Yet when huge doses of vitamins C and E were added to the diet, an extraordinary thing happened: The subjects' arteries responded to the high-fat meal as though they'd eaten a low-fat bowl of corn flakes.
Researchers caution that the small study's finding is preliminary, but it appears to bolster scientific thinking that antioxidant vitamins can decrease the heart-disease risk posed by a fatty diet.
The 20 subjects who ate the fat-packed McDonald's breakfast had impaired blood vessel function for up to four hours afterwards. But no such impairment was found on another day when they swallowed 20 times the recommended daily dosage of vitamins C and E immediately before eating the same meal.
The research appears in today's Journal of the American Medical Association.
"The predominant mechanism by which a high-fat (especially saturated fat) diet leads to atherosclerosis is by elevating serum cholesterol," wrote the authors, led by Dr. Gary Plotnick, a professor of medicine at the University of Maryland Medical Center.
Their findings provide further evidence of a "second potential pathway" to hardened arteries in which dietary fats damage the endothelium, the inner layer of cells that line the heart and the blood vessels.
The study may help explain why people sometimes have heart attacks right after eating a big, fatty meal, and why some people with normal cholesterol levels develop heart problems, said Dr. Kenneth Cooper, author of "The Antioxidant Revolution" and founder of the Cooper Aerobics Center in Dallas.
"This is just further information that documents the beneficial effects of antioxidants," Cooper said.
Antioxidants, such as the vitamins studied, work by soaking up dangerous byproducts that form when the body metabolizes oxygen. These byproducts, known as free radicals, can damage cells. They are thought to interact with some fats in a way that makes them clog arteries, and can produce cellular damage that sometimes leads to cancer.
Dr. Meir Stampfer, a professor of epidemiology and nutrition at Harvard's School of Public Health, called the study "a good piece of work and a nice step forward."
But, he added, "It was a small number of people and just one meal. We want to know what happens long term."
It's unknown if antioxidant vitamins could block the effects of a consistently high-fat diet. Long-term use of vitamins in such high doses also could be harmful.
Plotnick stressed that findings are "very preliminary."
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