The most unhealthy oil to fry in are highly refined seed and vegetable oils like corn oil, canola oil, sunflower oils and soybean oils.
Highly refined seed and vegetable oils are the unhealthiest oils to fry foods in because they are high in polyunsaturated fat or PUFA, which makes them unstable, prone to oxidation and can form harmful compounds like aldehydes at high heat, which can also lead to potential health issues and even inflammation.
Even walnut oil, flaxseed oil and extra virgin olive oil are also bad choices of oils for frying foods with as they have low smoke points and delicate structures which break down with heat.
Flaxseed oil, walnut oil and extra virgin olive oils have low smoke points, which means they burn and degrade quickly, losing nutrients and creating bad flavors and compounds.
The harsh refining of these oils strips the antioxidants from the oils and makes them less stable for high heat cooking, and at high temperatures, these oils also break down, creating inflammatory compounds and can also potentially increase your risk of heart disease cancer.
Soybean oil, corn oil, canola oil and sunflower oils are rich in omega-6 fatty acids, which also oxidize easily and produce toxic byproducts when they are heated excessively, unlike the more stable monounsaturated fats or oleic acid in avocado oils and olive oils.
Healthiest oils to fry and cook foods in are avocado oil, extra virgin olive oil, peanut oil and grapeseed oil.
Avocado oil is high in monounsaturated fats also known as oleic acid that is good for cholesterol, with a very high smoke point and a mild flavor.
Extra virgin olive oil is rich in antioxidants and monounsaturated fats, which are stable at high heat and a staple in the heart healthy Mediterranean diet, although some flavor is also present.
Peanut oil is popular for frying foods in as it has a high smoke point around 446 F and neutral taste, although it's often pricier.
And grapeseed oil is stable at high heat and provides a clean, neutral flavor good for high heat cooking.