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According to a team of Canadian researchers, eating egg yolks acts similarly to smoking and speeds up atherosclerosis. 

Atherosclerosis is the hardening of the arteries. It is common disorder and occurs when fat, cholesterol, and other substances build up in the walls of arteries and form hard structures called plaques, according to the U.S. National Library of Medicine. Plaque rupture is the usual cause of most heart attacks and many strokes. 

The research is published online in the journalAtherosclerosis.

Researchers examined data 1231 men and women, with an average age of about 62. Ultrasound was used to establish a measurement of total plaque area and questionnaires were filled out regarding their lifestyle and medications including pack-years of smoking (number of packs per day of cigarettes times the number of years), and the number of egg yolks consumed per week times the number of years consumed (egg yolk-years).

The researchers found that regular consumption of egg yolks is about two-thirds as bad as smoking when it comes to increased build-up of carotid plaque, a risk factor for stroke and heart attack.

Lead researcher  David Spence said the mantra 'eggs can be part of a healthy diet for healthy people' has confused the issue. 

"It has been known for a long time that a high cholesterol intake increases the risk of cardiovascular events, and egg yolks have a very high cholesterol content. In diabetics, an egg a day increases coronary risk by two to five-fold," Spence said. "What we have shown is that with aging, plaque builds up gradually in the arteries of Canadians, and egg yolks make it build up faster - about two-thirds as much as smoking. In the long haul, egg yolks are not okay for most Canadians."

The researchers found carotid plaque area increased linearly with age after age 40, but increased exponentially with pack-years of smoking and egg yolk-years. In other words, compared to age, both tobacco smoking and egg yolk consumption accelerate atherosclerosis. The study also found those eating three or more yolks a week had significantly more plaque area than those who ate two or fewer yolks per week.

Spence said the effect of egg yolk consumption over time on increasing the amount of plaque in the arteries was independent of sex, cholesterol, blood pressure, smoking, body mass index and diabetes. And while he says more research should be done to take in possible confounders such as exercise and waist circumference, he stresses that regular consumption of egg yolk should be avoided by persons at risk of cardiovascular disease.

According to Thomas Behrenbeck, a cardiologist with the Mayo Clinic, one large egg has about 186 mg of cholesterol, all of which is found in the yolk. 

"Therefore, if you eat an egg on a given day, it's important to limit other sources of cholesterol for the rest of that day, Behrenbeck said. "If you like eggs but don't want the extra cholesterol, use only the egg whites. Egg whites contain no cholesterol. You may also use cholesterol-free egg substitutes, which are made with egg whites."