How about bad enough to diminish your body's ability to defend itself against heart disease.

A recent study by researchers at the University of Sydney in Australia found just that reaction after 14 trial participants, all healthy and between the ages of 18 and 40, ate just one piece of high-fat carrot cake and drank a milkshake.

That fat-laden feast compromised the ability of the participants' arteries to expand to increased blood flow, the researchers found. The sudden boost in what's known as saturated fat hampered the effects of so-called "good" cholesterol, the high-density lipoprotein or HDL, from doing its job -- to protect the inner lining of the arteries from inflammatory agents that promote the build-up of fatty plaques. It's this plaque that, over time, clogs blood vessels and causes heart disease.

"Saturated-fat meals might predispose to inflammation of, and plaque buildup in, the vessels," said study leader Dr. David Celermajer, Scandrett professor of cardiology at the Heart Research Institute and the Department of Cardiology at Royal Prince Alfred Hospital.

Celermajer's team had the volunteers eat two meals, spaced one month apart. Each meal consisted of a slice of carrot cake and a milkshake. But, in one case the foods were made with saturated fat, and in the other case the meal was made with polyunsaturated safflower oil, a much healthier choice.

The high-fat meal, which contained about 90 percent saturated fat, had the equivalent of 68 grams of fat. In contrast, the meal made with polyunsaturated oil contained just 9 percent fat. The fat in the high-fat meal was equivalent to a 150-pound man or woman eating a double cheeseburger, a large order of french fries, and drinking a large milkshake, the researchers said.

Before and after each of the meals, the researchers obtained blood samples from the participants so they could evaluate whether the anti-inflammatory properties of the so-called good HDL cholesterol had decreased.

The anti-inflammatory properties did decrease after the saturated fat meal, the researchers said, but improved after the healthier polyunsaturated fat meal.

The effects may be temporary, Celermajer said. However, he's still concerned because the effect may be occurring over and over, each time a person eats a high-fat meal.

The study was published in the Journal of the American College of Cardiology.

The message is clear, Celermajer said: It's important to limit saturated fat intake as much as possible.

To do that, you've first got to know where saturated fat lurks, said Jeannie Moloo, a Sacramento, Calif., dietitian and a spokeswoman for the American Dietetic Association.

She suggests cutting down on meat, full-fat milk and full-fat dairy products as a way to reduce saturated fat. Those foods are all major sources of saturated fat, Moloo said. So are processed foods and snacks.

Switching to low-fat or non-fat dairy products can minimize your total saturated fat intake, Moloo said. Choosing foods wisely by reading the Nutrition Facts label can help, too. For instance, Moloo said, an ounce of regular cheddar cheese contains 6 grams of saturated fat, while an ounce of part-skim mozzarella contains less than half that, or 2.9 grams.

Ice cream contains a lot of saturated fat, Moloo tells her patients. For instance, she said, one cup of vanilla soft-serve ice cream has 13.5 grams of saturated fat. But some low-fat ice cream bars contain just 1.5 grams of saturated fat.

How much saturated fat per day is too much? Aim for 10 percent or less of your daily calories from saturated fat, Moloo suggested. The American Heart Association sets the bar for saturated fat at less than 7 percent of daily calories.

For instance, if your total calorie goal is 2,000 a day -- reasonable for moderately active adults -- you should aim for no more than 20 grams of saturated fat to keep your intake to 10 percent or so. While few people will take the time to add up their fat grams, doing so for a day or two can give you an idea of how you are doing.