"An egg a day can cut chances of suffering a fatal stroke," The Times reports. A new review of existing data covering around 300,000 people suggests eating up to one egg a day may lower stroke risk; but not the risk of heart disease.

The health effects of eggs have been debated for years. Eggs, which contain cholesterol, were thought to increase risk of heart disease by raising cholesterol levels.

But more recent studies show that cholesterol in food has little impact on the levels of cholesterol in your blood – most cholesterol in the blood is made by the liver.

Researchers wanted to carry out an updated analysis of the evidence on the link between eating eggs and the risk of stroke and heart disease.

The analysis found no link with heart disease and a small reduced risk (12%) of stroke for people who ate around one egg a day, compared to those who ate less than two a week.

The research supports the idea that eggs can be part of a healthy diet. However, it didn't look at people's whole diet, so we don't know what else they were eating, or how the eggs were prepared.

Also, the researchers didn't find that more was better – there was no evidence that people reduced their risk in line with the number of eggs they ate.

Eggs are a good source of protein, vitamins and minerals, so adding one a day to your breakfast could be a healthy way to start the day.

Where did the story come from?

The study was carried out by researchers from the EpidStat Institute in Michigan and DLW Consulting Services in Utah, both in the US, and was funded by the Egg Nutrition Center. This could be seen as a conflict of interest.

The study was published in the peer-reviewed Journal of the American College of Nutrition.

The news was greeted with enthusiasm and little criticism by the UK media. Most reported the study results reasonably accurately.

Both The Sun and the Daily Mirror described the modest 12% drop in relative risk as having "slashed" stroke risk, which is something of an exaggeration.

Although several reports included quotes from the Egg Nutrition Center, none pointed out that the centre had funded the study.

The Egg Nutrition Center is the self-styled "science and nutrition education division" of the American Egg Board (AEB), which is a trade association representing American egg farmers.

The Times' headline stated that eating eggs could cut chances of having a "fatal" stroke, but the study did not find a statistically significant difference in risk of fatal stroke from egg consumption.

What kind of research was this?

This is a systematic review and meta-analysis of prospective studies into egg consumption and heart disease and stroke.

A meta-analysis is a good way of summarising the research on a topic; however it's only as good as the studies included. In this case, all were prospective cohort studies.

Cohort studies can find links between factors (egg consumption and heart disease or stroke) but cannot show that one factor causes another.

What did the research involve?

Researchers identified all prospective studies published up to August 2015, which looked at egg consumption by adults and either heart disease or stroke.

They pooled the results and looked to see whether high egg consumption compared to low egg consumption had any effect on these outcomes. They also looked for a "dose response" – a suggestion that risk went up or down in line with the number of eggs people ate each week.

Most of the studies classified high egg consumption as about an egg a day, and low egg consumption as less than two eggs a week.

Most, but not all, adjusted their figures to take account of confounding factors that can affect risk of heart disease and stroke, such as:

weight

age

sex

smoking history

exercise

(in a few cases) how healthy participants' diet was overall

The researchers did standard tests to look for publication bias and to see whether the summary results were overly affected by one or more studies.

What were the basic results?

People whose egg consumption was high were no more or less likely to get heart disease (summary relative risk estimate (SRRE) 0.97; 95% confidence interval (CI) 0.88 to 1.07) than people whose egg consumption was low. This result was based on seven studies including 276,000 people.

However, people who ate an egg per day were 12% less likely to have a stroke than people who ate less than two eggs per week (SRRE 0.88, 95% CI 0.81 to 0.97). This was based on seven studies including 308,000 people.

The researchers found no sign that the stroke risk decreased in proportion to the number of eggs eaten.



How did the researchers interpret the results?

The researchers concluded: "consumption of up to one egg daily may contribute to a decreased risk of total stroke [all types of stroke] and daily egg intake does not appear to be associated with risk of coronary heart disease."

Conclusion

This research broadly supports previous studies in this area, which suggest eating eggs does not increase the chances of getting heart disease or stroke. It raises the possibility that eggs may decrease the risk of having a stroke, but there are limitations to the study, meaning this result may not be reliable.

It's interesting that researchers did not find a "dose response" between stroke risk and the number of eggs eaten. Usually, if something is having an effect on the chances of getting a condition, you can see a linear pattern – having more of that food or treatment increases or decreases chances of the disease, perhaps up to a certain point. But in this case, you can't see any clear pattern.

Studies that identify just one factor – in this case people's egg consumption –without balancing that with information about their overall diet and lifestyle, may find false associations that are actually explained by other factors. For example, people who eat eggs may be more likely to eat a generally healthy diet, or to exercise more, both of which would decrease the chances of stroke.

Another factor to be aware of is that the 12% risk reduction is quite small, and the confidence interval comes fairly close to the point at which the result is no longer statistically significant. This means it is close to the margin of error, so could be down to chance or a blip in the data.

It is important to remember to eat a balanced diet, rather than just assuming one type of food is best. There's a big difference between eating a daily boiled or poached egg with wholegrain toast and spinach, or eating an egg as part of a daily fry-up full of salt and fat.

Read more advice about preventing strokes.

Analysis by Bazian

Edited by NHS Website