The important thing about cooking with olive oil is not to heat the oil over its smoke point (also referred to as smoking point). The smoke point refers to the temperature at which a cooking fat or oil begins to break down. The substance smokes or burns, and gives food an unpleasant taste. The smoke point of oil varies with its quality. High quality extra virgin olive oils have a high smoke point, roughly between 400 and 365ºF (204 and 185ºC) depending on its free fatty acid content. When heated, olive oil is the most stable fat, which means it stands up well to high frying temperatures. The digestibility of olive oil is not affected when it is heated, even when it is re-used several times for frying.
One common myth is that heating olive oil will make it saturated or trans-fatty. This is not true. As far as making a saturated fat, all oils will oxidize and hydrogenate to a tiny degree if repeatedly heated to very high temperatures such as is done in commercial frying operations. Virgin olvie oils are highly monounsaturated oils and therefore resistant to oxidation and hydrogenation. Studies have shown oxidation and hydrogenation occurs to a lesser degree in olive oil than in other oils. But in any case, the amount of hydrogenation is miniscule and no home cook would ever experience this problem.
Another myth is that cooking in olive oil diminishes the nutritional value of the food. This is a misconception. The fact is that heating ANY food will break down its nutritional value. High heat such as frying is worse than moderate heat such as steaming, which is worse than eating vegetables raw. It is not the cooking oil per se, but the high heat of frying. We are not aware of any edible cooking oil, which by itself diminishes the nutritional value of the food cooked in it. Most nutritionists recommend lightly steaming vegetables or eating them. A touch of a flavorsome extra virgin olive oil added at the table will add taste and healthful anti-oxidants. Such is the Mediterranean diet, which has been shown to help prevent coronary disease and have other health benefits.