Note to medical researchers: correlation does not prove causation

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

17 Responses to Note to medical researchers: correlation does not prove causation

  1. nonegiven 15 August 2012 at 7:14 pm #

    So, people will cut back on eggs and the price will go down and I will buy more eggs. More for me!

  2. mark 15 August 2012 at 8:33 pm #

    some contact info here

  3. Phyllis Mueller 15 August 2012 at 9:21 pm #

    Jane Brody of the New York Times is confusing correlation with causation again:

  4. Frances K 16 August 2012 at 12:54 am #

    Mark Sissons has posted about this today too.

  5. Kristjan 16 August 2012 at 1:32 am #

    Great article (yours, not the study).

    A retrospective, highly-flawed study like this is basically meaningless when there are other prospective studies that show no association.

    Not to mention all the RCTs where low-carb, high-fat (usually high-egg) diets improve all the basic cardiac risk factors.

    I’m sure a lot of outdated nutritionists, vegans and fat-phobics will cheer when they see this study and use it to “confirm” their own biased beliefs about dietary fat and cholesterol.

    Too bad that this crap was posted in a respected journal and the media seems to be going haywire over it.

  6. Barbara Silverstein 16 August 2012 at 9:08 am #

    You mention that Dr. Spence has a conflict of interest in promoting cholesterol is bad theory. He was awarded money by Pfizer ( maker of Lipitor), but I was wondering if any of the large food companies i,e. the ones that make breakfast cereal could also be funding this research

  7. Kasey E. 17 August 2012 at 12:30 pm #

    What kind of eggs are we talking about? $.99/dozen mass-produced soy-fed chicken eggs, $6.00/dozen pastured chicken eggs, or something in between? I would love to see some nutritional data comparing the eggs (and meat, for that matter) of chickens raised in various conditions. A study which compared the effects of eating different qualities of eggs could be very useful. But to report about “eggs” as though they were all equal is not.

  8. Kris 17 August 2012 at 5:58 pm #

    Another study by the authors:

    At the bottom, it says:
    “CONFLICT OF INTEREST: None of the authors receives funding from purveyors of margarine or eggs. Dr Spence and Dr Davignon have received honoraria and speaker’s fees from several pharmaceutical companies manufacturing lipid-lowering drugs, and Dr Davignon has received support from Pfizer Canada for an annual atherosclerosis symposium; his research has been funded in part by Pfizer Canada, AstraZeneca Canada Inc and Merck Frosst Canada Ltd.”

    Somehow they forgot to mention the fact that they have received money from manufacturers of lipid-lowering drugs it in the new study, apparently.

  9. André 17 August 2012 at 7:03 pm #

    Causality is the only thing that can persuade me. And the causality on cholesterol is that it is good for you. The more the better. Sadly enough, it is hard to increase cholesterol levels by eating it. You can increase the bad cholesterol by eating sugar, but who wants that?

    I’m not religious but I’m almost inclined to ask : why would God put cholesterol in an egg – the beginning of a new life – when cholesterol kills you? Maybe this ‘scientist’ knows more than nature.

    As a silent protest I will have 4 eggs with bacon tomorrow at breakfast in stead of the usual 3.

  10. Linda Eckhardt 17 August 2012 at 9:28 pm #

    Only way to get a truly “clean” sample is to lock people up and totally control their food supply. i.e. put them in prison. None of these so-called studies about the dangers of a particular food substance means a damn thing except the researcher figured out a way to get his work paid for another semester. The real elephant in the living room is processed foods. It ain’t eggs folks. see Paleo Power for Seniors at EENews.

  11. Linda Eckhardt 17 August 2012 at 9:30 pm #

    Eat unprocessed foods. Real food. Eat eggs, meat, fish, vegetables, and fruits. Toss in a few nuts and berries. You’ll feel better, you’ll be healthier. Paleo power to the people!

  12. Helen 17 August 2012 at 9:48 pm #

    I had a two-egg omelette for my breakfast/lunch, filled with grated mature goat’s cheese and plenty of tomatoes (fried in butter). Delicious! I haven’t lost any weight since moving to a low-carb, higher protein, higher fat diet in March (doesn’t work so well for we menopausals), but I am enjoying my food so much more now, because I don’t worry or stress so much about what I eat. Food tastes so much better too, if you cut down on carbs of all kinds. To paraphrase Dr Strangelove, I have learned How to Stop Worrying and Love the Fat! Much of the indigestion I used to get, I now realise, was caused by the supposedly healthy wholegrain foods I used to consume in quantity. My skin and hair condition have greatly improved. I think I was actually malnourished after 30+ years on a so-called healthy diet. When I get my new cooker, I’ll also be able to include more fresh ingredients and less in the way of processed/pre-prepared foods.

