December 24, 2011

Claim: Mediterranean diet can add 3 years to your life... even if you don't start until you're 70 (?)

A load of old cobblers in the article below. Difficult to know where to start but I was amused to read in the journal abstract (also below) that they got their results by using "a refined version of the modified Mediterranean diet index". First you modify your index then you refine it to get the results you want, apparently.

I was however impressed that they went to great trouble to validate their diet questionnaire. Validation is routine in psychology but rare in medicine. None of the validation methods used would have distinguished Mediterranean from non-Mediterranean diets, however. The authors themselves admit the fallibility of their methods by excluding some "implausible" diet claims from their analysis. One wonders if some bias might have crept into that process.

Anyway, as usual, the results are explicable by social class. Middle class Swedes are more likely to say they eat the "correct" foods (whether they do or not) than working class ones are. And middle class people have better health anyway.

I note also that the failed but indestructible antioxidant theory is invoked.

And finally, how do they explain the fact that a traditional Australian diet is about as "incorrect" as you can get yet Australians live longer than Greeks? There are an amazing number of nonageneraians tottering around Australia who grew up on very fatty food accompanied by a few vegetables that had been boiled to death.

The traditional diet favoured in Greece, Spain and Italy provides a great health boost no matter when you switch. No one doubts that following a Mediterranean diet is the healthy option.

But researchers have calculated the regime could add an extra three years to your life. They say it is a rich source of chemicals called anti-oxidants that fight cancer, heart disease and can slow the ageing process.

Scientists who studied the eating habits of 1,200 over-70s found that those following a Mediterranean-style diet tended to live for two or three years longer. They examined surveys which had been carried out by all the adults on their eating habits.

This contained details of how much fruit, vegetables, cereals, meat and fish they ate as well as how much alcohol they drank.

Elderly men and women have been recruited for the rolling research programme since the 1970s. Those taking part were contacted by researchers every few years to find out about their general health.

The team from Sweden’s University of Gothenburg found participants whose eating habits followed a Mediterranean style diet were 20 per cent more likely to be alive eight years later. They calculated that on average these individuals lived for between two and three years longer than those who had a different eating regime.

The diet was inspired by traditional eating habits of Greece and Southern Italy, hence its name.

More HERE

Does the Mediterranean diet predict longevity in the elderly? A Swedish perspective

By Gianluca Tognon et al.

Abstract

Dietary pattern analysis represents a useful improvement in the investigation of diet and health relationships. Particularly, the Mediterranean diet pattern has been associated with reduced mortality risk in several studies involving both younger and elderly population groups. In this research, relationships between dietary macronutrient composition, as well as the Mediterranean diet, and total mortality were assessed in 1,037 seventy-year-old subjects (540 females) information. Diet macronutrient composition was not associated with mortality, while a refined version of the modified Mediterranean diet index showed a significant inverse association (HR=0.93, 95% CI: 0.89; 0.98). As expected, inactive subjects, smokers and those with a higher waist circumference had a higher mortality, while a reduced risk characterized married and more educated people. Sensitivity analyses (which confirmed our results) consisted of: exclusion of one food group at a time in the Mediterranean diet index, exclusion of early deaths, censoring at fixed follow-up time, adjusting for activities of daily living and main cardiovascular risk factors including weight/waist circumference changes at follow up. In conclusion, we can reasonably state that a higher adherence to a Mediterranean diet pattern, especially by consuming wholegrain cereals, foods rich in polyunsaturated fatty acids, and a limited amount of alcohol, predicts increased longevity in the elderly.

Age (Dordr). 2011 September; 33(3): 439–450.

UPDATE

Gianluca Tognon has replied to my comments as follows:

Hi, I’m Gianluca Tognon the first author of this article. On behalf of all the authors of this research, I would like to reply to some issues you mentioned in your article.

First of all thanks for having read it with criticism instead of just reporting the results, science is not based on absolute truths but it can just give the best estimation of the truth after a lot of discussions like this one.

