The big health/nutrition news this last week was about a study that supposedly showed how eating egg yolks is almost as bad as smoking. What??? The first account I read of the study was on Science Daily, and that story, published August 13, 2012, accepted the study results ‘hook, line, and sinker,’ without any apparent reservation.

The lead researcher for the study was Dr. J. David Spence, a professor of Neurology at Western’s Schulich School of Medicine and Dentistry in Canada.  In support of the study, Egg yolk consumption and carotid plaque, he makes such statements as “It has been known for a long time that a high cholesterol intake increases the risk of cardiovascular events, and egg yolks have a very high cholesterol content.” and “In diabetics, an egg a day increases coronary risk by two to five-fold.”  The study concludes that “eating egg yolks accelerates atherosclerosis in a manner similar to smoking cigarettes.” The press release about the study was reworded and repeated by many news outlets, mostly without questioning the conclusions: New York Daily News, CNN’s The Chart blog, USNews Health, CBC, the Daily Mail, plus many more.  I did find one story on NHS Choices  that listed several faults with the study.

When I first read the story in Science Daily, I knew immediately this ‘study’ didn’t make rational sense, so I started looking for the rebuttals which I knew would be coming.  And did they come!  I’ve collected some of the most significant points made in the rebuttals, many of them noted by more than one source.

Here’s what’s wrong with this ‘study’:

Understand Nutrition, August 14

The first response I read, Eating Egg Yolks as Bad as Smoking?, was posted the next day, August 14, by Bill Barrington on Understand Nutrition, and it was really good! He first points out that none of the participants in the study were healthy, all had already had a stroke or “mini stroke.”  Next, he mentions that the data collection was mostly through questionnaires asking how many eggs they had eaten and the number of packs of cigarettes they had smoked during their lifetime.  They could remember what they ate their whole lifetime?  The study assumed that the participants’ answers were both truthful and accurate, but no other diet, exercise, or stress-related questions were asked.  Questionnaires about eating habits are notoriously unreliable. The reviewer also noted that the group with the most atherosclerotic plaque averaged 15 years older than the group with the least plaque, and even the authors admit that plaque increases with age.  Although the authors claim that egg yolks are harmful because of their cholesterol content, “the group that ate the most eggs actually had the lowest cholesterol, lowest LDL cholesterol, and highest HDL cholesterol.”  Eating eggs would seem to be beneficial rather than harmful.  Finally the reviewer says that this was an epidemiological study that cannot show cause and effect.  Unfortunately the authors do make assumptions about cause and effect based on what appears to be a misunderstanding of their own data.

Dr. Briffa on egg yolks and smoking, August 15

Dr. Briffa makes some of the same points in his rebuttal, Note to medical researchers: correlation does not prove causation, but also mentions that researchers like to publish “‘impactful’ stuff” which may have led Dr. Spence to overstate the results.  Dr. Briffa also refers to a rebuttal he made of a similar study by Dr. Spence in 2010 where he also claimed that people at risk of heart disease should limit intake of cholesterol and stop eating egg yolks.  Dr. Briffa provides information that both Dr. Spence and fellow researcher, Dr. Jean Davignon, have received “honoraria and speaker’s fees from several pharmaceutical companies manufacturing lipid-lowering drugs.”  Also, Dr. Davignon’s research has been funded in part by Pfizer Canada, AstraZeneca Canada Inc, and Merck Frosst Canada Ltd.  These two researchers “have some potential interest in keeping the cholesterol theory alive and well.” [emphasis mine]

Mark’s Daily Apple, August 15

Mark’s Daily Apple addresses the debate in Are Eggs Really As Bad For Your Arteries as Cigarettes?  He mentions upfront the connections of Dr. Spence and Dr. Davignon with statin manufacturers.  He then emphasizes the unlikelihood of the participants remembering “their lifelong egg intake and smoking history.”  He notes that those who ate the most eggs were the oldest, smoked the most, and were the most diabetic.  Could those be “‘Possible'” confounders”?  He says that this is an observational study that depends on memory being infallible and completely ignores other possibly more significant factors, like what the stroke victims ate with their eggs–maybe pancakes, white bread, and vegetable oils.  This review makes a point not seen in other rebuttals.  What if the problem with atherosclerotic plaque might be caused by the quality of the eggs, which may have been from confined hens fed a high omega-6 diet of soy and corn rather than eggs from pastured hens?  He cites a study where the quality of the eggs eaten affected LDL cholesterol levels.

Chris Masterjohn, August 16

Chris Masterjohn thoroughly rebuts the egg yolk/smoking claims in Does Eating Egg Yolks Increase Arterial Plaque?  He also notices the problem with questionnaires of lifetime eating habits and that plaque increases with age.  He adds some points not seen elsewhere.  He says “Are we really supposed to believe that the switch in the year 2000 to ‘urgent’ clientele who had just suffered from transient ischemic attack or stroke makes no difference? Is it not possible that these ‘urgent’ cases tended to have more plaque?”  He also notes (and includes a graph) that Canadian egg consumption varied widely during the period of the study, falling to a low in 1999, then beginning a steady increase around 2000.

