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Eating an egg a day does not raise risk of heart attack or stroke

Thursday 5 March 2020

"Eggs are NOT bad for your heart: Thirty-year study finds eating one a day does not raise the risk of stroke or heart disease," reports the Mail Online.

While we know eggs contain many nutrients, they are also quite high in cholesterol. Because of the link between cholesterol and heart disease, doctors once thought eggs might raise heart disease risk. Later research showed that cholesterol in the diet is not such a problem. Studies into eggs and heart disease have had conflicting results, perhaps because people who eat a lot of eggs might have a less healthy diet overall.

In a new study, researchers used dietary information from 215,618 people in the US, dating back to 1980. They found no evidence that people eating an egg a day had a higher risk of heart attack or stroke than people who rarely or never ate eggs, once their overall diet and lifestyle was taken into account. The researchers pooled their study results with those of 27 other studies from around the world. The pooled results also found no increased risk of heart attack or stroke for people eating eggs.

You can reap the benefits of the nutrients of eggs, while reducing any risk posed by cholesterol, by cooking them without adding salt or fat and avoiding frying them. Frying eggs in oil can increase their fat content by around 50%.

Read more about the healthy way to eat eggs.

Where did the story come from?

The researchers who carried out the study were from the Harvard TH Chan School of Public Health in the US. The study was funded by the US National Institutes of Health. It was published in the peer-reviewed British Medical Journal on an open-access basis, meaning it is free to read online.

The UK media sources that carried the story provided reasonably accurate and balanced reports about the research.

What kind of research was this?

This was a prospective cohort study, using 3 large ongoing cohorts of healthcare professionals in the US. Cohort studies are good ways to find links between risk factors (such as regularly eating eggs) and outcomes (such as heart attack and stroke). However, they cannot prove that one directly causes the other. There may be other factors that affect both egg consumption and heart disease, which have not been fully taken into account.

What did the research involve?

The research was carried out in 2 parts: a cohort study and a meta-analysis.

Cohort study

The researchers used information from 3 large cohort groups of healthcare professionals, which ran from 1980 to 2012, 1991 to 2013 and 1986 to 2012.

2 of the studies included only women and 1 contained only men. At the start of each study, people filled out questionnaires about their health and lifestyle, including dietary questionnaires. These were repeated every 2 years. People were also asked about diagnoses of heart attack, diabetes and stroke. Deaths from heart disease or stroke were also recorded.

Researchers grouped people according to how many eggs they ate, from fewer than 1 per month to at least 1 each day. They then looked to see how likely people who ate varying amounts of eggs were to have had a heart attack or stroke, or to have died from heart disease, compared to people who ate fewer than 1 egg a month. They also looked at the risk of adding 1 extra egg a day to the diet.

Because they had regular data on diet, the researchers were able to keep updating the information, so they accounted for any change in diet. They adjusted their figures to take account of a wide range of possible confounding factors, including:

  • age and sex
  • family history of heart attack
  • high blood pressure or high cholesterol at the start of the study
  • body mass index (BMI)
  • whether they smoked
  • how much physical activity they did
  • how much alcohol they drank
  • whether they had been through the menopause, and whether they took hormone replacement therapy (HRT)
  • whether they took hormonal contraceptives
  • how many calories they ate in total
  • how often they ate unhealthy foods such as fried potato, red meat or bacon (often consumed with eggs in the US and the UK)

Systematic review and meta-analysis

The researchers wanted to see how their results fitted with other studies. They searched for other prospective cohort studies that also looked at egg consumption and heart attack or stroke. They then pooled the results of all the studies they found with their own study results, to get an overall picture of the state of evidence. They compared the risks of heart attack and stroke between the people who ate most and least eggs and looked at the risk of 1 additional egg per day.

What were the basic results?

In the cohort study, 14,806 of 215,618 (6.8%) people had a heart attack, stroke, or died from heart disease or stroke, during 22 to 32 years of follow up.

After taking account of potential conflicting factors the study found:

  • people who ate at least 1 egg a day had the same chance of heart attack, stroke, or of dying from heart disease as people who ate fewer than 1 egg a month
  • eating an additional egg each day did not increase the chance of having a heart attack or stroke, or of dying from heart disease

In the systematic review, 139,195 people (8%) from 28 studies had a heart attack or stroke or died of heart disease, out of a total 1,720,108 people.

The pooled results found:

  • people who ate the most eggs had the same chance of heart attack or stroke as people who ate the least eggs
  • eating an additional egg each day did not increase the chance of having a heart attack or stroke

The results varied from study to study, however. The researchers said the variation was most noticeable between studies carried out in Asia (including a big study from China) and those from Europe or the US.

The Chinese study found people who ate more eggs had a lower risk of heart attack or stroke. They say this could have been because eating eggs regularly suggested a better standard of living overall, for this population.

How did the researchers interpret the results?

The researchers said: "The results from our cohort study and updated meta-analysis show that moderate egg consumption (up to 1 egg per day) is not associated with cardiovascular risk overall." They added that most of the people in the studies ate fewer than 1 egg a day, so the results should be interpreted with caution.

Conclusion

Overall, this study suggests that eggs can be part of a healthy, balanced diet. The researchers did not find any evidence that eating eggs raises the risk of heart attack or stroke, once you take into account a person's overall diet and lifestyle.

However, they did find that people who ate a lot of eggs tended to have less healthy diets overall. They were more likely to be overweight, did less exercise and were more likely to smoke. They also ate more red meat, processed meat (such as bacon), refined flour, potatoes and sugary fizzy drinks.

This highlights the problems with looking at just 1 food in someone's diet – it only makes sense if you look at what else they are eating, and their general lifestyle. People should not take this study on eggs as a green light to have a full fried breakfast every morning.

There are a few issues that mean we should be cautious about the results. The cohort study was carried out in the US, among healthcare professionals who were all well-educated and healthier than average. It's possible the results would not hold true for other population groups.

The results could be affected by unmeasured confounding factors, although the researchers did take a wide range of these into account. Dietary questionnaires rely on people reporting what they ate accurately, which is not always the case. In addition, variation in study findings in the meta-analysis are not completely explained, which makes the results a little less reliable.

Overall, the study supports advice to eat eggs in moderation as part of a balanced diet. Find out more about a eating a healthy diet.

Analysis by Bazian
Edited by NHS Website