We all know that eating too many high-fat breakfasts will clog our arteries and increase our heart attack risk.

Now, a study finds that even a single, high-fat meal can be damaging to cardiovascular function by making us more reactive to stress.

Researchers at the University of Calgary looked at the stress responses of 30 students who are two different meals after fasting the night before. On the first day, the students consumed a fast-food breakfast from McDonald's, including two hash brown patties, a Sausage McMuffin and an Egg McMuffin.

Several days later, the same students ate a low-fat breakfast of Kellogg's Frosted Flakes with skim milk, Source fat-free yogurt, a Kellogg's Fruit Loops fruit bar and Sunny Delight orange juice.

Both meals contained the same number of calories. The low-fat breakfast included supplements to balance it for sodium and potassium.

Two hours after eating the breakfasts, the students were subjected to standard physical and mental stress tests while having their cardiovascular responses measured.

They performed a mathematical test designed to be stressful, completed a public speaking exercise about something emotionally provocative, held an arm in ice water, and had a blood pressure cuff inflated around an arm, which gradually causes a dull ache.

The researchers then measured the subjects' blood pressure, heart rate and the resistance of blood vessels.

After consuming the high-fat breakfast, the students had higher blood pressure, a faster heart rate and hardened blood vessels -- all risk factors for heart disease and stroke.

"It was about 25 per cent more, which is a big effect -- a whopping effect -- especially when you consider the only difference was one meal," says Dr. Tavis Campbell, the study's senior author.

"Regardless of the task, we recorded greater reactivity among those who consumed the high-fat meal in several cardiovascular measures we recorded," added Fabijana Jakulj, a University of Calgary student who used the study as the basis for her honours thesis.

The study is published this month in the Journal of Nutrition.

• Read the full Journal of Nutrition study

Campbell says the study suggests a new and damaging way that a high-fat diet affects cardiovascular function.

"What's really shocking is that this is just one meal," he says.

Dietician Rosie Schwartz say this is the first study showing that two modern realities -- high-fat foods and stress -- together can harm our bodies, and it may explain why even young people are developing high blood pressure.

"I think it is a wake-up call. Take the time to eat a balanced breakfast especially if you have a fast-paced life," she says.

Campbell cautions that more research is needed to fully understand how the mechanisms work.

"Telling people to never eat something is probably not a good way to promote a better diet," he says.

"At the same time, we do have an epidemic of obesity in North America and it's important that people try to make informed choices."