Myth Busting: Egg Yolks

In the middle of August a study came out claiming that eating egg yolks regularly is about 2/3 as bad as smoking for building up carotid plaque in the arteries, which can cause atherosclerosis. The study was published online in the journal Atherosclerosis. The study was an observational study or epidemiological study, involving 1231 men and women, who were patients attending vascular prevention clinics at London Health Sciences Centre’s University Hospital. They were given questionnaires and asked how many packs of cigarettes and eggs they eat in a week, then multiplied it to come to a yearly amount. Ultrasound was used to look at the plaque build up in the arteries. All the people surveyed either had a stroke or a “mini Stroke”.

The group that ate the most eggs also smoked the most, had the highest rate of diabetes and were the oldest (average age 69.77). The people who consumed the least amount of eggs also smoked less (average age 55.70). The study also took into account various factors of sex, cholesterol, blood pressure, smoking, body mass index and diabetes. What they found is that the average plaque in the carotid arteries of patients eating 2 eggs or less a week was 125mm2 and patients were eating eggs 3 times or more a week is 132mm2, which is a statistically significant difference.

This study has severe limitations. First of all the study was done with people who had already had strokes or “mini strokes”. The study was not done on healthy people and if egg yolks could increase risk of strokes, these people had already had a stroke regardless of egg consumption.

Next, let’s consider the questionnaire (where the researchers got their data). It has already been well established that exercise, waist circumference, stress and a patient’s diet are confounding variables that have an effect on atherosclerosis. In this study, however, participants were never asked about any of these, adding to the questionable validity of study’s findings. Lastly, age was never taken into consideration when the data was being analyzed. Given that age has been linked to plaque increase, and this presents another problematic omission. There is almost a 15 year age gap between those patients who ate the most eggs and those who ate the least. Interestingly enough, the group that ate the most eggs had the lowest total cholesterol, lowest LDL cholesterol, highest HDL cholesterol, and lowest body mass index. The data from this study could be interpreted completely differently. Headlines might read “Live longer without a stroke by eating egg yolks” or “Egg yolks make you old”.

Both of these statements are supported by the data in this study. It is important to realize that observational studies show correlation NOT cause and effect. A prospective study needs to be done to prove causation. When we read a study like this, we must always remember that correlation does not imply causation

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