A recent study conducted in Spain has been hailed by many commentators in the media as further evidence of the benefits of a Mediterranean-type diet (1). This is the PREDIMED trial in which the effects of 3 different diets were compared with respect to the incident rates of various diseases and al-cause-mortality. The participants were men (55 to 80 years of age) and women (60 to 80 years of age) with no cardiovascular disease at the outset but who had either type 2 diabetes mellitus or at least three of the following major risk factors: smoking, hypertension, elevated low-density lipoprotein cholesterol (LDL) levels, low high-density lipoprotein cholesterol (HDL) levels, overweight or obesity, or a family history of premature coronary heart disease. Initially there were 7447 men and women who were allocated at random to one of3 different diets. These were a usual Spanish diet(the control) or one supplemented with extra virgin olive oil (EVOO) or another which was given an extra supplement of 30 g of mixed nuts per day (15 g of walnuts, 7.5 g of hazelnuts, and 7.5 g of almonds). There was no attempt to control the intake of calories.
It was found that during the follow-up period of almost 5 years those in the experimental groups increased weekly servings of fish (by 0.3 servings) and legumes (by 0.4 servings) in comparison with those in the control group.
The results reported the changes in the primary end point (based on the incidence of heart attacks, stroke and the death rate for cardiovascular disease) and in the all-cause mortality. The authors concluded that:
“an energy-unrestricted Mediterranean diet supplemented with either extra-virgin olive oil or nuts resulted in an absolute risk reduction of approximately 3 major cardiovascular events per 1000 person-years, for a relative risk reduction of approximately 30%, among high-risk persons who were initially free of cardiovascular disease. These results support the benefits of the Mediterranean diet for cardiovascular risk reduction”
This message was enthusiastically taken up by the press. For example The Guardian(2) stated that the results:
“offer hope to those in danger of a heart attack or stroke because they smoke, have type 2 diabetes or exhibit other unhealthy characteristics”
While the above statements are true in so far as they go, this is certainly not the whole story. First of all it is important to understand that the 30% reduction is a relative value which is obtained by relating the absolute rate of 4.4% in the control to 3.4% in the experimental diets. This is of course a significant improvement but only if it is not compromised by an increase in the death rate from other causes. Unfortunately this is actually what happened. There was no significant reduction in the all-cause mortality rates. Essentially what this means is that the time of your death will still be the same but the cause of death may be different.
Any reader of the original paper will have to be very thorough in order to detect this information because the authors do gloss over this information with all the emphasis on the benefits related to heart disease.
This story is typical of the way in which scientific information can be twisted and misrepresented.
In reality this study does not tell us very much about the role of a Mediterranean diet as all 3 diets could be defined as such. A much more convincing result was obtained in the Lyon Heart Study which compared a Mediterranean type diet with a conventional Western diet typical of the USA or the British Isles. This confirmed that the incidence and death rates attributed to heart disease are reduced very substantially. But of even greater significance was the fact that the total mortality was also reduced by at least 50%. The key differences between the diets were in the polyunsaturated fatty acids (PUFAs)(See Blog 15). An increase in omega3s and a reduction in the omega6s meant that the omega6:omega3 ratio fell from 18 to 4. The authors of the report concluded that this difference between the diets was the critical factor contributing to the much lower death rates in the experimental diet (3,4).
For those who wish to make changes in their diet along these lines then clearly the omega3s can be increased by increasing the consumption of oily fish. However it is crucial to appreciate that reducing the intake of foods which are high in omega6s is just as effective. This can be achieved by avoiding products such as vegetable oils and spreads. Invariably these are promoted as “healthy” on the grounds that they lower cholesterol. This rationale is fundamentally flawed since the “evidence” that lowering blood chlolesterol reduces heart disease does not stand up to rigorous examination (See Blog 6)
REFERENCES
- R Estruch et al (2013) New England Journal of Medicine February 25, 2013DOI: 10.1056/NEJMoa1200303
- Guardian (2013) 26th February
- M de Logeril et al(1994) Lancet 343 (8911) pp454-459
- M de Logeril et al (1999) Circulation 99 (6) pp 779-785.
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