There is much confusion about which oils are best for you. Some oils should be avoided in the diet. For this discussion, I will only focus on dietary use and not cosmetic/topical use.
The Seed Oils
Seed oils are often referred to as industrial seed oils and include Canola (Canadian food-grade rapeseed), corn, cottonseed, vegetable shortening, rapeseed, grapeseed, rice bran, safflower, cottonseed, soybean, vegetable oil, and sunflower. Vegetable oil is mostly soybean oil in combination with corn oil, Canola, peanut, olive, sunflower, palm, or cottonseed. All other plant-based oils, including flaxseed, are not considered seed oil.
Seed oils undergo heavy processing and have limited health benefits. They are often refined, meaning the extraction process involves high heat, which removes any beneficial compounds. The more refined they are, the more likely they will be labeled as "light." Consider seed oils to be processed or ultra-processed foods.
Inflammation and Seed Oils
Except for lab-controlled rodent studies, it is difficult to find good studies on different seed oils and their health effects. These oils only became part of the human diet in the 1910s, mainly in the southern states. By the 1920s, they were mass-produced for consumption and quickly overtook tallow and lard. Many studies touting health benefits have been funded by big food. Further, I found studies comparing these oils to each other, such as soy to Canola, Canola to sunflower, and other odd combinations. In every study, one oil performed better than the other, but that is not the same as saying that the better-performing one is healthy. Beware of this as you do your own reading.
Seed oils contain high amounts of omega-6 fatty acids, which are inflammatory, and zero omega-3 fatty acids. Human consumption of more plant oils over several decades has resulted in a high increase in the intake of inflammatory omega-6 linoleic acid (LA). Although linoleic acid is an "essential" nutrient, we get plenty from natural foods. Is is not a good idea to supplement it with oils. Very little is known about excessive LA's effect on the brain. There is some evidence that excessive dietary LA makes the brain more vulnerable to inflammation.
As oil changes, for any reason, abnormal compounds form, including free radicals. Free radicals are always inflammatory. It is best to store all oils in a dark spot and in glass. If possible, only buy oils that are sold in glass. If an oil smells rancid, it probably is.
(See Omega-3 Fish Oil blog post for more on omega-3 vs. omega-6 fatty acids: https://www.savannahfunctionalmedicine.com/post/omega-3-fish-oil
Carcinogenicity and Seed Oils
Various studies in rodents show that the ingestion of corn oil itself as well as the quantity results in increased colon cancer. 1,2 The risk of lung cancer in smokers is increased by corn oil consumption. Several studies used rodent models to show the increased risk and to try to find the mechanism.3
Several rodent studies show liver inflammation and cancer growth to be increased with various seed oils. One study compared soybean oil to ghee and showed downregulation of carcinogenic activation enzymes and no increased risk to the ghee-fed mice while the soybean oil-fed mice has signicant increases of GGT (a liver enzyme) and a lack of downregulation of carcinogenic activation enzymes.4
Smoke Point
If you choose to cook with seed oils or any oil, it is essential to know the smoke point. This is the point at which the oil burns, changes color (usually darkens), smokes, and changes its structure. The more refined or processed the oil is, the higher the smoke point. Although seed oils tolerate higher temperatures, there are more natural oils that tolerate high heat that may be a better substitute.
Wikipedia has an excellent template of all of the oils at different levels of refinement and their smoke points. https://en.wikipedia.org/wiki/Template:Smoke_point_of_cooking_oils
Although deep-fried foods are never good, they cannot always be avoided, and sometimes, the occasion calls for them. Instead of using corn or soybean oil for the deep frier, a better choice would be palm oil. Unfortunately, palm oil must be ordered online as it does not exist at most local big box stores.
1. Reddy BS, Tanaka T, Simi B. Effect of different levels of dietary trans fat or corn oil on azoxymethane-induced colon carcinogenesis in F344 rats. J Natl Cancer Inst. 1985 Oct;75(4):791-8. PMID: 3862909.
2. Wu B, Iwakiri R, Ootani A, Tsunada S, Fujise T, Sakata Y, Sakata H, Toda S, Fujimoto K. Dietary corn oil promotes colon cancer by inhibiting mitochondria-dependent apoptosis in azoxymethane-treated rats. Exp Biol Med (Maywood). 2004 Nov;229(10):1017-25. doi: 10.1177/153537020422901005. PMID: 15522837.
3. Chang SI, El-Bayoumy K, Sinha I, Trushin N, Stanley B, Pittman B, Prokopczyk B. 4-(Methylnitrosamino)-I-(3-pyridyl)-1-butanone enhances the expression of apolipoprotein A-I and Clara cell 17-kDa protein in the lung proteomes of rats fed a corn oil diet but not a fish oil diet. Cancer Epidemiol Biomarkers Prev. 2007 Feb;16(2):228-35. doi: 10.1158/1055-9965.EPI-06-0567. PMID: 17301254.
4. Rani R, Kansal VK. Effects of cow ghee (clarified butter oil) & soybean oil on carcinogen-metabolizing enzymes in rats. Indian J Med Res. 2012 Sep;136(3):460-5. PMID: 23041740; PMCID: PMC3510893.
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