Every fat, whether it's oil or butter, has both saturated and unsaturated fatty acids—it's the ratio that makes a difference for your health.
Polyunsaturated (omega-3 and omega-6) and monounsaturated (omega-9) fatty acids help improve blood cholesterol levels and help reduce your risk of cardiovascular events. These healthy fatty acids are more prevalent in vegetable oil and olive oil, as opposed to butter and lard (which are famous for their saturated fatty acids). Saturated fats and trans fats have long been found to increase your risk of heart disease, but studies have shown that replacing saturated fats with the polyunsaturated fats found in vegetable oil actually reduces your risk of coronary heart disease.
Omega-3 fatty acids, found in fatty fish such as salmon, have received a lot of attention for improving blood cholesterol and preventing heart disease. But the average American's diet, which is known to be high in processed foods, is likely much higher in omega-6 fatty acids, which are found in processed oils like canola oil and vegetable oil. Don't panic: This isn't necessarily a bad thing. A recent study suggests that linoleic acid, a member of the omega-6 family, may help reduce fat surrounding your organs, improve body composition, and reduce inflammation and insulin resistance (which can lead to diabetes). Researchers did not find the same benefits with omega-3 fatty acids.
In short: Every fat has something to offer, and a diet with diverse healthy fats will give you a full array of nutritional benefits.
When choosing which cooking oil to use, you might also want to consider how it will be used. Oils high in monounsaturated fat have low smoke points and will become rancid at high heat. Use olive oil and avocado oil for dressings, sautés, pizzas, and any meal that needs a finishing flavor. Vegetable oils, on the other hand, tend to have a more neutral flavor and higher smoke point, which makes them a better choice for high heat cooking. Use safflower, canola, peanut, and sunflower oils to pan-fry, sear meat, or roast vegetables.