Polyunsaturated fats are very similar to monounsaturated. They're abundant in many plant-based foods, and consuming them in moderation helps keep cholesterol and blood sugar in line. In fact, our body needs polyunsaturated fats, but it can't produce them on its own, as it does with other kinds of fat. So we need to get them from food.
There are two types of polyunsaturated fat: omega-6 and omega-3 fatty acids. Soybean, corn, and safflower oil are especially solid sources of omega-6s. Since so many processed foods are made with these oils, omega-6s have become much more abundant in the standard American diet than omega-3s. Rich sources of omega-3s include walnuts, flaxseeds, organic milk, and cold-water fish such as salmon, mackerel, and herring.
There's been some controversy over whether omega-6s are actually good for you or if they cause inflammation, but most health experts now agree that they're beneficial – although you need to consume roughly an equal amount of omega-3s, which too few people do since they aren't as ubiquitous. Doctors are realizing more and more that upping your omega-3 intake has countless benefits beyond protecting your heart. Studies show that these essential fatty acids may help stave off Alzheimer's, cancer, autoimmune diseases, and more.