The British medical journal Heart recently published an article titled: “Associations of egg consumption with cardiovascular disease in a cohort study of 0.5 million Chinese adults.” An impressive title, for sure–over 500,000 Chinese adults–but let’s take a closer look. What exactly did it find?
The authors conclude that the consumption of up to one egg per day was associated with lower risk of heart disease. Predictably, this conclusion sparked considerable excitement, as displayed by headlines around the world.
BBC.com led with a provocative question: “An egg a day to keep the doctor away?”
The Daily Mail weighed in with: “An egg a day could keep the doctor away, research suggests”
TIME magazine went a step further: “Eating an Egg a Day May Keep Heart Disease Away, a New Study Says”
EurekAlert.org’s press release exclaimed: “Daily egg consumption may reduce cardiovascular disease”
And finally, USnews.com boldly stated: “Eggs Not Harmful for Heart Health”
Not harmful for heart health? A serious claim, no doubt, but does it withstand scrutiny?
Before getting into the nitty gritty details of the study, let’s take a look at the its methodology. It used the same methods as the PURE study, which I wrote previously about. (That is, both are observational and prospective study designs. In its simplest terms, a prospective study is one in which the researchers measure certain factors and some behaviours (e.g. diet, smoking, etc.) and then track the health outcomes of the study subjects for a period of time (in this case, an average of nine years). In effect, such a study watches for certain outcomes over time, in order to discover various associations (but never causation). Outcomes measured in this particular study were cases of cardiovascular disease, including ischaemic heart disease, major coronary events, haemorrhagic stroke and ischaemic stroke.
At the start of the study, 13.1% of the subjects reported consuming eggs regularly (an average consumption of three quarters of an egg per day, or a little more than five eggs per week). On the other hand, 9.1% reported either no or very rare consumption (an average consumption of less than a third of an egg per day). The study reports that over time, the higher egg consumers had lower risks of total cardiovascular disease (11%), ischaemic heart disease (11%), major coronary events (14%), haemorrhagic stroke (26%) and ischaemic stroke (10%). They also had less risk of total cardiovascular death (18%) and haemorrhagic stroke death (28%) compared to rare consumers.
These findings sound very impressive, on the surface, but it is very difficult to account for confounding variables. Similar to the PURE study, those who ate the fewest eggs differed from the those who ate the most eggs in many ways. For example, lower egg consumers had, on average, lower education levels and lower household incomes. They also had less access to what the researchers termed a “new affluence dietary pattern.” What, you might ask, is a “new affluence dietary pattern?” As with any dietary pattern, there are several components. In this case, those who consumed fewer eggs were, on average, also consuming fewer fruits. Lastly, they also suffered from higher rates, on average, of both hypertension and smoking. In other words, this study compared people with a higher rate of hypertension and smoking but lower education/income level and lower consumption of fruit and eggs to those with less hypertension and smoking but higher education/income level and higher fruit/egg consumption. And that’s not even to mention other confounding variables.
Is that convoluted enough for you? Well hold on, because that’s not even the whole story.
The study’s upper threshold–that is, its so-called “high consumption” group–only ate 5.32 eggs per week. Unfortunately, this doesn’t quite match the reality of high egg consumption in many parts of the world. In fact, this is a lower consumption than many regular egg consumers in America! Likewise, throughout the original study report, the authors refer to those who ate the least eggs as “non-consumers.” And yet they categorize this group as 0.29 eggs per day (or 2.03 eggs per week). Can this really be classified as non-consumption? Rather than comparing high egg intake (e.g. 14 eggs per week) with no egg intake, the study instead compared those who ate 2.03 eggs per week with those who ate 5.32 eggs per week. Such a narrow scope is both limiting and misleading. With this in mind, the study reported that those consuming 0.76 eggs per day actually had more cardiovascular disease and death than those consuming 0.56 eggs!
The study reported that those 0.76 eggs per day actually had more
cardiovascular disease and death than those consuming 0.56 eggs!
In all fairness, and despite the media headlines listed above, the actual journal publication had a far more accurate and less sensationalist title: “Associations of egg consumption with cardiovascular disease in a cohort study of 0.5 million Chinese adults.” And if we read past the sensationalist headlines, some of the media reports actually did mention the study’s shortcomings. While the press release via EurekAlert.org lead with the headline “Daily egg consumption may reduce cardiovascular disease,” the article on their website notes, “This was an observational study, so no firm conclusions can be drawn about cause and effect.” The TIME magazine headline stated that “Eating an Egg a Day May Keep Heart Disease Away, a New Study Says,” but also hedged that “The study doesn’t prove that eating eggs can protect against heart disease or stroke.” The Daily Mail even quoted two independent experts: professor of cardiovascular medicine at the University of Sheffield Tim Chico, and senior dietitian at the British Heart Foundation Victoria Taylor. The first said, “It is important to stress that this does not prove that eating eggs protects against these diseases, as there may be other differences between the people eating more eggs that cause these differences,” and the second: “It’s possible that the lower risk of cardiovascular disease seen in those who consumed eggs on a daily basis may have been caused by something else in their diet or lifestyle – rather than a specific cause and effect.” Similarly, NewsWeek quoted Gavin Sandercock from the University of Essex, U.K., who urged caution when reading the results: “If you study enough people you can find a correlation between almost anything. To say that eating eggs is good (or bad) for you based on a study like this would be foolish as diet is much more complicated than picking on one foodstuff like eggs.”
Focusing more on the study’s obvious flaws, and in contrast to the headlines above, other outlets had different views:
HealthNewsReview.org led with: “BMJ’s scrambled message on eggs and heart disease: a recipe for rotten news coverage”
MedPage included a vlog by F. Perry Wilson assistant professor of medicine at the Yale School of Medicine titled: “Eggs Are Good for the Heart… or Not. Here’s why dietary epidemiology research can be rotten.”
Finally, Newsweek wrote: “SCIENTISTS LINK EGGS TO LOWER HEART DISEASE RISK—BUT ARE THEY ALL THEY’RE CRACKED UP TO BE?”
Although this study reports that eggs may be beneficial for heart disease, many others have reported that eggs may not be beneficial. Others still have reported that eggs may even be harmful! Until randomized, controlled trials prove that eating eggs is healthful with regards to heart disease, we should remain cautious about large studies which have inherent flaws, and so too remain cautious about the news outlets reporting on them…eggxactly!
- Qin C, Lv J, Guo Y, Bian Z, Si J, Yang L, Chen Y, Zhou Y, Zhang H, Liu J, Chen J, Chen Z, Yu C, Li L; China Kadoorie Biobank Collaborative Group. Associations of egg consumption with cardiovascular disease in a cohort study of 0.5 million Chinese adults. Heart. 2018 May 21. pii: heartjnl-2017-312651. doi: 10.1136/heartjnl-2017-312651. [Epub ahead of print]