Why we can’t rely on epidemiological evidence

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I came across this post recently, on the website of a Tom Naughton – a comedian and now nutritional commentator based in the US. Now I should imagine that comedians don’t necessarily make for the best nutritional commentators ordinarily. But Tom is an exception to this rule – he does a great job, in my opinion, of presenting truthful insight regarding the food we put into our mouths and how it affects our health. And he’s a comedian too, which means that much of what he writes at least raises a smile.

The post I have linked to is Tom’s attempt, I think, to reveal just how misleading so-called ‘epidemiological’ evidence can be. Epidemiological studies, also known as ‘observational’ studies, essentially look at the relationships between things. For example, epidemiologists might look at the relationship between smoking and lung cancer or saturated fat and heart disease.

The fundamental issue with such studies is that even if two things are found to be linked, it does not mean that one must be causing the other. To use an example from Tom’s blog post, imagine if we find that egg-eating is linked with an enhanced risk of heart disease. Well, seeing as eggs have generally had an unhealthy reputation for the last 20-30 years, it’s entirely possible that health conscious individuals have gone off eggs, while those who don’t care much to act on health advice have not. And so any apparent link between egg-eating and heart disease might have nothing to do with eggs at all, but to do with the un-health-conscious behaviours of those who eat eggs (e.g. smoking and being sedentary).

There is a huge amount of epidemiological data in the scientific literature, and I would most certainly not dismiss all of it out-of-hand. But I most certainly take far less notice of it than intervention studies – studies where individuals are subjected to something (e.g. a drug, a food or a lifestyle change) to see what effect it has compared to a group who do not experience this intervention. It is these studies that tend to allow us to dissect the realities about what is healthy and what isn’t, and what works and what doesn’t.

I came across a study recently which showed that women eating more wholegrains had lower levels of something called C-reactive protein (CRP) in their bloodstreams [1]. CRP is a marker of inflammation, and higher levels of it are associated with enhanced risk of heart disease. This finding, on the face of it, might be used by some to justify perpetuating the notion that ‘wholegrains are good for the heart’.

Now, wholegrains enjoy a healthy reputation generally. I don’t care much for them myself, as evidenced by the fact that I eat hardly any grain at all (wholegrain or otherwise). But those who are health-conscious and believe wholegrains are healthy will tend to make some effort to include them in their diets. Might the lower CRP levels seen in those eating wholegrain have nothing to do with the wholegrains per se, and more to do with other habits associated with health-consciousness?

One way to perhaps find out would be to perform an intervention study. And as it happens, such a study has been done and published recently and I reported on it here. It found that supplementing individuals with wholegrain did not improve any risk factors for heart disease, including markers for inflammation. We should not conclude from this one study that wholegrain do not benefit heart health, but as I pointed out in the blog post I’ve linked to, there is also evidence that blood sugar-disruptive carbs (including some wholegrains) have been found to worsen heart disease risk markers.

Another supposed health benefit of eating wholegrain is that it reduces the risk of cancer of the colon. There is some epidemiological research to support this. But, again, when the theory has really been put to the test in the form of intervention studies, wholegrains have again been found wanting [2,3].


1. Gaskins A J, et al. Whole Grains Are Associated with Serum Concentrations of High Sensitivity C-Reactive Protein among Premenopausal Women.. Journal of Nutrition 2010;140(9):1669-1676

2. Jacobs ET, et al. Intake of supplemental and total fiber and risk of colorectal adenoma recurrence in the wheat bran fiber trial. Cancer Epidemiol Biomarkers Prev. 2002 11(9):906-14

3. Alberts DS, et al. Lack of effect of a high-fiber cereal supplement on the recurrence of colorectal adenomas. Phoenix Colon Cancer Prevention Physicians’ Network N Engl J Med. 2000 20;342(16):1156-62

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