The Secret of Olive Oil's Success

Olive oil is one of the cornerstones of the healthy Mediterranean diet. Researchers have long believed that the most important health-promoting substance in olive oil is oleic acid, a monounsaturated fat.

But if the researchers were right, then all monounsaturated fats -- such as canola and safflower oil -- should be just as good for you.

However, repeated studies have suggested that olive oil is superior to other monounsaturated fats, and a European research team has just figured out why.

Olive oil contains lots of polyphenols, plant substances that appear to also protect the heart, according to a study published in the Annals of Internal Medicine.

And, in particular, virgin olive oil has much higher amounts of polyphenals than refined oil olive. (So-called virgin olive oil comes from the first pressing of the olives. Refined olive oil is derived from later pressings.)

As part of the study, 200 middle-aged male volunteers consumed about two tablespoons of virgin oil a day for three weeks. The men were then switched to various mixtures of non-virgin oils for similar periods.

The results revealed polyphenol-rich virgin olive oil did a better job at boosting the men's level of "good cholesterol," which clears artery-clogging fats from the bloodstream.

At the same time, it also seems to mitigate the potential harm done by "bad" cholesterol.

"Our study shows that olive oil is more than a monounsaturated fat . . . and the polyphenol content can account for the further [health] benefits," according to the research team led by Maria-Isabel Covas at the Municipal Institute for Medical Research in Barcelona, Spain.

Virgin oil tends to be more expensive than the non-virgin variety, but Dr. Covas believes it's worth it.

But what virgin olive oil should you buy? In stores, consumers are faced with a choice of either "extra virgin" or "virgin" oil. Both come from the first pressing and have similar levels of polyphenols, Dr. Covas said in an e-mail.

The two taste slightly different because they have different levels of acidity. But "there are no nutritional differences" between them.