Note to medical researchers: correlation does not prove causation

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Someone recently sent me a link to this report of a study which warns us of the supposed perils of eating eggs. The study on which the report is based [1] looked at the association between the amount of egg yolks people ate and the amount of ‘atherosclerotic plaque’ in the main arteries supplying blood to the head (the carotid arteries). The authors tells us that an association was found, even when other potential ‘confounding’ factors were taken into account such as smoking and body mass index. By their own admission, though, the authors suggest they should have perhaps included some other confounding factors too, including exercise and waist circumference. Nevertheless, the end result is the authors warning us off eating eggs. Is there concern and advice justified?

OK, let’s get something straight from the start: this study is what is referred to as an epidemiological study, which looks at associations between things, but can’t be used to determine causality (in this case, that eating eggs accelerates atherosclerosis). One relevant factor here is that eggs have suffered from an unhealthy reputation for years now, and it may be that those who eat eggs are less health-conscious on-the-whole, and perhaps are more likely to engage in genuinely life-threatening behaviours such as filling up on processed food or being very sedentary. I actually wrote about these issues most recently here, in a post entitled ‘Note to medical journalists: correlation does not prove causation’. It seems some researchers need to be reminded of this too, hence the title of this blog post.

Another fundamental problem with research of this nature as it relies on individuals reporting how much and/or often they eat of specific foods. As a patient remarked to me yesterday, most people find it difficult to recall what they ate even a couple of days ago. Self-reporting of diet is generally recognized to be hopelessly prone to error.

So, let’s summarise here the essentials of this study:

  1. it found an association between egg eating and the amount of atherosclerosis in the carotid arteries
  2. it controlled for some relevant confounding factors but not others
  3. it relied on self-reported dietary data which is very prone to error
  4. even if the study was really well done and the dietary reporting accurate, it’s still epidemiological in nature which tells us, in the end, little or nothing

Now, bear this in mind when you read this quote from the one of the study authors – Dr J David Spence – as it appears in the report I link to above:

What we have shown is that with aging, plaque builds up gradually in the arteries of Canadians, and egg yolks make it build up faster…

This stance clearly gives the impression that eating eggs yolks cause atherosclerosis, but this claim simply cannot be made on the basis of this study.

Here’s another quote from Dr Spence:

In diabetics, an egg a day increases coronary risk by two to five-fold


He’s referring to other epidemiological research here, and again his assertion is indefensible.

But maybe we shouldn’t be too surprised, here, because Dr Spence has form in this area. In a previous blog post here I detail how he, along with a co-author, makes strong claims about eggs eating based on weak evidence. What would cause a ‘scientist’ to overstate the relevance of his or her research? Many things, but here’s two:

1. Ego
Look, researchers generally like to publish ‘impactful’ stuff. Too bad that Dr Spence engages in ‘research’ that simply can’t be very impactful on the basis of it epidemiological nature. Even unconsciously there can be a tendency to ‘over-egg’ (sorry, couldn’t resist) one’s findings.

2. Conflicts of interest
As I detail in the blog post I link to above, Dr Spence has been rewarded financially in a way that gives him a vested interest in keeping the ‘cholesterol is bad’ theory alive.


1. Spence JD, et al. Egg yolk consumption and carotid plaque. Circulation epub 31 July 2012

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