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.