You criticized the fact that we used a refined version of the Mediterranean diet score in order to get an association. The other “unrefined” score did not show an opposite association compared to this one. It was inversely associated with mortality but not statistically significant. This means that what it was measuring was not the whole story. [Why?]

What we did during refinement was to replace total cereals with unrefined cereals based on the fact that low glycaemic index food items are considered healthier than refined one. Then we included eggs, as a possible marker of a “western” diet (together with meat products) and polyunsaturated fats likely to be the kind of unsaturated fats in the healthy Nordic diet. Finally, we included alcohol intake because it’s a hallmark of the Mediterranean diet.

So, since we did not produce several random scores and then report the one that was associated, but based the refinement on previous knowledge, we do not consider this cheating. It is not the first time that this score is modified or adapted to different contexts: development and regional adaptation is a constant and evidence-based process.

Anyway, in the article we reported a sensitivity analysis which showed that none of the factors included in the score we used was able alone to explain our result. This is reported in the article we published.

What is important to say, is that our research was based on food groups (vegetables, cereals, fish products, etc.). Which food items are included in a specific food group can vary from country to country. The important thing is that the dietary pattern inspired by the traditional Mediterranean diet can be exported in other contexts without losing its healthy properties.

We think that explaining this result by just social class would be too simple. Our statistical model was adjusted for smoking, marital status, education and weight status, all possible mediators of the influence of social class on health.

So the association we obtained was independent of potential confounding factors (which, by the way, could also explain the differences in survival between Australian and Greeks since diet is not the only factor affecting survival).

We excluded “implausible” diet reporters by excluding subjects with extreme ratios between the energy calculated from their reported diet and their calculated basal metabolic rate. This did not modify the association making it positive instead of negative, but allowed us to get a better estimation of how much the effect was in terms of longevity increase.

The questionnaire was validated, and I have to tell you that validation of an instrument (like diet history here) with another one is absolutely the rule in epidemiology and not just in psychology as you reported. Diet assessment is subject to errors, validation allows you to reduce them as much as possible.

Finally, regarding antioxidants as a possible explanation, you’re correct that the discussion on this issue is still open. This is an observational study, so we cannot provide a biological explanation of what we observed. But we can speculate what the mechanisms could be. Antioxidants might or might not be part of the story, other researchers will then try to demonstrate if this is the truth or not.

Scientific evidence is not based on a single study, but it’s based on several results, obtained in different contexts by different independent researchers.

I am aware that this will not reply to all possible criticisms about this research, but again thanks for giving us the opportunity to clarify some important issues.

Prof. Tognon is a very civil and polite man so I am reluctant to be too hard on him but his reply really only amplifies my original criticisms. He used an existing index of the Mediterranean diet and found that it predicted nothing. He then dreamt up his own version of the Mediterranean diet and that predicted something. So which is the true measaure of a Mediterranean diet? Surely the first one. The second one could more accurately be called the Tognon diet!

The only real social class marker he used was education but education is only a start. Many highly educated people are poor and many rich people are not well educated. Bill Gates never got a degree. A proper study of social class would also require inclusion of income, occupational status, self-perceived class and IQ. So his findings are very much open to explanation in terms of a class effect.

Finally, Prof. Tognon in his bracketed comment above does something I have been waiting for advocates of the Mediterranean diet to do. He admits that national diets tell us nothing certain about lifespans because other factors could explain long life. I have made that point myself in the past but I have mainly gone along with the joke and pretended that the longer lifespan of Australians (when contrasted with Mediterranean lifespans) tells us something. It does not, of course.

But the same applies to the claim of benefits from the Mediterranean diet. Mediterranean people get fewer heart attacks. So what? How do we know that that is due to their diet? We don't. It could be some genetic factor at work, for instance.

Prof. Tognon's admission has thrown the whole body of epidemiological assertions about the benefits of the Mediterranean diet out the window -- including his own assertions. Correlation is not causation.





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