A single meal high in saturated fat can decrease the ability of high-density lipoprotein cholesterol to protect arterial endothelium from inflammation, whereas a meal high in polyunsaturated fat can increase its anti-inflammatory activity, according to an article in the August 15 issue of the Journal of the American College of Cardiology.

“The take-home, public-health message is this: It’s further evidence to support the need to aggressively reduce the amount of saturated fat consumed in the diet,” said researcher Stephen J. Nicholls, MB, BS, PhD, leader of the Australian study and now a cardiologist at Cleveland Clinic in Ohio. “This study helps to explain the mechanisms by which saturated fat supports formation of plaques in the arterial wall, and we know these plaques are the major cause of heart attack and stroke.”

The American College of Cardiology and the American Heart Association recommend that people limit their intake of saturated fat to no more than 7 percent of their total daily calories. Polyunsaturated fats, on the other hand, come mainly from plants and are liquid at room temperature.

For the study, Nicholls and his colleagues recruited 14 healthy volunteers and supplied them with two meals, eaten one month apart. The volunteers, ranging in age from 18 to 40 years, were examined and had blood drawn before eating (following an overnight fast), three hours after eating and again six hours after eating their supplied meals. Neither participants nor researchers knew which meal was eaten during which visit.

The meals were identical except that one was high in saturated fat (coconut oil), whereas the other was high in polyunsaturated fat (safflower oil). Each meal consisted of a slice of carrot cake and a milkshake. All meals were specially prepared so that each participant consumed 1 gram of fat per kilogram of body weight.

In examining the volunteers, the researchers found that after three hours, the saturated fat meal had reduced the ability of endothelium to expand arteries in order to increase blood flow. The researchers determined this by using a blood pressure cuff to restrict blood flow and then monitoring the body’s response. The polyunsaturated meal also reduced this ability slightly, but the results were not statistically significant.

After six hours, researchers found the meal high in saturated fat had diminished the protective qualities of high-density cholesterol, allowing more inflammatory agents to accumulate in arteries than had been present before volunteers ate. The polyunsaturated meal, however, seemed to boost the anti-inflammatory abilities of high-density cholesterol, with the researchers finding fewer inflammatory cytokines in the arteries than before the volunteers ate.

“In putting this all together,” Nicholls said, “we have a difference between the two meals regarding a number of factors that influence the early stages of plaque formation. We have a situation where consumption of a single meal containing a high level of saturated fat is associated with impairment of vascular reactivity and impairment of a normal protective property of high-density lipoprotein cholesterol (HDL). In contrast, consumption of a meal high in polyunsaturated fat results in [HDL] that is more protective.

“It is a small study,” he concluded, “but I think the findings have broad implications because diet and exercise are the cornerstones of all strategies for preventing heart disease.”

Robert Vogel, MD, a cardiologist and professor of medicine at the University of Maryland Medical Center, did not participate in the research, but agreed it provided “one more nail in the coffin” against eating diets high in saturated fat.

“This study helps to flesh out just why we shouldn’t eat too much saturated fat,” Vogel wrote. “Traditionally, we think of unhealthy foods as raising cholesterol or raising blood pressure, but this demonstrates that depending on what you eat, you can actually change the effect of HDL ? typically thought of as ‘good’ cholesterol ? from protective to detrimental. This opens up new insights and avenues for research.”