The prevalence of processed vegetable and seed oils — like canola, safflower, and soybean oil — in our food supply has increased dramatically since the 1900s. [1,2] Seed oils initially got their “heart healthy” stamp of approval when studies discovered replacing saturated fats (like butter) with fats high in omega-6 polyunsaturated fats (like vegetable oils) can reduce cardiovascular disease risk. [3,4,5]
Despite potential health benefits, scientists have raised concern about the drastic increase in omega-6 polyunsaturated fatty acid (PUFA) consumption from refined seed oils over the past century.  This is primarily due to the backlash omega-6 seed oils have received for their potential to promote inflammation. 
So are these oils healthy or not? Here’s everything you need to know about vegetable and seed oils, why oxidation of these fats matter, their impact on inflammation and heart health, and whether you should avoid them.
What are vegetable and seed oils?
Soybean, canola, palm, and corn oil are examples of vegetable oils — extracted from the seeds of those plants — that are commonly consumed in the United States.  These oils are suitable for baking due to their neutral flavor profile, but they are also frequently added in large amounts to processed foods like salad dressings, margarine, mayonnaise, and baked goods.
These oils are also known as “refined” oils because manufacturing involves using a chemical solvent to extract the oil from the plant or seed, which is then refined and deodorized to counteract the taste and smell of the chemical solvent.  The technology for this type of refinement wasn’t available until the twentieth century.
Other oils are also extracted from plants (think olive oil, coconut oil, etc). But the term “vegetable oil” has specifically come to encompass plant oils high in omega-6 polyunsaturated fatty acids (PUFAs). So fats like olive oil and coconut oil don’t fall under these umbrella terms due to the type of fatty acids they contain.
What is the fatty acid profile of seed oils, and how does it differ from other oils?
Fats can either be classified as saturated or unsaturated (mono- or polyunsaturated) based on the number of double bonds in their chemical structures. These bonds influence the health effects of the types of fat. Here’s a quick breakdown of the primary types of fat found in commonly consumed oils. 
Saturated fats: No double bonds (butter, lard, coconut fat)
Monounsaturated: One double bond (Olive oil, avocados)
Polyunsaturated: Multiple double bonds (fish oil, vegetable oils)
All oils include a mix of these types of fats; however, different varieties vary in the proportion of fat they contain. Seed and vegetable oils typically have the highest levels of polyunsaturated fats (PUFAs).
PUFAs can be split into two categories: omega-6 and omega-3 fatty acids. Both must come from the diet and are therefore termed essential fatty acids. Refined seed oils such as soybean, corn, and safflower oil are considerably high in linoleic acid (LA), the primary dietary omega-6 PUFA. 
The impact of omega-6 fatty acids on health
Linoleic Acid (LA) — an omega-6 fatty acid found primarily in vegetable oils as well as in some nuts and seeds — can also be beneficial for heart health when eaten in moderation and in place of saturated fats. [1,3,5] High‐quality evidence suggests increasing omega‐6 fat consumption reduces total serum cholesterol slightly in the long term, which can be protective of heart health. 
A systematic review of 49 randomized controlled trials found that increasing total PUFA intake may reduce the risk of coronary heart disease and cardiovascular disease events.  However, each PUFA is processed differently by the body, which is why more recent studies have looked at the influence of omega-6s in isolation on risk factors for cardiovascular disease, like inflammation.
Omega-6s from vegetable and seed oils are especially susceptible to oxidation
Because of both the refinement process and the multiple double bonds found in PUFAs, vegetable and seed oils are sensitive to heat, light, and oxygen. When exposed to these elements, PUFAs may go through a process called oxidation — more so than other types of fat or whole food sources of omega-6s like nuts or seeds. 
Oxidation of oils leads to 
- Undesirable tastes
- Decomposition of nutritional quality
- The production of harmful compounds like free radicals
Oxidation can occur when oils are sitting on your shelf waiting to be used, or during the cooking process. They can also oxidize and deteriorate within the body, which forms harmful free radicals. An increase in free radicals without an increase in antioxidants —compounds that can neutralize or stop them — leads to oxidative damage of molecules in the body and inflammation. That damage can lead to the production of more free radicals, creating more damage within the body. It’s a vicious cycle.
Low-density lipoproteins (LDL) are especially vulnerable to oxidation because PUFAs comprise their cell walls. LDL transports cholesterol throughout the body, and LDL levels influence the risk for heart health complications.
When dietary PUFAs are consumed from refined seed oils in large quantities, LDL’s sensitivity to oxidation increases, and particles degrade. High levels of oxidized LDL particles are particularly detrimental to heart health. [7,9] Medical professionals may recommend consuming refined seed oils in moderation in conjunction with increasing the amount of anti-inflammatory fats rich in antioxidants to protect heart health and from excess oxidative damage from occurring.
The relationship between seed oils, inflammation, and heart health
Fatty acids serve different functions in the body. Some are believed to cause inflammation, but others (particularly omega-3s and monounsaturated fats) seem to have anti-inflammatory properties. Researchers have discovered that when consumed in high amounts, omega-6s from refined seed oils can promote oxidative stress and chronic low-grade inflammation, which can lead to poor heart health outcomes. [3, 9,10] This is known as “the oxidized linoleic acid hypothesis.” 
Though chronic inflammation is a major contributor to heart disease, there are currently no controlled human studies that have analyzed the effects of omega-6 fatty acids on heart disease. [11,12] It is important to note that there are other characteristics of some dietary patterns that also promote inflammation including high intakes of saturated fat, refined grains, meats, and refined sugars.  These diets are also low in many “heart healthy” nutrients that act to reduce bodily inflammation such as fiber, antioxidants, and omega-3 fatty acids.  Therefore, looking at ways to improve dietary patterns as a whole can be more beneficial for improving health outcomes.
Despite the concerns raised by the oxidation sensitivity of omega-6 PUFAs in seed oils, larger randomized controlled trials are needed to determine the effects these oils have on heart disease risk factors and other health outcomes. 
You don’t need to avoid all omega-6s?
While high omega-6 rich seed oil consumption can lead to poor health outcomes when accompanied by low omega-3 intake, there is no need to avoid all omega-6 fatty acids in your diet. 
In fact, they can improve heart health when coming from whole foods such as nuts and seeds.  So focus on moderating intake of omega-6s from seed oils in processed foods and getting adequate levels of PUFAs from whole foods sources including nuts and fatty fish rich in omega-3s in your diet.  The American Heart Association, along with the Institute of Medicine, recommends getting 5% to 10% of your daily calories from omega-6 fats from whole foods. 
How to choose the right oil
Oils are a common pantry staple used to prepare a variety of different types of recipes. But cooking oils can have differential impacts on your health, and should be chosen accordingly.
Choosing the right oil can be overwhelming, especially with so many options out there. But, understanding the different types of fats that make up these oils can help you choose one that provides the most health benefits.
Limit oils high in saturated fats like coconut, palm, palm kernel oils: The US Dietary Guidelines recommends limiting saturated fat intake to less than 10% of calories to reduce the risk of cardiovascular disease.
Opt for oils high in monounsaturated like extra virgin olive oil: Oils high in monounsaturated fats can improve blood cholesterol levels.  In addition, extra-virgin olive oil has the lowest oxidation rate of cooking oils and is a great source of antioxidants, both of which can help reduce inflammation. 
Make sure your oil is fresh: Check the best-by date on oils before purchasing them as they should be used within 30 to 60 days after opening to prevent them from going rancid.