Fast-food breakfast sandwiches are typically doughy bread concoctions featuring some type of meat and a heap of cheese. So, yeah, they're not the healthiest food choice to start your day. Now research shows just what a high-fat breakfast sandwich does to your body before the clock strikes noon.

The health impact of breakfast sandwiches is of interest because they're a booming segment of the fast-food industry. Almost every fast-food chain is now open early with breakfast offerings. According to the market research firm NPD Group, breakfast meals now account for 12 percent of fast-food sales—about $42 billion annually. An estimated 80 percent of breakfast meals purchased outside the home come from fast-food restaurants.

It turns out that kind of popularity can have a big impact on heart health. Canadian researchers asked 20 healthy, college-age students who did not have any type of heart disease to eat a high-fat, commercially available breakfast sandwich that had 50 grams of fat. Researchers measured the participants' endothelial function before the meal and two hours after the sandwich. Endothelial function explains just how well your blood vessels are working. If arteries are not functioning well, there is an interruption or slowing down of blood flow. If they are working well, the vessels open wide and blood vessel hormones are at their optimal levels.

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The study showed that two hours after eating the sandwich, blood flow in the arteries had slowed by 15 to 20 percent. The slow-down is temporary but is a stark sign that the blood vessels were "unhappy," says the lead author of the study, Dr. Todd Anderson, director of the Libin Cardiovascular Institute of Alberta. Anderson presented his research this week at the Canadian Cardiovascular Congress.

The study shows that a high-fat breakfast sandwich can do more harm, more quickly, than anyone imagined.

"This study reminds us that our behaviors are the backbone of preventing heart disease," says Dr. Beth Abramson, of the Heart and Stroke Foundation of Canada. "Remember that whether you eat at home or go to a restaurant, you're still in charge of what you eat. So consider all the choices, and try to cut down on saturated and trans fats, calories and sodium. That's one of the keys to decrease your risk of heart disease and stroke."

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Many breakfast sandwiches are high in those categories. A review of popular sandwiches by Eating Well magazine found several big offenders:

Dunkin' Donuts Sausage, Egg & Cheese on Croissant has 710 calories, 20 grams of saturated fat, 0.5 grams of trans fat, 1,370 milligrams of sodium and two grams of fiber. Au Bon Pain Sausage, Egg and Cheddar on Asiago Bagel has 810 calories, 20 grams of saturated fat, 1,340 milligrams of sodium and two grams of fiber.

The website acaloriecounter.com cited Arby's Sausage Gravy Biscuit as an artery-killer with 961 calories and 3,755 milligrams of sodium while Jack In The Box Spicy Chicken Biscuit has a whopping seven grams of trans fat.

Bear in mind that the federal government recommends that your daily limit on saturated fats each day should be less than 10 percent of your daily calories. So, if you eat 2,000 calories a day to maintain your weight—a typical adult male—than you should consume under 200 grams of less of saturated fat in a day. Trans fats should be avoided whenever possible, and sodium should be held to less than 2,300 milligrams daily.

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The breakfast-sandwich raters have found some healthy choices, too. Eating Well praises Starbucks Spinach & Feta Breakfast Wrap, which has 290 calories, 3.5 grams of saturated fat and 830 milligrams of sodium. Consumer Reports cites Subway's Egg White and Cheese on Flatbread. Consumer Reports advises consumers to substitute egg whites for whole eggs when ordering breakfast sandwiches or leaving off the cheese or meal to avoid excess fat.

Question: Do you think about the nutrition value of a fast-food breakfast sandwich before purchasing one? Tell us what you think in the comments.

Shari Roan is an award-winning health writer based in Southern California. She is the author of three books on health and science subjects.