The Effect of Virgin and Refined Olive Oils on Heart Disease Risk Factors

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The summary below is from the full report titled "The Effect of Polyphenols in Olive Oil on Heart Disease Risk Factors. A Randomized Trial." It is in the 5 September 2006 issue of Annals of Internal Medicine (volume 145, pages 333-341). The authors are M.-I. Covas, K. Nyyssönen, H.E. Poulsen, J. Kaikkonen, H.-J.F. Zunft, H. Kiesewetter, A. Gaddi, R. de la Torre, J. Mursu, H. Bäumler, S. Nascetti, J.T. Salonen, M. Fitó, J. Virtanen, and J. Marrugat, for the EUROLIVE Study Group.

What is the problem and what is known about it so far?

A Mediterranean diet is an approach to eating that includes food choices common among people in countries around the Mediterranean Sea. The main fat in the Mediterranean diet is olive oil. The Mediterranean diet also includes fish, vegetables, fruits, whole grains, and nuts. Research has shown a strong connection between Mediterranean diets and lower rates of heart disease. Most researchers believe that the most important health-promoting substance in olive oil is oleic acid, which is a monounsaturated fatty acid. People want to know whether other substances in olive oil also promote health. Researchers think that polyphenol might be another health-promoting substance in olive oil. Virgin olive oil comes from the first pressing of olives. Refined olive oil comes from later pressings. Virgin olive oil has higher amounts of polyphenols than refined olive oil. If the amount of polyphenol in olive oil was important to health, people might prefer to use olive oil that was rich in polyphenols.

Why did the researchers do this particular study?

To determine whether olive oil containing different amounts of polyphenols affected risk factors for heart disease.

Who was studied?

200 healthy men 20 to 60 years of age who lived in 1 of 6 European cities.

How was the study done?

The researchers compared virgin olive oil (high level of polyphenols), refined olive oil (low level of polyphenols), and a mixture of equal amounts of the 2 olive oils (medium level of polyphenols). Each participant consumed about a tablespoon of one of the olive oils each day for 3 weeks. Over the course of the study, each participant consumed each of the 3 olive oils for a 3-week period. The researchers took blood samples to measure cholesterol levels and other chemicals before and after each 3-week period and compared the results after each type of olive oil.

What did the researchers find?

Virgin olive oil (high in polyphenols) increases the level of high-density lipoprotein cholesterol ("good cholesterol") the most. It also increases the body level of substances that prevent a chemical reaction (oxidation) that may damage low-density lipoprotein cholesterol ("bad cholesterol") and make it more likely to promote clots in blood vessels that can lead to heart disease.

What were the limitations of the study?

The study was too brief to determine whether consuming olive oil that contained different amounts of polyphenols changed people's risk for heart disease events.

What are the implications of the study?

Virgin olive oil seems to have a bigger effect on health-promoting substances in the body than refined olive oil. More research is needed to find out whether people who use virgin olive oil have a lower chance of developing heart disease than people who use refined olive oil.