 Zoe Harcombe, August 8

This may have been the first rebuttal published, but I didn’t find it until I had read most of the others. Zoe Harcombe’s article was probably in response to the publication online July 31, 2012, of the study itself, not the later press release about the study, published August 13, 2012, which prompted the widespread media reports.  Here’s what Zoe Harcombe says in Egg yolk consumption, carotid plaque & bad science.  She says she had access to the full article, which is not available for free.  She mentions some discrepancies between the abstract and the full article.  For example, the abstract said there were 1,262 participants while the full study says 1,231 people were involved.  Other interesting bits of information: the average age of the participants was 61.5 years and 47% were women.  She says “The entire study is about egg yolks. [why–conflict of interest, perhaps?] There is no evidence presented in the paper that these 1,231 people throw away egg whites.  The paper has simply assumed that there can be nothing in egg whites that could cause any concern whatsoever so we don’t need to even think about egg whites.” [emphasis mine] She provides a table with extensive details about the nutrition in a whole egg, egg yolk, and egg white and concludes “So the egg yolk is where we find the nutrition in an egg.” [For more information about the nutritive value of eggs, see my previous post, Why would anyone bother to eat just the white of an egg?]  She observes, as most reviewers did, “that plaque increases directly with age–this is not surprising. . .”  She does add in a note that “plaque itself is not the root cause of problems. Plaque forms over damage to the arterial walls in much the same way that a scab forms over a cut on the skin. The original source of that damage is what we need to understand.”   She discusses the composition and our digestive processing of eggs quite extensively and questions the validity of any claim that egg yolk could clog our arteries.

Tom Naughton, August 16

Tom Naughton, in The Anti-Egg Bad Scientist Strikes Again, has an interesting, humorous way of expressing himself.  He covers many of the same faults in the study that others have noted.  He notes that this was an observational study that could not justify the researchers’ conclusions about eggs causing acceleration of arterial plaque.  I especially love the way he describes those conclusions:

Here’s what you can reasonably conclude about cause and effect from a study like this:

[nothing]

Tom Naughton says that we are not just “witnessing the usual media interpretation of an observation study. But in this case, the lead (ahem) researcher has been aiding and abetting that misinterpretation.”  Dr. Spence claims that the “effect of egg yolk consumption over time on increasing the amount of plaque in the arteries was independent of sex, cholesterol, blood pressure, smoking, body mass index and diabetes.” [emphasis mine] However, how can “the artery-clogging effects of eggs” be independent of cholesterol? when “the whole reason Dr. Spence has been warning us against consuming eggs is that they contain too much cholesterol.”

Tom Naughton’s review has much more to say about this ‘study.’  In fact, all of these responses have much more information and are worth reading entirely, if you can. They each look at different factors, but all come to the same conclusion–the study does not and cannot prove that eating egg yolks causes accelerated arterial plaque or is similar to smoking in its effect on arterial plaque.

Update August 27, 2012: 

Another article questions the claims of the egg yolk study and further expresses fears that no amount of rebuttal will stop the damage already done.  The article reports an interview with Dr. Stephanie Seneff, MIT senior scientist, who is working on compelling research that indicates we may actually be “suffering from cholesterol deficiency rather than excess.”

Update August 27, 2012:

Additional details about the study are revealed in  “Is It True that Eggs are as Bad for Your Arteries as Smoking?”  The egg yolk study was funded by the Heart & Stroke Foundation of Ontario and the Heart & Stroke Foundation of Canada, both organizations funded by the pharmaceutical industry by at least $7 million a year.  The conflicts of interest of two of the three researchers was mentioned by several of the rebuttals summarized above; however, this article adds that the third researcher, David Jenkins, is one of the creators of the vegan ‘Portfolio Diet,’ which recommends 42.8 grams of soy protein a day.

Sources:

Egg Yolk Consumption Almost as Bad as Smoking When It Comes to Atherosclerosis, Study Suggests
Egg yolk consumption and carotid plaque
Research finds egg yolks almost as bad as smoking
Eating Egg Yolks as Bad as Smoking?
Note to medical researchers: correlation does not prove causation
Are Eggs Really As Bad For Your Arteries as Cigarettes?
Does Eating Egg Yolks Increase Arterial Plaque?
Egg yolk consumption, carotid plaque & bad science
The Anti-Egg Bad Scientist Strikes Again
Sunny-Side Up: In Defense of Eggs
Is It True that Eggs are as Bad for Your Arteries as Smoking?

This post is linked in Monday Mania 8/20/2012 on The Healthy Home Economist