet affects the risk of metabolic syndrome and some of its separate components. Although 50 studies were included overall, the different outcomes they each addressed meant that fewer studies could be included in the meta-analyses relating to each specific outcome. For example, in total, only eight studies assessed the effect of Mediterranean diet on the development or progression of the entire set of metabolic syndrome risk factors. Only two of these were randomised controlled trials, two were cohort studies and four were cross-sectional studies.
What were the basic results?
The Mediterranean diet was found to protect against the development or progression of metabolic syndrome, reducing the risk by about 50%. The Mediterranean diet was also protective against some of the individual components of the syndrome, with people who consumed it having, on average, a 42cm smaller waist circumference, higher levels of good cholesterol (1.17mg more), lower blood triglycerides (-6.14mg lower), lower blood pressure and lower blood glucose.
How did the researchers interpret the results?
The researchers conclude that their results “are of considerable public health importance” because the dietary pattern can be easily adopted by all population groups and is a cost-effective approach to the primary and secondary prevention of metabolic syndrome and its individual components.
This was a well conducted systematic review and meta-analysis, although the interpretation of some of its extensive results is not straightforward. The researchers have performed different sub-analyses, each separately pooling all cross-sectional studies, all cohort studies and all controlled trials. They then reported the results of these groups separately and also combined the results from cohort studies and trials in some cases.
The most noteworthy results are probably those obtained from pooling randomised controlled trials. Randomised controlled trials have the most appropriate study design for assessing the effects of receiving an intervention compared with not receiving it. Pooling just those results from randomised controlled trials showed Mediterranean diet reduced the risk of developing or progressing metabolic syndrome overall, as well as all of the individual components that make up the syndrome. These are the important results of the study, as combining the results of cohort studies and cross-sectional studies has limitations. Neither cohort studies nor cross-sectional studies can prove cause and effect.
There are some other points to keep in mind when interpreting the results:
- Although the researchers excluded studies that did not describe the full Mediterranean diet, the precise pattern of food was likely to vary across the included studies as was the way it was administered and the recommendations given. There were also differences in the diets consumed by the control groups and in whether dietary change was being recommended as part of wider lifestyle changes or not.
- Importantly, some of the analyses combined studies that were very different from one another in terms of the sample size, study duration, trial quality and context of intervention. These analyses had a high ‘statistical heterogeneity’, which is a way of measuring whether it is appropriate to pool them or not (higher heterogeneity means pooling is less appropriate). The researchers say that this “introduces a warning about the generalisation of the present results”.
- The outcomes were related to risk factors for cardiovascular disease, not the disease itself. It is, therefore, an extrapolation, although perhaps not an unrealistic one, to claim that this study proves that the Mediterranean diet has an effect on cardiovascular disease outcomes.
Overall, this research provides further evidence of the benefits of eating a Mediterranean-style diet and quantifies the benefit in terms of the individual risk components of metabolic syndrome.