Wednesday August 15 2012
Egg yolks are known to have a high cholesterol level
"Eating egg yolks is as bad as smoking in speeding up coronary heart disease" the Daily Mail says, reporting that egg yolks contribute to the clogging up of arteries which, in turn, can increase the risk of heart disease.
The news is based on a Canadian study which used ultrasound to look at the fatty build-up in the arteries of around 1,200 adults who were attending a clinic because they had pre-existing risk factors for heart disease.
The adults were questioned on their smoking history, the number of egg yolks eaten per week and how long they had eaten this amount of egg yolks.
They found that a combination of smoking and egg yolk consumption was related to a fatty build-up in the arteries, which could increase the risk of heart disease as well as other conditions that can affect the heart and blood vessels (cardiovascular diseases or CVDs).
This study does contain some important limitations, such as:
- the accuracy of the participants’ recollections of their egg yolk consumption
- a lack of detailed information on how the eggs were cooked
- there may have been additional risk factors contributing to artery ‘clogging’, not assessed by the study, such as lack of exercise or alcohol consumption
- while it is reasonable to assume that fatty build-up in the neck arteries can increase the risk of heart disease, it is uncertain exactly what the increased level of risk would be
This study perhaps best supports the notion of “all things in moderation”. Eggs are a good source of protein. Without further study, there is no firm evidence that egg yolks are as bad for you as smoking.
Where did the story come from?
The study was carried out by researchers from the Stroke Prevention & Atherosclerosis Research Centre, Robarts Research Institute, and other research institutions in Canada and was funded by the Heart & Stroke Foundation of Ontario.
The study was published in the peer-reviewed medical journal: Atherosclerosis.
While the Mail reports that eating egg yolks is two-thirds as bad for you as smoking when it comes to artery build-up, this cannot be concluded when you consider the limitations of this single piece of research. Also, the headline does not make it clear that the researchers were only looking at people with pre-existing risk factors for heart disease and not the population at large.
What kind of research was this?
The authors report that although high cholesterol contributes to heart disease, there is a general lack of consensus on whether eggs actually raise blood cholesterol and contribute to heart disease.
This study aimed to examine whether egg yolk consumption was related to the fatty (plaque) build-up in the arteries of a consecutive series of adults attending a vascular clinic in Canada.
The design of this study includes several limitations:
- it recruited a small, selective sample of adults
- egg yolk consumption is being estimated through questionnaire responses which may contain inaccuracies
- other factors not studied by the researchers, such as less exercise or a diet higher in other saturated fats, may also contribute towards a build-up of plaque
What did the research involve?
The observational study included 1,231 consecutive patients (average age, 62) referred to a vascular prevention clinic at a hospital in Canada. The total plaque area of their carotid arteries (the main arteries in the neck supplying blood to the head) was measured by ultrasound scan. At the time of referral lifestyle information was also measured by questionnaire. This included smoking history and the frequency of egg yolk consumption. From these responses the researchers calculated:
- pack-years of smoking: the number of packs of cigarettes per day multiplied by the number of years of smoking
- egg-yolk years: the number of egg yolks per week multiplied by the number of years consumed
The researchers specifically say that they did not assess:
- alcohol intake
- exercise taken
- liquorice consumption (high intake of liquorice can increase blood pressure and cause problems in people with heart disease)
What were the basic results?
Overall the researchers found that, as would be expected, the extent of plaque build-up in the carotid arteries increased with age. They also found that both increased smoking and increased egg yolk consumption were associated with more plaque build-up.
Average plaque area in the carotid arteries of patients consuming less than two eggs per week (388 people) was 125mm2 compared to 132mm2 in those consuming three or more eggs per week (603 people). This was a statistically significant difference (not the result of chance).
The association was not affected by adjustment for age.
The researchers did additional analysis adjusting for other cardiovascular risk factors, such as:
- total blood cholesterol
- blood pressure
- body mass index (BMI)
The researchers do not give figures for the build-up in the arteries associated with smoking, but say that the increase in total plaque area with increasing egg consumption followed a similar, linear pattern to that of cigarette smoking.
The highest consumption of eggs (eating more than 200 yolks per year) was said to be equivalent, in terms of plaque build-up, to two thirds of the effect of the highest amount of smoking – the figure quoted by the Mail.
How did the researchers interpret the results?
The researchers say that their findings suggest that regular consumption of egg yolk should be avoided by people at risk of cardiovascular disease. They do acknowledge though, that their theory should be tested in a prospective study that includes more detailed information about diet and other possible confounding factors, such as exercise and waist circumference.
This study found that egg yolk consumption was associated with increased fatty build-up in the arteries of the neck, though this was small when compared to the build-up expected with age. This study has important limitations which mean that it cannot be concluded that egg yolks are as bad for you as smoking:
- Average egg yolk consumption per week and duration was evaluated through a questionnaire response. These are only estimates and may include a considerable degree of inaccuracy. Consumption may vary over time. We also don’t know how these eggs were prepared (boiled, fried in oil, scrambled in butter, etc).
- This wasn’t a trial, and so people are choosing the number of egg yolks they eat. People who ate more egg yolks may differ in other health and lifestyle factors from people who ate less, and this may account for their different artery build-up. For example, as the researchers rightly acknowledge, they did not thoroughly assess other dietary factors, exercise or waist circumference. It is possible that higher egg yolk consumption could be associated with less exercise and higher overall saturated fat intake – both well known risk factors for heart disease. The small changes in fatty build-up in the arteries seen with higher egg yolk consumption could have been accounted for by these other factors.
- None of the participants in this study were reported to be suffering from heart disease and the heart arteries were not examined.
- We do not know how or whether the extent of fatty build-up in the neck arteries was associated with build-up in the heart arteries.
- This is a relatively small, select sample of people attending a vascular clinic in Canada, and further quality studies would be needed to better assess the question.
This study perhaps best supports the notion of all things in moderation. Eggs are a good source of protein in addition to other vitamins and minerals and most experts advise that they can form part of a healthy, balanced diet.
If you have been told you have pre-existing risk factors for heart disease, or other CVDs, your GP will be able to provide more detailed advice about a recommended diet.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on twitter.