In a study on blood vessel function in human volunteers, they found that a polyphenol-rich olive oil caused a significant improvement but there was little effect after subjects consumed an oil that had many of the phenols removed.
Healthy blood vessels ensure proper blood flow around the body whereas unhealthy vessels can eventually lead to blockages with implications for the heart.
The work by a team from the Reina Sofia University Hospital in Córdoba is one of a growing number of investigations going on around the world to better understand why olive oil protects the heart.
Several epidemiological studies have linked high consumption of olive oil, a key ingredient in the traditional Mediterranean diet, to lower incidence of heart disease in many south European countries. But only in recent years have researchers begun to investigate how the plant oil exerts these beneficial effects.
Identifying a mechanism for a food's health effect is key to confirming its healthy properties and gaining scientific credibility for its consumption as a health food.
The work on olive oil is complex however as it contains hundreds of potentially active compounds, many of which are not well-known. Moreover with numerous varieties of oil available, studies cannot be easily compared.
Yet there have been some recent breakthroughs. In September, US scientists identified a previously unknown chemical, which they called oleocanthal, that has a strong anti-inflammatory action.
They compared its action on COX enzymes to that of drugs like the common painkiller ibuprofen.
The new research, to be published in the 15 November issue of the Journal of the American College of Cardiology (vol 46, pp 1864-1868), is also important as it is the first to show a direct benefit of an olive oil with high content in phenolic compounds on endothelial function in humans, according to the authors.
Phenols are a large group of compounds that include flavonoids such as anthocyanins and quercetin, phenolic acids like ellagic acid, fibres such as lignans and vitamins. Many of these have antioxidant, anti-inflammatory and anti-clotting properties, all of which are known to benefit cardiovascular health.
The Spanish scientists looked particularly at the response of the endothelium, or inner lining, of small blood vessels in the fingers of subjects to sudden changes in blood flow, which were produced by inflating and then deflating a blood pressure cuff.
Poor responsiveness to this sort of blood flow test is considered an early warning sign of cardiovascular disease.
Previous studies have linked high-fat meals to poor endothelial function lasting for several hours after eating. So the team designed a study on a relatively high-fat meal - 60g of white bread with 40ml of olive oil.
The subjects, 21 adults with high cholesterol levels, were randomized to receive either an olive oil high in phenolic compounds (400 parts per million) or the same brand of oil that had been processed to remove most of the phenolic compounds (80 parts per million remaining). Then they crossed over and repeated the study to consume the other oil.
Consumption of the polyphenol-rich breakfast was associated with an improvement in endothelial function, as well as a greater increase in concentrations of nitric oxide, report the researchers.
"Virgin olive oil is more than fat because it is a real juice with other healthy micronutrients," said author author Dr Francisco Pérez Jiménez.
"We think, looking at our results, that the reduction in oxidative stress and the increase in the nitric oxide bioavailability are behind the observed improvement in ischaemic reactive hyperemia," he said.
But he cautioned that further studies in "appropriate populations, or with a large sample size, are required to definitively establish the in vivo antioxidant properties of these components in relation to cardiovascular disease outcomes."
Moreover, the scientist noted that not all olive oils have a high phenolic content.
But the results add up to the evidence to support increasing consumption of olive oil as way of preventing progression of atherosclerosis.