Picture: ISTOCK
Picture: ISTOCK

It is enough to scramble your brain. One minute, experts say eggs are bad for you, the next that they are good for you. Now, eggs are bad again, say US scientists.

They say three to four eggs a week could significantly raise your risk of cardiovascular disease “incidents” (heart attack or stroke) and premature death.

Their study, in the Jama (Journal of the American Medical Association), undermines the reputation rehabilitation that has restored eggs’ pride of place on breakfast tables globally.

Nutrition experts moved quickly to defend the humble egg and debunk the study. They said the researchers’ conclusions are not all they’re cracked up to be.

The Jama study seems impressive: it is large and long term; and the researchers have combined and analysed findings from six studies, involving 30,000 subjects between 1985 and 2016, with an average 17-year follow-up.

They made two major conclusions:

  • Eating three to four eggs per week is associated with 6% higher risk of cardiovascular disease (CVD) and 8% higher risk of any cause of death (all-cause mortality, in medical jargon).
  • Eating 300mg of dietary cholesterol daily is associated with 17% higher risk of incident CVD and 18% higher risk of all-cause deaths.

The researchers, led by Dr Victor Zhong, of Northwestern University Feinberg School of Medicine, said cholesterol was “the driving factor independent of saturated fat consumption and other dietary fat”.

They noted that egg yolks are one of the richest sources of dietary cholesterol among commonly consumed foods and that one large egg yolk contains 186mg of dietary cholesterol.

The study’s take-home message is “really about cholesterol, which happens to be high in eggs and specifically yolks”, said co-corresponding study author Dr Norrina Allen, Northwestern associate professor of preventive medicine, in a university release.

Nutrition experts moved quickly to defend the humble egg and debunk the study.

“As part of a healthy diet, people need to consume lower amounts of cholesterol. People who consume less cholesterol have a lower risk of heart disease.”

The researchers take aim at the radical 2015 change to the influential US dietary guidelines, which SA closely follows, making dietary cholesterol “no longer a nutrient of concern”. After warning against eggs for 40 years, official advice became — and remains — that eggs “should be considered part of a healthy diet”.

The guidelines should be revised, the researchers say. 

They acknowledge study limitations, including data collection methods — use of notoriously unreliable, subjective food-frequency questionnaires and taking diet histories. The study is also associational and, thus, not causational.

Canadian nutrition professor Andrew Mente has “major concerns” about the study. For starters, it disagrees with other notable cohort (group) studies, though Mente acknowledges that, in itself, this doesn’t mean it’s wrong.

However, the conclusions “lack coherence”.

“The researchers found that a higher egg intake is associated with lower levels of LDL (low-density lipoprotein, so-called bad) cholesterol and lower blood pressure but, somehow, an increased CVD risk,” Mente said

He finds it “curious” that researchers did not address these findings.

“This raises the question: what’s the basis of the harmful effect of eggs and dietary cholesterol on CVD risk? Evidently, [the researchers’] own data challenge the basic framework for reducing egg and dietary cholesterol intake.”

This makes the study “unconvincing”.

Mente also has problems with research using a single-slope estimate to characterise the entire relationship between eggs and outcome events. 

“A more informative, now commonly used approach is to show separate estimates of association at different levels of egg intake, whether it’s tertiles, quartiles, quintiles or other categorical scales,” he said.

“It’s important to show readers the magnitude of association at different levels of egg intake, otherwise it can be misleading.

“For instance, the overall association may be positive but it’s driven mostly by people at the very highest levels of egg intake. It’s important to see whether these estimates at different levels of intake are consistent after full multivariable adjustment for known confounders.”

 A major factor in any study in Western populations is “health consciousness”, Mente said. “Since the most health-conscious people who take great care of themselves may minimise eggs, eggs are then a marker, not a cause related to worse health.

“Even full multivariable adjustment is probably not enough to eliminate this source of bias.”

Another factor is “collinearity with other foods”. Collinearity is the special case in which two variables correlate exactly, making interpretation of results difficult.

“For example, if eggs are eaten with a host of bad foods, it may be difficult to tease apart their separate effects,” Mente said.

 Heart and Stroke Foundation of SA (HSFSA) dietitian Bianca Tromp, said that as the study was conducted in the US, the guidelines “might not be applicable for other countries and cultures”.

The HSFSA promotes a diet low in saturated fat and dietary cholesterol, Tromp said.

“Therefore, we recommend limiting egg consumption to seven eggs a week and reducing other foods high in saturated fat and cholesterol.”

South Africans diagnosed with CVD, hypercholesterolemia or diabetes should be “cautious when consuming foods high in cholesterol and should limit egg consumption to three eggs a week”, she said.