Eating up to an egg a day does not increase the risk of developing cardiovascular disease, according a study published this week in The BMJ.

The study, which involved both a large cohort study and a meta-analysis, found no evidence that people who consume an egg daily are at any greater risk of heart attacks and stroke than people who never or rarely eat eggs — that is, after their overall diet and lifestyle are considered.

For it appears that people who eat a lot of eggs tend to have less healthful lifestyles, including less nutritious diets and lower levels of physical activity.

Those factors may explain the conflicting results of past studies on this topic. An unhealthy diet and physical inactivity are both major risk factors for cardiovascular disease.

Eggs have been suspected of contributing to cardiovascular disease because of their cholesterol content. An egg yolk from a single large egg contains about 200 milligrams of cholesterol. In the past, health guidelines recommended that dietary cholesterol be limited to 300 milligrams per day.

More recent studies, however, have found only a weak (at best) association between the cholesterol we consume in our foods and the cholesterol that builds up in our blood vessels. As a result, most health officials no longer deem eggs a concern, and current U.S. dietary guidelines do not recommend limiting their consumption.

Still, the debate over eggs continues, which is why the authors of the new study — a team from the Harvard TH Chan School of Public Health — decided to do an updated review and analysis of the evidence.

A study in two parts

First, the Harvard researchers analyzed data collected between 1980 and 2012 from three large U.S. cohort studies: the Nurses’ Health Study I, the Nurses’ Health Study II and the Health Professionals’ Follow-Up Study. The studies involved 215,618 participants, of whom 80 percent were women. When recruited, all the participants were free of cardiovascular disease, type 2 diabetes and cancer.

The participants filled out detailed questionnaires about their health and lifestyle,  including their dietary choices, every two years. The researchers grouped the people together based on how many eggs they ate (from less than one per month to at least one per day). Most of the participants ate one to five eggs per week.

During the three decades of the study, almost 7 percent of the participants (14,806) had a heart attack or stroke or died from some form of cardiovascular disease.

The researchers then looked to see if there was an association between the consumption of eggs and the risk of heart attack, stroke or death from cardiovascular disease.  They first adjusted the data, however, to take into account major known risk factors, such as age, body mass index (BMI), physical activity, smoking, family history of heart attack and the regular consumption of red meat. (Although this latter risk factor is a matter of debate.)

Their analysis found no association between the amount of eggs eaten and the risk of cardiovascular disease.

The researchers did find, however, that people who ate a lot of eggs tended to be less healthy than those who ate few. On average, they consumed more calories, had a higher BMI, engaged in less physical activity and were more likely to be current smokers. They also ate more red meat, processed meat (such as bacon), refined grains, potatoes and sugar-sweetened beverages.

Pooling data

For the second part of their study, the researchers pooled data collected from almost 140,000 people who participated in 28 observational studies conducted in the United States, Europe and Asia. The analysis of this data also found a lack of a link between egg consumption and cardiovascular risk.

The results of the Asian studies tended to differ, however, from those conducted in Europe and the U.S. In one large Chinese study, for example, people who ate more eggs had a slightly lower risk of heart attack and stroke than those who ate fewer.

The researchers cite several possible explanations for this finding, including a cultural one.

“In Asian cultures, eggs are typically incorporated into various cuisines,” they note, “while in Western populations, eggs are typically consumed with red and processed meats and refined grains.”

Limitations and implications

These findings are based on self-reported dietary data. Such self-reports can be inaccurate.

In addition, all the studies that were analyzed were observational, and thus can’t prove a cause-and-effect relationship (or the lack of it) between two things  — in this case, between egg consumption and cardiovascular disease.

As Andrew Odegaard, an epidemiologist at the University of California-Irvine, writes in an editorial that accompanies the study, “We should not put all our eggs in this observational basket for formal guidance on eating eggs.”

Still, the findings from the study are “convincingly null,” he says, to form “an evidence informed take-home message.”

“If frequent egg consumption is occurring in the context of an overall dietary pattern known to be cardioprotective, or eggs are being consumed for essential nutritional needs,” Odegaard writes, “then it is probably nothing to worry about.”

FMI: The study and the editorial can be found on The BMJ’s website.