A Mediterranean diet , typically thought of as rich in vegetables, fish, whole grains, nuts and olive oil, has long been hailed as a means to healthy living. The potential benefits of this diet were tested in the long-term nutritional intervention study PREDIMED (‘PREvención con DIeta MEDiterránea’), which began in 2003 in Spain. Researchers and clinicians across 16 Spanish communities investigated the effects of a Mediterranean diet on the prevention of cardiovascular diseases in those at high risk, keeping track of their diets and health status for at least five years. The researchers particularly focused on comparing a low fat diet to a Mediterranean diet supplemented with extra virgin olive oil and tree nuts. BMC Medicine published several of the studies resulting from the PREDIMED data, from exploring how a Mediterranean diet impacts blood pressure and risk of depression, to investigating the effect of nut consumption on cardiovascular disease mortality, and wine consumption on depression.
We spoke to nutritional scientists David Jenkins and Cyril Kendall from the University of Toronto, Canada, about how data from the PREDIMED study adds to our understanding of the relationship between nutrition and disease, specifically implications for cardiovascular and mental health.
How do the results of the PREDIMED study add to our knowledge of the benefits of a Mediterranean diet?
The PREDIMED study provided encouragement for those who have been interested in the potential value of diet for the treatment and prevention of chronic disease. Those investigators involved in the study are to be congratulated for their insight and understanding of the urgent need and therapeutic potential of plant based interventions. These findings were particularly important at a time when the LOOKAhead study and Women’s Health Initiative had provided less encouragement than had been anticipated. Furthermore, the emphasis on Mediterranean diets has been a recurrent theme related to beneficial cardiovascular outcomes with studies including the Lyon heart study and the Indo Mediterranean diet study. Both of these studies demonstrated a beneficial effect on coronary heart disease outcomes (30 percent event reduction) of added vegetable oils (with increased n-3 fatty acid intake) when added to a Mediterranean diet. The present study also demonstrated a 30 percent cardiovascular disease reduction (significant for stroke) together with reduction in blood pressure, and improvement in diabetes risk when olive oil or nuts as sources of vegetable fats were added to a Mediterranean diet.
The extent that the Mediterranean diet is responsible for these benefits is difficult to determine, or whether vegetable oils are indeed responsible (increased in Mono or n-3 fatty acids or both). However this point may be academic since the Mediterranean diet, itself, encourages the use of plant oil, epitomized by the original Cretan diet of Ancel Keys’ Seven Countries study that began in the late 1940’s – a flagship diet to demonstrate the beneficial effect of high vegetable oil intake (as extra virgin olive oil).
The result of the PREDIMED study should therefore be no surprise given the past history of added vegetable oils in prevention of cardiovascular disease. Furthermore, there is a long history of nuts showing significant cardiovascular benefits in cohort studies for both cardiovascular disease and diabetes prevention. Indeed nuts may also improve glycemic control in diabetes and are known to lower serum cholesterol and hence reduced the Framingham cardiovascular risk score.
However, the lack of success of other equally ‘obvious’ interventions including weight loss and exercise in diabetes or more fiber and less saturated fats for post-menopausal women, make the success of the PREDIMED study so timely and encouraging.
Why do you think nuts and olive oil have beneficial effects on cardiovascular health and longevity?
Both nuts and olive oils have been shown to reduce cardiovascular risk factors, to lower low-density lipoprotein cholesterol (LDL-c), to raise high-density lipoprotein cholesterol (HDL-c), to improve glycemic control, and in some studies, to lower blood pressure. The inclusion of nuts and olive oils in a diet have the overall potential of reducing the dietary glycemic load with advantages for cardiovascular disease, diabetes incidence and treatment.
Many studies have suggested various diets are good for cardiovascular health, with results that can be confusing and contradictory. Do you think a particular diet, such as the Mediterranean diet, should be recommended for good cardiovascular health?
The Mediterranean diet has been promoted as a diet rich in plant foods, starchy staples (e.g. beans, chickpeas, lentils and pasta), high in plant oils, typically extra virgin olive oil, and low in intake of red meat – all of these dietary changes have been associated with a reduction in major chronic diseases. Currently the dietary guideline of the Canadian Cardiovascular Society (2012) recommends the Mediterranean diet for cardiovascular disease risk reduction along with the dietary portfolio and the DASH (Dietary Approaches to Stop Hypertension) diet. The recommendation of the American Heart Association/ American College of Cardiology (AHA/ACC) committee guideline (2013) recommends the DASH diet, which in many ways is the reflection of a low salt Mediterranean diet. The European Society of Cardiology emphasizes fat types, fiber, fruit and vegetables, and although they do not recommend the Mediterranean diet specifically, it is discussed in a positive light (European Heart Journal; 2012; 33: 1635-1701).
In addition to the cardiovascular effects, the PREDIMED study has also linked moderate wine consumption to a reduced risk of depression. How do these findings further our understanding of the complex link between alcohol and mental health?
Alcohol consumption has been linked to reduced incidence of cardiovascular disease. However, increased alcohol consumption is also linked to hypertension and to cancer, especially of the gastrointestinal tract. Moderate alcohol intake may have a positive impact on mood. However, there may also be other features related to a lifestyle of moderation which may also be associated with better stress management and perhaps overall reduction in environmental stresses (e.g. the well to do who have a glass of wine or two with their dinner). Obviously excessive alcohol consumption may be harmful to mental health and overall well-being.
Do you think the findings of the PREDIMED study occur as a direct result of diet or do you think cardiovascular and mental health are more likely to be influenced by other positive health behaviors?
The PREDIMED study demonstrated the advantages of nuts or olive oil intake on cardiovascular events. There are reasons as discussed above why this effect may be a direct one. However, such diets may also encourage improvement in mental health and well-being, which themselves may have benefits on cardiovascular risk factors (such as blood pressure etc). It is possible therefore that one is looking at a positive cycle where dietary benefits are reflected in overall health, which in turn improves the sense of well-being and further enhances cardiovascular risk reductions.
How should the results of the PREDIMED study be extended?
The PREDIMED study has indicated an advantage for nuts and olive oil as feasible interventions that enhance adherence to a Mediterranean lifestyle. This lifestyle has resulted in reduced cardiovascular events. Exercise along with diet has also been demonstrated to reduce cardiovascular disease to an extent which has been suggested to be comparable to pharmacotherapy. PREDIMED2 will logically extend PREDIMED1 findings by adding exercise to the intervention. Such a study is obviously essential.
Nevertheless, at a time, when we are recommending statin therapy for those whose coronary heart disease risk score is more than 7.5 percent (AHA/ACC 2013), there seems to be an urgent need for a solid body of large randomized controlled trials (RCTs) to extend the findings of the PREDIMED study to a wide range of additional plant food components, to demonstrate their benefits on cardiovascular risk and so reduce the need for statin therapy. At present we have pitifully few large RCTs of dietary interventions to rival the over a quarter of a millions participants who have taken part in long term statin RCTs. The balance of evidence therefore at this point must lie in favor of pharmacotherapy in the absence of an equal volume of large dietary trials.
Questions from Claire Barnard, Senior Editor for BMC Medicine.