Can the Mediterranean diet help us ward off heart disease? The latest evidence

Key message: Limited evidence suggests that a Mediterranean dietary pattern reduces some cardiovascular disease (CVD) risk factors. Existing evidence is promising but more trial evidence is needed to establish the role of the Mediterranean dietary pattern in preventing CVD in the general population and high-risk individuals.

Back in 1970s Britain, I grew up with a Mediterranean diet, thanks to my French step-father, though I didn’t always appreciate it. Strawberry Mivvis for my friends, something (everything?!) in olive oil for me. I particularly hated a concoction called tian, which combined courgettes, eggs and rice to produce something truly unpleasant, particularly when served cold. In 21st century Britain, Marseilles has come to Morrisons and many of us eat Med-style. We hear much conflicting advice about what we should and shouldn’t eat, the benefits of this over that, so I was interested in the latest Cochrane evidence on the so-called Mediterranean diet.

A study in the 1960s showed that populations in the Mediterranean region had fewer deaths from cardiovascular disease (CVD), which affects the heart, the blood vessels, or both and is a leading cause of death worldwide. It was thought this could be a result of different dietary patterns and more recent evidence has suggested it has beneficial effects on risk factors for CVD, such as high blood pressure. However, the bulk of the evidence has concerned people with existing CVD and has come from observational studies rather than randomised trials. A new review from the Cochrane Heart Group has now brought together trials exploring the potential benefits of a Mediterranean diet for the primary prevention of CVD.

Interventions included in this review were provision of dietary advice that recommended at least two of these seven components of a Mediterranean-style diet:

• high monounsaturated.saturated fat ratio (use of olive oil as main cooking ingredient)
• low to moderate red wine consumption
• high consumption of legumes (such as lentils and chickpeas)
• high consumption of grains and cereals
• high consumption of fruit and vegetables
• low consumption of meat and meat products and increased consumption of fish
• moderate consumption of milk and dairy products

Eleven randomized controlled trials with over 52,000 people were included. They varied a lot in the types of people recruited, the length of follow-up and the number of dietary components. Seven described the interevention as a Mediterranean diet. One of the trials, the Women’s Health Initiative (WHI), involved over 48,000 postmenopausal women and didn’t describe the diet as Mediterranean but it included two of the components: increased fruit and vegetable and cereal intake. This was the only trial that reported clinical events; the others measured risk factors for CVD, such as cholesterol levels. The majority of participants (over 49,000, in five RCTs) were healthy; the rest had increased risk of CVD or cancer.

What did they find?

• Small reductions in total cholesterol and in the harmful LDL-cholesterol, with greater reductions with diets described as ‘Mediterranean’ (results could be pooled for this outcome)
• Blood pressure was reduced in 3 of the 5 trials reporting it
• The WHI trial found no significant effects on death, diabetes rates or other outcomes after 8 years follow-up
• None of the trials reported on health-related quality of life, harmful effects or costs

How good was the evidence?

The results need to be treated with caution, owing to differences between studies, problems with risk of bias in the studies and short-term follow-up in some studies.

Where does that leave us?

The evidence for the Mediterranean diet is promising and suggests that this dietary pattern is good for us in terms of its effects on individual risk factors such as blood pressure and potentially illness and death from CVD. This is backed up by biologically plausible explanations for the benefits of the diet as a whole and its individual components and indeed some aspects, such as the five-a-day recommendation for fruit and vegetable consumption, already appear in clinical guidelines for the prevention of CVD. Future research needs to fill in some of the gaps (such as costs, any ill effects, and sustainability over time and in different populations) but I’m going to feel good about cooking ratatouille for tea. There’s a lot of debate about whether to cook the vegetables separately and then combine them or throw it all in together and no evidence to help us sort it out. I shall feel at liberty to keep experimenting. If you have a favourite ratatouille recipe or one for another Mediterranean diet type of dish, do share it please! Just don’t send me any recipes for tian.

Links:

Rees K, Hartley L, Flowers N, Clarke A, Hooper L, Thorogood M, Stranges S. ‘Mediterranean’ dietary pattern for the primary prevention of cardiovascular disease. Cochrane Database of Systematic Reviews 2013, Issue 8. Art. No.: CD009825. DOI: 10.1002/14651858.CD009825.pub2.

Cochrane summary http://summaries.cochrane.org/CD009825/mediterranean-diet-for-the-prevention-of-cardiovascular-disease