— Dilraj Kahlon, Delta, British Columbia
A: Like all oils, olive oil is a mixture of monounsaturated, polyunsaturated, and saturated fatty acids. What makes olive oil so healthful is its high percentage of oleic acid, a monounsaturated fat that helps protect the heart and in some research seems to lower cancer risk. The amount of oleic acid varies from 55 percent to 85 percent of the total fat content, depending on the type of olives and the site and soil in which they were grown. But oleic acid isn't the only reason to like olive oil: It also contains heart-healthy flavonoid antioxidants and vitamin E. Extra-virgin olive oils—they're made from the first pressing of the choicest olives—contain the highest levels of these beneficial substances.
As far as cooking is concerned, when any oil reaches its smoke point (about 410 degrees for olive oil), it will degrade, oxidize (a process that damages the fat molecules), and partially hydrogenate, creating harmful trans fats. But you don't need to get oil that hot to sauté vegetables (300 degrees) or even to fry breaded items (340 degrees). Although those lower temperatures may damage some of the flavonoids, the loss will be trivial. And by starting with a healthy oil, your dish will still be more nutritious than if you cooked with, say, butter.
As for storage, olive oil keeps well—and maintains its healthful properties along with its great taste—for up to two years if unopened. Once opened, it can last a year if kept well sealed in a cool, dry, dark cabinet (even longer if refrigerated or frozen). If the oil smells like old French fries or has a buttery taste, it has probably gone rancid and should be tossed.
My advice is to use it routinely as part of a healthful diet, and then you won't have to worry about long-term storage.
David L. Katz, MD, is director of the Yale-Griffin Prevention Research Center and president of the nonprofit Turn the Tide Foundation.