Contemporary understanding of the association between diet and cardiovascular disease (CVD) stems from the last century, with significant observations on dietary patterns and disease prevalence. The evidence-based review of the current Australian Dietary Guidelines included a number of specific questions on this topic and the evidence-based statements included in the guidelines are informative for practice. The scientific literature continues to add to this evidence base and it behoves practitioners to keep up to date and learn from new insights. Broad frameworks for incorporating new knowledge into practice can be helpful. In this Editorial, we focus on two dietary patterns for which recent major publications have provided further evidence of protection from CVD: the Mediterranean diet and vegetarian diets. In discussing these dietary patterns, we look at research on related foods and nutrients to piece together the evidence and show where gaps might create further thinking spaces for researchers and practitioners alike.
Mediterranean diets, olive oil, nuts, fish and fatty acids
Early observations on diet and CVD had their origins in the Mediterranean diet pattern. The most recent study examining effects of the Mediterranean diet was the PREDIMED study. This randomised controlled trial compared advice to lower total fat intake (which did not occur) to advice to use 1 L/week of virgin olive oil or advice to eat 30 g of mixed nuts per day, both of which were supplied. Results showed a 30 and 28% reduction in CVD events in the latter two groups. The main dietary differences between the two intervention groups and the control group were a 32–50 g/day increase in virgin olive oil, a 0.9–6 servings per week difference in nuts, and 0.3 and 0.4 more servings of fish and legumes. Thus, while the study focused on a particular dietary pattern, there was strong evidence surrounding some of the foods central to the overall diet. For example, the results were consistent with epidemiological data showing that frequent nut consumption (>1 per week) is associated with a 37% reduction in CHD mortality.
There is also other evidence supporting the effects of the dietary pattern itself. The Mediterranean diet is a low meat, relatively low-dairy diet enriched in olive oil, fruits and vegetables, and legumes with regular alcohol use. In the Spanish-EPIC cohort, the highest quintile of Mediterranean diet had a 27% lower number of CHD events (both fatal and non-fatal) compared with the highest quintile but significant reductions were seen in all quintiles (trend P = 0.0013). A 1 unit increase in the score was associated with a 6% reduction in CHD events. In contrast, the Westernised diet pattern of meat and refined cereals was not associated with increased disease, and this is an important point. This research really serves to illuminate the beneficial dietary elements, and care needs to be taken with making assumptions beyond that. Again, evidence for key foods was made available. In this cohort, a 7% reduction in CHD events was seen for each 10 g/day increase in olive oil consumption, a 22% reduction comparing top and bottom quartiles. The reduction was greater in non-smokers (11%) and in virgin olive oil consumers (14%), which say something about broader lifestyle factors. Age may be another consideration. In the EPIC Elderly study, adherence to modified Mediterranean diet by 2 units was associated with 18% lower overall mortality rate in survivors of myocardial infarction followed up for 6.7 years.
Interestingly in this group, the intake of monounsaturated fat (the major fatty acid group in olive oil) was not related to mortality but the intake of saturated fat was. Here we can see that moving from a food to a nutrient perspective poses new questions. High-saturated-fat diets were not associated with a significantly increased CVD risk, although a 7% (non-significant, P = 0.2) increase seen was the expected change based on LDL cholesterol changes. Small, old and very unphysiological interventions show that replacement of saturated fat with large amounts of n6 and short-chain n3 fats (8–21%) reduces CHD events by 19% overall. More recent studies, the Alpha Omega study and the Omega study,[10, 11] have not confirmed the apparent benefits of fish oil in secondary prevention nor does there appear to be benefit in primary prevention in high-risk individuals as shown in the GRACE study, and the Italian General Practice study.[8, 12] Combined cohort analyses suggest that a higher n3 fat intake is only associated with a reduction in sudden death. It may be that dietary pattern research is able to provide more consistent outcomes than research focused on individual nutrients, for multiple reasons worth having a think about.
Vegetarian diets, fruits and vegetables
The evidence supporting the protective value of vegetarian diets has also been recently strengthened. A meta-analysis in 2012 of seven studies with 125 000 participants found a 29% reduction in mortality from ischaemic heart disease (CHD) in vegetarians and a 12% reduction in mortality from cerebrovascular disease. As key foods in this cuisine, vegetables and fruits appear to play a large part, just as olive oil and nuts may do in the Mediterranean diet. For example, one study showed that meat eaters consuming 3–5 servings of fruits and vegetables per day had reduced heart disease by 7% and consuming more than 5 servings per day reduced it by 17%. This was compared with consuming less than 3 servings per day. From the perspective of meat itself in the diet, a recent meta-analysis showed no association between unprocessed meat intake and CHD, but for processed red meat a 50 g/day increase was associated with a 42% increase in CHD, and two-thirds of this was accounted for by the salt content, although salt is mostly associated with stroke and fatal CHD. Thus, it is not a simple scenario to grapple with, but overall dietary patterns are clearly important.
As to the distinction between fruits and vegetables, Dauchet et al. looked at fruits and vegetables separately and found that the risk of CHD was decreased by 4% (P = 0.0027) for each additional portion per day of fruit and vegetable intake and by 7% (P < 0.0001) for fruit intake. The risk of stroke was decreased by 11% for each additional portion per day of fruits, by 5% for fruits and vegetables, and by 3% (NS) for vegetables. The association between fruits or fruits and vegetables and stroke was linear, suggesting a dose–response relationship (4). Again, the evidence was towards the value of the individual food group, in this case fruits and vegetables. Further evidence could be found in a study using a dietary score which showed that a high plant intake was associated with a 14% reduction in total mortality in the EPIC Elderly study (>60 years old at recruitment) per standard deviation of the score. Interestingly, the apparent association was stronger in Greece, Spain, Denmark and The Netherlands, but absent in the UK and Germany. This suggests there is always more to think about in the broader context of environmental exposures in which dietary patterns develop.
In conclusion, CVD rates are clearly lower in vegetarians and consumers of the Mediterranean diet. Protective foods in these diets include fruits and vegetables, legumes, nuts and fish. Issues that are currently creating a great deal of debate include the adverse role of saturated fat, the protective roles of both n6 and n3 polyunsaturated fats, and olive oil and the problematic position of meat and alcohol. Studying these areas in more detail from both a research and practice perspective will no doubt present interesting challenges for all.