The research is preliminary and does not compare the effect of eating a couple of fast-food breakfast sandwiches to a healthier choice such as yogurt and a banana. Still, the study hints at the dangers of unhealthy food, said co-author Dr. Todd Anderson, director of the University of Calgary's Libin Cardiovascular Institute of Alberta.

"Even one episode of eating something unhealthy can have an effect," he said, "and cumulatively, that can have an effect. Think before you eat."

The purpose of the study was to examine what happens in the blood vessels after someone eats a high-fat, high-salt meal.

The researchers recruited 20 healthy people, with an average age of 23, to undergo tests that measure how efficiently blood travels through the vessels in their forearms. Some students fasted while others ate two fast-food breakfast sandwiches, and then the groups switched.

Anderson declined to identify the brand of breakfast sandwich because, he said, its identity should not be a focus of the attention given to the research. However, he did say it is a ham, cheese and egg sandwich -- with 25g of fat and 450 calories -- that "you could buy anyplace in America or Canada."

Compared to the participants who hadn't eaten recently, the blood vessels of those who ate two breakfast sandwiches worked less efficiently two hours after the meal, Anderson said. "There was a decrease in the ability of the small vessels of the forearm to dilate and generate blood flow," he explained.

This dilation -- widening of the vessels -- helps the blood get more oxygen to the body when you exercise. Other research has suggested that disruptions in this process over time make people more susceptible to develop strokes and heart attacks, Anderson said.

The study did not examine what would happen if someone ate a healthy meal. Anderson said previous research suggests healthy food would not have had a negative effect.

It is also not clear what would happen if people ate this kind of food continuously, nor what different kinds of fast food (say, a hamburger and fries) would do to the blood vessels.

So what does the research mean? It suggests how trouble in the vessels begins due to an unhealthy diet, Anderson said. "High levels of fat, salt or cholesterol can cause blood vessels to be abnormal, which can increase the risk of heart attack or stroke," he said.

The study was released at the Canadian Cardiovascular Congress meeting in Toronto.

Chris Fahs, a graduate assistant at the University of Oklahoma's department of health and exercise science who studies blood vessels, cautioned against taking general messages from the study.

"It's hard to say how one specific food impacts a person's risk for health problems. With nutrition there are so many variables that can be changed," Fahs said. "Too much of any one food will probably have negative consequences in the long term, but eating that food in combination with other foods which meet a person's nutritional needs may not have same impact on the vascular system."

In addition, Fahs said, "the field of nutrition is very conflicting when it comes to dietary recommendations for the prevention of cardiovascular disease."

Because this study was presented at a medical meeting, the data and conclusions should be viewed as preliminary until published in a peer-reviewed journal.