Mediterranean diet cuts heart and stroke risk

"Mediterranean diet 'cuts strokes and heart attacks in at-risk groups'," The Guardian advises. Along with much of the global media, The Guardian reports on a study that found that eating a diet rich in fruit, vegetables, fish, olive oil and nuts cuts the risk of heart disease and stroke by 30%.

The story is based on an impressive trial looking at the effects of a Mediterranean diet on people at risk of heart disease and stroke, compared with a standard low-fat diet.

Researchers found that after nearly five years people who followed a Mediterranean diet supplemented with either extra-virgin olive oil or mixed nuts were around 30% less likely to have had a heart attack or stroke, or to have died from one.

It should be noted that the number of strokes, heart attacks and deaths that occurred in the study was fairly small. Nevertheless, this large and well-conducted study supports previous research on the benefits of a Mediterranean-style diet for the heart and circulation.

Where did the story come from?

The study was carried out by researchers from academic institutions across Spain, including the Universities of Barcelona, Valencia, Malaga and Navarra. It was funded by the Spanish government and other public sources.

Olive oil and nuts used in the trial were donated by commercial sources of these foods. Many of the researchers disclosed grants and fees for work done with agricultural and food industry firms and groups, as is common for research in this field.

It was published in the peer-reviewed New England Journal of Medicine.

The study did not compare the Mediterranean diet with statins, as The Daily Telegraph and Daily Mail's headlines imply. The claim that this diet is better than a drug appears to be an opinion of one of the researchers, rather than a statement of fact.

The current study cannot be used as a way of assessing the effectiveness of statins, not least because some of the people in the Mediterranean diet intervention group were also taking statins.

What kind of research was this?

This was a randomised controlled trial (RCT) involving people at risk of cardiovascular disease. It compared the effects of two variations of the 'Mediterranean diet' – one with extra-virgin olive oil and one with nuts – with a standard low-fat diet.

As the authors point out, previous research has suggested the Mediterranean diet may protect against heart disease and stroke. Helpfully, in this study the researchers have defined what they consider a Mediterranean diet to be and, as the paper is open access, you can view their Mediterranean dietary recommendations for free online.

A well-conducted RCT is the best way of examining the effects of a particular intervention (in this case a Mediterranean diet) compared with a control condition (in this case a standard low-fat diet) on a health outcome.

Randomisation helps iron out other factors that may affect cardiovascular risk, balancing them out between groups. For example, many observational studies of specific diets have been conducted. However, observational studies cannot necessarily prove that the particular diet was responsible for the outcomes seen. This is because people who choose to eat a healthier diet may also choose other healthier lifestyle options, such as exercising more or drinking less alcohol.

What did the research involve?

The trial began in October 2003. Eligible participants included men aged 55-80 and women aged 60-80. Participants did not have a history of heart attack or stroke, but were considered to be at future risk of having cardiovascular disease.

This was because they either had type 2 diabetes or at least three of the following major risk factors for cardiovascular disease:

smoking
high blood pressure
high cholesterol
being overweight or obese
having a family member who developed heart disease at a young age

Participants were randomly assigned to one of three groups:

one group was advised to follow a Mediterranean diet supplemented with extra-virgin olive oil
a second group was advised to follow a Mediterranean diet supplemented with mixed nuts (walnuts, almonds and hazelnuts)
the third control group were advised to follow a low-fat diet

Participants in the two Mediterranean diet groups received extra olive oil or nuts at no cost, while those in the control group received free non-food gifts.

All of the groups received a dietary training session at baseline (study start). The Mediterranean groups received further sessions every three months afterwards. This included an assessment of their adherence to the diet, while the low-fat group received a leaflet each year for the first three years explaining the low fat diet. In October 2006, this protocol was amended and the control group got the same intensity of diet advice and assessment as the other two groups.

