In an effort to understand what makes olive oil so good for heart health, a study from Europe’s Cardiovascular Risk and Nutrition Research Group and the U.S. National Institutes of Health has found that olive oil’s polyphenols significantly increase the size of high-density lipoprotein-cholesterol (HDL) in the blood and enhance the HDL’s ability to inhibit formation of the abnormal fatty deposits, known as plaque, within the walls of arteries. Polyphenols are natural compounds from plants known to help prevent cancer and heart disease.

In the three-week study, researchers isolated the effect of polyphenols by dividing 47 healthy European men into two groups: one ate a diet containing polyphenol-poor olive oil and the other consumed polyphenol-rich olive oil. The enriched diet resulted in increased size, fluidity and stability (resistance to oxidation) of the HDL molecules by reducing their triglyceride core. The researchers note that the oxidation of cholesterol lipids such as low-density lipoprotein (LDL) is linked with arteriosclerosis.