In the 1980s, eggs were the dietary equivalent of slow-acting poison.
Over the next 20 years, a mountain of research showing that eggs do not have a major impact on cholesterol levels, combined with pricey marketing campaigns from egg producers, helped to clear their name. Now, eggs occupy coveted positions on brunch menus across the country and are even growing in popularity as producers fortify them with omega-3 fatty acids and offer organic, cage-free and free-run varieties.
Then, a few days ago, Canadian researchers made waves when they said one egg yolk is worse, cholesterol-wise, than KFC's Double Down sandwich, a notorious fast-food legend that replaces the traditional bun with two pieces of chicken slathered with bacon, sauce and cheese. The researchers published a report in the Canadian Journal of Cardiology warning that egg consumption can be dangerous to a person's health and that it is wrong to assume that dietary cholesterol from eggs is harmless.
"Dietary cholesterol, including egg yolks, is harmful to the arteries," the report says. "Stopping the consumption of egg yolks after a stroke or [heart attack]would be like quitting smoking after a diagnosis of lung cancer: a necessary action, but late."
The more things change, it seems, the more they stay the same.
But does this new report mean that egg-yolk naysayers were right all along?
Cholesterol emerged as a nutritional bogeyman in the 1960s, pushing some Canadians to reject butter in favour of margarine and to abandon eggs altogether. Sweeping public-health campaigns were launched to warn citizens of the perils of egg consumption amid fears that cholesterol in the yolks was a major contributor to cardiovascular problems.
The thinking was that the dietary cholesterol found in eggs could significantly boost levels of blood cholesterol found naturally in the body and consequently raise an individual's risk of developing heart disease.
Over time, however, a growing amount of research began to suggest that dietary cholesterol - or the cholesterol found in food we eat - has less of an impact than originally believed on the body's overall levels.
Most of the cholesterol circulating in the bloodstream is actually produced in the liver, research has shown. The rest is derived from dietary sources, such as the cholesterol found in egg yolks, dairy products, meats and other foods.
Although cholesterol has long suffered from a negative public image, it plays an essential role in the proper function of cell membranes and helps the body produce important vitamins and bile and perform other processes.
There are two types of cholesterol: low-density lipoprotein (LDL) and high-density lipoprotein (HDL). LDL carries cholesterol to the body's cells from the liver and is known as "bad" cholesterol because when there is too much of it, it promotes the build-up of plaque in arteries, which is a risk factor for cardiovascular disease. HDL cholesterol, on the other hand, is considered "good" because it helps to carry LDL cholesterol away from artery walls.
A study published in the Journal of the American Medical Association in April, 1999, found that people who consumed an egg a day did not face an increased risk of heart attack or stroke. It was a definitive piece of evidence that helped to repair the reputation of eggs as part of a healthy diet, when consumed in moderation.
David Spence, a stroke-prevention expert at the University of Western Ontario and co-author of the new report questioning the value of eggs, said the study, and others like it, are flawed. He pointed out that diabetics in the 1999 study faced higher cardiovascular risks with increased egg consumption. Similar problems may not have been detected in healthy patients because the study did not follow them long enough, he added.
Dr. Spence said marketing campaigns have wrongly convinced Canadians that they can safely consume eggs without a fear of long-term health risks.
But his opinions clash with a wider consensus in the medical community.
In the years since the 1999 study was published, more research has shown that saturated and trans fats are much more likely to raise an individual's blood cholesterol levels and fuel the risk of heart problems.
Of course, there are some caveats. Certain people are more sensitive to the effects of dietary cholesterol than others and need to watch their consumption to avoid potential problems. People who are at an increased risk of heart attack, stroke or other cardiovascular problems should also watch their intake of dietary cholesterol, said Rosie Schwartz, a Toronto-based dietitian and author.
But she emphasized that Canadians should not fixate on cholesterol alone. High blood pressure, a diet high in fat, being overweight and a sedentary lifestyle are all factors that can contribute to serious health problems.
"We need to really look at all the issues of heart disease," Ms. Schwartz said.