    As well as Dr Briffa’s latest book, I would also recommend Dr Malcolm Kendrick’s ‘The Great Cholesterol Con’, which demolishes all the standard objections to a fatty diet and explains how fat and cholesterol cannot form arterial plaques, and posits a likely mechanism by which plaques are actually formed: a combination of poor carbohydrate metabolism, and a disruption of the hypothalamic-pituitary-adrenal axis. The latter will ring bells with anyone who has ME. A piece of research undertaken by a team at Dundee University a few years ago showed a correlation (not sure what kind though) between ME and hardening of the arteries. ME is thought by researchers interested in the condition’s physiological causes – as opposed to the psychiatric lobby which has claimed clinical ownership of this condition – to be induced by just such a disruption to the HPA axis. I have ME, so I’m now wondering if my risk of CVD has been raised for reasons unconnected with my diet. I have a familial risk too. I have concluded that anything that reduces physical and mental stress, like a low-carb diet, must be worth doing for this reason.

  13. Monique Palmer 17 August 2012 at 9:51 pm #

    Its a shame these ‘ findings’ were also put as a headline on the NHS Choices website where many people will not read beyond the intial damning headline… ! some people may be tempted back to their sugar laden breakfast cereal

  14. John Walker 17 August 2012 at 10:00 pm #

    You could lock me up if you wish, but you would never get me eating avocado. I hate the ‘non’ taste, and the texture of them. Same goes for aubergine. Despite the knowledge these foods are not going to harm me, I just won’t eat them, cos I don’t like ’em! It’s good that I like eggs, meat, fish and cheese then! 🙂 !

  15. Sue Gooch 18 August 2012 at 3:39 pm #

    ” I haven’t lost any weight since moving to a low-carb, higher protein, higher fat diet in March (doesn’t work so well for we menopausals),”

    An interesting remark Helen, do you have a link to anything that backs this up? I started back in February and lost a couple pounds and no more despite still sticking to it longer than any other “diet”. I am post menopausal and am quite disappointed. I am moving on with fasting following the Horizon programme with Dr. Mosley. Some initial loss which is promising, maybe this is suggesting that although I do not eat huge portions or snack, it is the amount of food we eat that should be reduced. I wonder who came up with the statistics for ideal calorie consumption for men and women? Were they a product of the same think tank that brought us 5 a day and alcohol units, none of which seem to be supported by good hard evidence?

  16. Helen 19 August 2012 at 1:22 am #

    “An interesting remark Helen, do you have a link to anything that backs this up? I started back in February and lost a couple pounds and no more despite still sticking to it longer than any other “diet”. I am post menopausal and am quite disappointed.”

    I’m afraid my remark was based on anecdotal evidence – my own, and quite a few other women I know who have tried this approach over the past 10-12 years, since the Atkins Diet came to public attention in the UK. I have seen similar remarks on discussion forums (sorry, can’t remember which), but no-one has offered a really cogent explanation that I’ve read. I find menopause books just offer the bog standard advice, without explaining why. I’m hoping Dr Briffa will turn his attention to this problem, because it seems there are few options for avoiding post-menopausal bloat. I have had a thyroid problem since my teens, which obviously doesn’t help, but I have sought good advice in the private sector for this, and many of the hypo problems receded some years ago after I moved on to T3-only treatment – this occurred well before of the menopause.

    I don’t know what the evidential basis is for the current calorie limits for men and women, but John Briffa tells us that not all calories are equal, so I take less notice of that now. I’m not a big eater anyway and frequently don’t eat before lunch, which I keep light. Occasional snacks are limited to the least sweet fruit, or nuts, and I’m happy with that. Sometimes I tot up the calories in my evening meal, just out of interest, but they rarely exceed 400, and are often lower. I’ll keep on with the low-carb diet because I feel better on it, but for weight loss I haven’t a clue! I can’t do a lot of exercise, but I do my best, and that’s all anyone can do.

  17. John Walker 19 August 2012 at 4:01 pm #

    I first heard of Dr. Atkins in the late 1970s. At that time, I lost weight in remarkably quick time, and quite a lot of weight too. I didn’t exercise, as such, but I did take to cycling to work, and going for ‘leisure’ rides whenever I could. Why I didn’t stay with the diet, I don’t know, but maybe it was because, much as I like meat, I found it difficult to keep to the diet way from home. There were also times when I found the choices of mix and match of proteins a little bland. But the moment I started eating bread and potatoes again, the weight came back on, almost as quickly as I lost it. So I am convinced. I used to keep track of how many carbs I could tolerate, using ‘ketostix’, but to be honest, they are ridiculously expensive these days. So I just don’t eat starch or sugar (Not that I ever did eat much sugar!) The weight is going, but much more slowly, but at least I am having to buy smaller clothes!

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