Eating egg yolks as bad as smoking?

This article refers to the article and paper below:
http://www.sciencedaily.com/releases/2012/08/120813155640.htm
http://www.sciencedirect.com/science/article/pii/S0021915012005047

egg yolksOn August 13, 2012, ScienceDaily.com published an article entitled, “Eating Egg Yolks as Bad as Smoking?” ScienceDaily.com concludes “eating egg yolks accelerates atherosclerosis in a manner similar to smoking cigarettes.”

Unfortunately, ScienceDaily.com and many other news networks failed to accurately describe the details and outcomes of the study. Here, I carefully examine the study and suggest an alternative conclusion from the data.

First, it is important to look at the participants of the study. The data was collected from individuals soon after they had a stroke or transient ischeamic attack (known as a “mini stroke”). This study is not examining healthy individuals or comparing the number of strokes in people who ate lots of eggs vs. those who ate few eggs. All participants in the study already had a stroke regardless of their egg consumption.

Next, the means of data collection is important to consider. Participants were given questionnaires and asked to recall the number of eggs and packs of cigarettes they had smoked during their lifetime. Questions regarding exercise, stress levels, and other aspects of the diet were not asked. The researchers relied on the participants to be both truthful and accurate in their memory of egg consumption and smoking history during their lifetime.

There are important differences in the makeup of each group in the study. The group that ate the most eggs had an average age of 69.77 years compared to only 55.70 years for the group who ate the least eggs. The group who ate the most eggs also smoked the most and had the highest rate of diabetes. Surprisingly, the group that ate the most eggs had the lowest total cholesterol, lowest LDL cholesterol, highest HDL cholesterol, and lowest body mass index.

The difference in age between the groups eating the most and least eggs is important, as the authors themselves admit that plaque area increases with age. Although raw data is not presented in the paper, based on figure 1A, it appears that individuals with an age of 69.77, solely based on age, would have an average cartoid plaque area of 170 mm2. The group that ate the most eggs had an average carotid plaque area of about 175 mm2, while the group that smoked the most cigarettes had an average carotid area of about 240 mm2. This indicates that the group that ate the most eggs had an average carotid plaque area that was 2.94% greater than expected, while the group that smoked the most cigarettes had an average carotid plaque area 40.00% greater than expected. Unfortunately, figure 1C, comparing eggs eaten and plaque area, does not correct for age so the effect of egg consumption appears to have much more of an effect than it would if age was eliminated as a variable.

The authors promote the idea that egg yolks are bad because they are high in cholesterol and they argue eating foods high in cholesterol increases serum cholesterol in the blood. However, as stated earlier, the group that ate the most eggs actually had the lowest total cholesterol, lowest LDL cholesterol, and highest HDL cholesterol.  According to their data, it seems that eating lots of eggs actually promotes a healthier cholesterol profile and lower body mass index. Amazingly, the authors do not address this in their paper nor do they hypothesize on what mechanism is causing high egg consumption to increase plaque buildup.

Egg consumption is likely not the cause of increased plaque. Too many other variables are at play (exercise, stress, dietary factors). Interestingly, the authors note that these patients had been advised to lower egg consumption and had not complied prior to their stroke. Perhaps they were less concerned with their health than the group that had complied and lowered egg consumption, causing them to make a variety of poor health choices unrelated to egg consumption.

Alternatively, this data could be interpreted completely differently. The data shows the individuals who ate the most eggs were the oldest and had the most plaque after their stroke. Perhaps the eggs actually had a protective effect allowing those who ate the most eggs to withstand more plaque buildup and live the longer before having a stroke. Those individuals who ate the fewest eggs had a stroke an average of 14 years earlier than those who ate the most eggs. Perhaps if they would have been eating more eggs, they would have lived longer without a stroke.

It is important to remember that these epidemiological studies do not show cause and effect. They merely show correlation. They serve as a point of reference to form a hypothesis to be tested in prospective studies. Unfortunately, the authors say, “Our findings suggest that regular consumption of egg yolk should be avoided by persons at risk of cardiovascular disease.” This is not true. That statement is merely their hypothesis that needs to be tested in prospective studies. Based on my logic in the previous paragraph, with the same data I could have concluded, “Our findings suggest that regular consumption of egg yolk should be encouraged for persons at risk of cardiovascular disease.”

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22 thoughts on “Eating egg yolks as bad as smoking?

  1. This is a great review and I am glad you took the time to further analyze it. Although, how can I verify from the study what you stated above “the group that ate the most eggs actually had the lowest total cholesterol, lowest LDL cholesterol, and highest HDL cholesterol” Also the data for the carotid plaque area? Did you have to pay $31.50 from sciencedirect.com to actually see this data that was collected?

  2. THANK YOU for posting this review. From the sciencedirect site, I can’t see the data that was actually collected, nor can I see the charts, and it’s been bugging me. What *were* the survey questions? While it’s apparent that this survey was poorly done and in a way that specifically skews the numbers, I’d still like to see the survey. This article is being quoted all over the major news sites and it’s disheartening because it spreads such misinformation.

