The news headlines on Egg yolks almost as bad as smoking (Murray, 2012) certainly caused a flurry in the nutrition and food manufacturing world. I must admit that I was not particularly phased by this gloomy claim and actually had a feeling of déjà vu.
Readers may also have realised that this type of headline recurs regularly - probably every 2-3 years we have an egg scare, then there is the artificial sweeteners scare, the margarine scare, and so on. So should we actually be concerned by these research findings?
The study which was conducted by Dr David Spence and his colleagues at the Stroke Prevention & Atherosclerosis Research Centre, in London, Canada (not the UK), compared the size of plaques (fatty deposits in the arteries) related to egg intake to the plaque related to smoking in terms of "pack-years". The data that was obtained for the total of 1 262 patients, both male and female, with an average age of 61.6 years, showed that the size of the plaques (as determined by duplex ultrasound) increased with age, pack-years of smoking and egg yolk intake, as follows:
- In the 388 patients who consumed less than 2 eggs per week, the size of the plaque area was 125 mm2, compared to a plaque area size of 132 mm2 in the 603 patients who consumed 3 or more eggs per week. This difference in the size of the plaque areas was found to be highly significant even when other factors that could influence plaque buildup were eliminated by means of statistical methods.
(Spence et al, 2012)
The publication does not provide similar concrete results for size of plaque area due to smoking in pack-years, but the authors do state categorically: "Our findings suggest that regular consumption of egg yolk should be avoided by persons at risk of cardiovascular disease."
Understandably the results of this study were snapped up by the media. After all, it’s been a while since we had a "cholesterol - egg - heart disease" scare story to enliven our reading.
While most newspapers and electronic news services carried the story, a number of pro-egg or pro-natural food websites have come to the defence of the humble egg.
For example, Adam Bornstein of Livestrong reports that he consulted a number of prominent nutritionists who pointed out that the Canadian study was flawed by factors such as its observational nature, use of subjects who were not in good health, lack of exercise data, lack of data relating to dietary intake of saturated fat and sources of cholesterol other than eggs, as well as dependence on self-reporting by the subjects.
Examples of earlier studies
Because eggs and particularly egg yolks have a high cholesterol content, they have been the subject of research studies into the causes of arteriosclerosis (heart disease) for many years.
Let’s consider two of these studies:
a) South African Study:
A South African study conducted by leading nutrition researchers at the then University of Potchefstroom in 1992 investigated the effect of eating 3-14 eggs per week on biochemical risk markers of coronary heart disease in 70 young men who also ate a high-fat diet (Vorster et al, 1992).
In this study, all the participants first ate 3 eggs per week for 2 months in the run-in phase. During the 5 month experimental phase the control subjects continued to eat 3 eggs per week, while the two experimental groups of subjects ate either 7 or 14 eggs per week.
None of the risk markers for heart disease such as lipoprotein in the blood or coagulation factors were different in the control and the two egg-eating experimental groups. Vorster and her co-authors (1992) recommended that dietary advice to prevent cardiovascular disease, should emphasise reducing total fat intake, rather than to concentrate only on dietary cholesterol intake in general, and eggs in particular.
b) American Study
Another study conducted in the USA nearly a decade later, investigated the link between egg consumption and the risk of coronary heart disease (CHD) and of stroke in men and women (Hu et al, 1999). A total of 37 851 men between the ages of 40 to 75 years, and 80 082 women between the ages of 34 to 59 years who were free of cardiovascular disease, high blood cholesterol, diabetes or cancer, were included in the study. Egg intake was determined with the aid of a food-frequency questionnaire.
The men were followed up for 8 years, while the women were followed up for 14 years, with 866 cases of CHD and 258 cases of stroke occurring in males and 939 cases of CHD and 563 cases of stroke in the female subjects during the two different follow-up periods.
When the occurrences of CHD and stroke were compared to egg consumption no overall association between eggs intake and cardiovascular diseases in these very large populations could be identified.
Hu and his coworkers (2012) concluded that, "These findings suggest that consumption of up to 1 egg per day is unlikely to have substantial impact on the risk of CHD or stroke among healthy men and women." The only link that was identified was in diabetic men and women in the study groups, where higher egg consumption appeared to be linked to an increased risk of CHD.
To answer the question if we should be concerned by the findings of the latest egg vs. smoking trial conducted by Spence and his co-workers (2012), I think it is feasible to say that their study may well have had some flaws and that the basic recommendation made by South African diet and nutrition experts many years ago that South Africans should not ingest more than 4 eggs per week because of our tendency to developing cardiovascular disease, still holds true.
It is not necessary to remove all eggs from our diets, but it is also not a good idea to consume large quantities of eggs every single day of the week. Moderation remains the key to good health.
Any questions? Ask Diettitians
(References: Bornstein A (2012). Do eggs cause hear disease? http://www.livestrong.com/blog/do-eggs-cause-heart-disease/; Hu F B et al (1999). A prospective study of egg consumption and risk of cardiovascular disease in men and women. Journal of the American Medical Association, 281, 15(1387-1394; Murray R (2012). Eggs yolks almost as bad as smoking. http://articles.nydailynews.com/2012-08-17/news/33252380_1_yolks-hollandaise-spence New York Daily News. Spence JD et al (2012). Egg yolk consumption and carotid plaque. Atherosclerosis (Ahead of print); Vorster HH et al (1992). Egg intake does not change plasma lipoprotein and coagulation profiles. American Journal of Clinical Nutrition, 55(2):400-410)
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Dr Ingrid van Heerden is a registered dietician and holds a doctoral degree in Nutrition and Biochemistry. She believes that "we are what we eat" and offers free nutrition and weight loss advice via her DietDoc service on Health24.com. Read more of her articles.