Participants also filled in a general medical questionnaire, a food frequency questionnaire and a physical activity questionnaire every year. Their weight, height and waist circumference was measured. The researchers also measured certain biomarkers (chemicals in the blood or urine) in random subgroups of participants in the Mediterranean diet groups at one, three and five years to see if they were sticking to the advice to supplement their diet with extra-virgin olive oil or nuts.

Over the period of the study, they looked at the main (primary) outcome of interest, which was the number of participants who had suffered either a heart attack or stroke or who had died from any cardiovascular cause. Other (secondary) outcomes the researchers examined were the number of people who had suffered these individual events and those who had died from any cause. They obtained this information from:

repeated contact with participants
contact with family doctors
yearly review of medical records
the national death index

The researchers initially estimated they would need a sample of 9,000 participants to detect any significant differences in outcomes between the groups. However, this figure was recalculated in April 2008 to 7,400 participants.

What were the basic results?

A total of 7,447 people were enrolled in the trial. The researchers report that the people in the two Mediterranean diet groups said they adhered to their diets, which was confirmed by biomarkers in the blood or urine.

After an average follow-up of 4.8 years, they found that in total 288 people either had a heart attack, stroke or died from a cardiovascular event. Of these:

96 (3.8%) events occurred in the Mediterranean diet group with extra olive oil
83 (3.4%) occurred in the Mediterranean diet group with extra nuts
109 (4.4%) occurred in the control group on a standard low-fat diet

After adjusting for baseline risk factors (such as diabetes), the researchers calculated that, compared with those who followed the standard low-fat diet, those assigned to a Mediterranean diet with extra-virgin olive oil had a 30% reduced risk of suffering a heart attack, stroke or dying from a cardiovascular event (hazard ratio 0.70, 95% confidence interval (CI), 0.54 to 0.92).

Similarly, those assigned a Mediterranean diet with nuts had a 28% reduced risk of suffering a heart attack, stroke or dying from a cardiovascular event (hazard ratio 0.72, 95% CI 0.54 to 0.96).

No diet-related adverse effects were reported.

How did the researchers interpret the results?

The researchers say that among people at high cardiovascular risk, a Mediterranean diet supplemented with olive oil or nuts reduced the number of cardiovascular events over the study period.

They suggest there is a "synergy" in the diet's nutrient-rich foods that fosters favourable changes to some risk factors, including blood fats, insulin sensitivity and inflammation. However, they say that in this trial olive oil and nuts were probably responsible for most of the benefits.

Conclusion

The results of this randomised controlled trial appear to confirm previous studies that there are benefits to following a Mediterranean diet. The trial has many strengths, including its large size, long period of follow-up, thorough assessment of medical outcomes (including reviewing medical records and having contact with the family doctor), and careful attempts to assess whether the diets were being followed.

As this is a randomised controlled trial, it should also balance out other health and lifestyle differences between the groups that may influence cardiovascular risk. This avoids the limitations of many previous diet observational studies, where participants choose which diet to follow.

However, there are still several limitations to bear in mind:

The protocol for the control groups was changed halfway through the trial. This group did not receive the same intensity of dietary advice as the other two groups, a factor which could have affected their compliance with the diet.
Despite careful attempts at screening and measuring biomarkers, it is still difficult to know how far participants stuck to their assigned diets.
The control group had a higher dropout rate (11.3%) compared with the Mediterranean diet groups (4.9%).
The participants were at high risk of cardiovascular disease at study start, but had not yet suffered any cardiovascular events. It is not certain if the results are generalisable to other groups, including those with no risk factors for cardiovascular disease and those who have already suffered from a heart attack or stroke.

The 30% reduction in risk may sound impressive but, as the authors point out, these results mean that following a Mediterranean diet would mean that about three major cardiovascular events would be avoided per 1,000 person-years. This means that if 1,000 people at high risk of cardiovascular disease ate a Mediterranean diet for one year, there would be three fewer 'events' (such as stroke) than there would be if they ate a standard low-fat diet.

Despite these limitations, this large and well-conducted study adds to the body of previous research on the benefits of a Mediterranean style diet for the heart and circulation.