For the study, Dr. 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, 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 the participants nor the researchers knew which meal was eaten during which visit.
The meals were identical, except that one was high in saturated fat (coconut oil), while 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 or 1 gram of fat for every 2.2 pounds. (For a 150-pound person, that's nearly the fat equivalent of eating a double cheeseburger, a large order of french fries and a large milkshake at one meal.)
In examining the volunteers, Dr. Nicholls and his colleagues found that after three hours, the saturated fat meal had reduced the ability of the endothelium to expand the 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 HDL, allowing more inflammatory agents to accumulate in the arteries than had been present before the volunteers ate. The polyunsaturated meal, however, seemed to boost the anti-inflammatory abilities of the body's good cholesterol, with the researchers finding fewer inflammatory agents in the arteries than before the volunteers ate.