Eating egg yolks is similar to smoking for the damage it does to your arteries, according to a Canadian study published in the scientific journal Atherosclerosis. But how seriously should you take this finding?
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The study concluded that regular consumption of egg yolk should be avoided by those at risk of cardiovascular disease. A similar conclusion was reached in an earlier review article written by the same authors.
But before you consider cutting eggs out of your diet, let's look at this study more closely. It has a number of weaknesses that might make you pause before changing your egg intake.
The study subjects are patients of a heart disease prevention clinic and the design is cross-sectional. This means that factors suspected of leading to disease are measured at the same time as the signs of the disease itself.
In the course of their examination, patients were asked to recall their liquorice intake, their alcohol intake and their intake of egg yolks over their lifetime. The average age of the patient group was 62 years, and only the information on lifetime egg yolk intake was used, because the other dietary information (and also exercise pattern) proved too difficult to quantify.
Damage to arteries was assessed by using ultrasound to measure the total area of plaque in the carotid artery. Plaque is a waxy substance that builds up on the inside of arteries, hardens over time and narrows the artery. Its presence is associated with future cardiovascular events, although plaque thickness and type are also thought to play a role in this.
In the study, many factors were found to be associated with damage to arteries. A larger plaque area was independently associated with being male, having diabetes, having lower total blood cholesterol, a lower body mass index, higher systolic blood pressure, greater lifetime smoking, and greater lifetime consumption of egg yolks. But the role of exercise or other aspects of dietary intake could not be examined because they were not measured.
The investigators acknowledge their weak study design, stating that the "hypothesis should be tested in a prospective study with more detailed information about diet, and other possible confounders such as exercise and waist circumference."
Most studies on egg intake and cardiovascular disease have not found evidence that the consumption of up to one egg a day has any adverse effect. These include a study of 912 subjects who were followed for 24 years, a Harvard study of 37,851 men and 80,082 women over eight years, the Physicians' Health Study of 21,327 over 20 years, a Spanish cohort of 14,185 university graduates followed over six years, and a US cohort of 6833 men and 8113 women followed for six to 12 years for coronary heart disease and stroke mortality.
An analysis of these studies reveals that people with diabetes have an increased risk of cardiovascular disease when their egg consumption is high (more than about five eggs a week). This finding is worthy of further investigation, as the metabolism of people with diabetes is different to that of people without diabetes.
Nutritional discussions of eggs often focus on their cholesterol content. But eggs are also an inexpensive source of high quality protein, as well as being rich in minerals, folate and B group vitamins. For people without diabetes, there's no strong evidence that consuming up to one egg a day is at all harmful.
Malcolm Riley is Nutrition Epidemiologist at CSIRO. He was employed at Dairy Australia from 2006-2010.