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  4. Hey guys, as far as obtaining the actual study- the journal Atherosclerosis currently holds the rights to it. Often times public libraries have access to these databases, so you could access it print it there. Any college or university should have access to this paper too. Alternatively, you can pay for it- although $31.50 seems like a lot for a bad study. I think most publicly funded research becomes free to everyone a year after it’s release, but that doesn’t do much good now.

    Sometimes the journals will allow reprints of certain pieces of the study. I’ll ask permission to reprint the graphs and data here. I’ll keep you updated.

  5. Welcome to blogging and WordPress. I thought your critique of the study was excellent and posted it for my readers on Food, Facts, and Fads (foodworksblog.wordpress.com). Check out my post entitled, Eggs, Cholesterol, Bad Science? and note the section on Conflict of Interest!! Gives some obvious clues as to why they reached their bizarre conclusions. Keep up the good work.

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  7. Mr. Barrington, Excellent rebuttal and thank you so much for putting this together and posting. When I first heard this I was motivated to write something as well. I’m just an IT guy, no medical or nutrition training, no PHD… but for the last year and a half I’ve been digging and learning more and more about my health our food and health industry, and it’s been an eye opener. I have a blog on WordPress as well and if your interested you can see what I wrote on the subject here. More of a mocking than anything else, but wanted to know I linked to your post. I would also like to add your blog to my links with your permission. Here’s my post: http://lonbeyond.wordpress.com/2012/08/15/egg-yolks-same-as-smoking/
    More in laymen’s terms and lots of parody. Thanks, I appreciate and will follow your blog.

  8. Dear Bill. I am not at all surprised that a sensationalist article like the one I saw in Science Daily News and other news sources around the world recently on this British study that included an evaluation of egg yolks is creating a lot of debate. I am yet to read the research article fully so I cannot comment properly but I hope to soon (have been away from my desk in the field). Except to say that I am very pleased that you have responded and examined the paper yourself and have made what look like reasonable comments on the methodological and probably value-laden limitations of the paper. If as you suggest the claims made about the ill-health value of the cholesterol of eggs rely on opinions (via a survey) data collection methods, including their lack of rigor in controlling the variables then there is nothing new to report and this research may be limited or open to different interpretations and speculation and may as you suggest actually be a misreading and mis-analysis of their own data. Previous research findings that I am aware of indicate that (a) humans produce their own cholesterol maybe up to around 50% and (b) cholesterol that is ingested really only has an effect on lipo-proteins (LDL’s) and small blood vessels (e.g., coronary, cerebral, ocular arteries) if dietary saturated fats are consumed in moderate to high amounts. Thank you for your overview.

  9. It seems to me that a great deal of “science” is done poorly by vested interests so that they can make claims like “Studies show [our product works]” They know people buy into that.

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  12. Really nice to see someone doing the legwork that the news outlets reporting this should’ve been doing. Great analysis and timely. Looking forward to more stuff like this!

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  14. Dear Bill and all:
    There is yet another part of this incredibly bad article that should be mentioned since the statistics are totally wrong:

    From the abstract:
    “Data were available in 1262 patients; mean (SD) age was 61.5 (14.8) years; 47% were women. Carotid plaque area increased linearly with age after age 40, but increased exponentially with pack-years of smoking and with egg-yolk years. Plaque area in patients consuming <2 eggs per week (n = 388) was 125 ± 129 mm2, versus 132 ± 142 mm2 in those consuming 3 or more eggs per week (n = 603); (p < 0.0001 after adjustment for age). In multiple regression, egg-yolk years remained significant after adjusting for coronary risk factors."

    My comment:
    125+/-129 versus 132+/-142 is a totally insignificant result, since the difference between the mean result (132-125=7) is hugely smaller than the standard deviations. Nevertheless they say that it is exceedingly significant with a very VERY small probability of getting this result by chance (p<0.0001). How could this be? With the difference between eating more or fewer than 3 eggs a week being much smaller than the standard deviation (7<<0.96, not p<0.0001!.

    How did they obtain the absurdity that the results are very significant? They got it "after adjusting for age". As you pointed out, the people that ate eggs lived longer had a stroke 14 years later (on the average) than the ones that didn't eat eggs. How could the age adjustment make the results significant?

    I've never seen a worst use of statistics in my whole life!

    Eugenia Kalnay
    Professor, University of Maryland

    • Thanks for your comments Eugenia.

      It’s seems to me that the researchers were aiming to show this result so they found the statistics to do so. On closer review, their analysis simply doesn’t hold up.

      • Hi Eugenia,

        If you’d like to write up a discussion on their paper I’d be glad to post it on my site and give you credit. It’s awesome that you take an interest in sciences outside of your main field. How did you find my blog?

        Bill

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