Are Seed Oils Toxic?

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Alanna O'Keefe, UNH Extension Intern, Nutrition Connections
  • Bottles with different kinds of seed oils.

If you've been online recently, you might have seen discussion about seed oils. Some people claim that these oils cause all sorts of chronic diseases, and buzzwords like "toxic" and "inflammatory" can make these claims feel scary and overwhelming. Before rushing to your kitchen and throwing out these oils, let's look at what the science has to say about the safety of seed oils.

What are seed oils?

Seed oils are a group of vegetable oils which come from the seed of a plant. These oils include:1

  • Canola
  • Corn
  • Cottonseed
  • Grapeseed
  • Peanut
  • Rice bran
  • Safflower
  • Sesame
  • Soybean
  • Sunflower

While all oils are made up of fats, not all oils are created equally. All oils contain different amounts of saturated fats (SFA), monounsaturated fats (MUFA) and polyunsaturated fats (PUFA). See the chart below for a more in-depth breakdown of the fat content of common cooking oils.2

Compositional differences between various fats and oils bar graph.

Figure 1: ©2024 Fredrik Rosqvist and Sari Niinistö

Compositional differences between various fats and oils. Values are taken from the Swedish, Finnish and US food databases which showed the amount of different types of fatty acids as grams per 100g fat or oil which equals to percentages.

Here is a text version of the fat and oil composition chart shown in the image. Each fat or oil is broken down into percentages of the following types of fatty acids:

  • SFA: Saturated Fatty Acids (Red)
  • MUFA: Monounsaturated Fatty Acids (Yellow)
  • n-6 PUFA: Omega-6 Polyunsaturated Fatty Acids (Light Green)
  • n-3 PUFA: Omega-3 Polyunsaturated Fatty Acids (Dark Green)

 

Fatty Acid Composition by Oil Type (Approximate %):

1. Coconut fat

  • SFA: ~90%
  • MUFA: ~6%
  • n-6 PUFA: ~2%
  • n-3 PUFA: ~1%

2. Palm kernel oil

  • SFA: ~80%
  • MUFA: ~12%
  • n-6 PUFA: ~6%
  • n-3 PUFA: ~1%

3. Butter

  • SFA: ~65%
  • MUFA: ~30%
  • n-6 PUFA: ~4%
  • n-3 PUFA: ~1%

4. Lard

  • SFA: ~40%
  • MUFA: ~45%
  • n-6 PUFA: ~10%
  • n-3 PUFA: ~2%

5. Palm oil

  • SFA: ~50%
  • MUFA: ~40%
  • n-6 PUFA: ~8%
  • n-3 PUFA: ~1%

6. Soybean oil

  • SFA: ~15%
  • MUFA: ~25%
  • n-6 PUFA: ~50%
  • n-3 PUFA: ~10%

7. Sesame oil

  • SFA: ~15%
  • MUFA: ~40%
  • n-6 PUFA: ~40%
  • n-3 PUFA: ~1%

8. Olive oil

  • SFA: ~15%
  • MUFA: ~75%
  • n-6 PUFA: ~10%
  • n-3 PUFA: ~1%

 

9. Corn oil

  • SFA: ~15%
  • MUFA: ~30%
  • n-6 PUFA: ~55%
  • n-3 PUFA: ~1%

10. Grapeseed oil

  • SFA: ~10%
  • MUFA: ~20%
  • n-6 PUFA: ~70%
  • n-3 PUFA: ~1%

11. Sunflower oil

  • SFA: ~10%
  • MUFA: ~20%
  • n-6 PUFA: ~70%
  • n-3 PUFA: ~1%

12. Hempseed oil

  • SFA: ~10%
  • MUFA: ~15%
  • n-6 PUFA: ~55%
  • n-3 PUFA: ~20%

13. Walnut oil

  • SFA: ~10%
  • MUFA: ~25%
  • n-6 PUFA: ~55%
  • n-3 PUFA: ~10%

14. Safflower oil

  • SFA: ~10%
  • MUFA: ~15%
  • n-6 PUFA: ~75%
  • n-3 PUFA: ~1%

15. Linseed oil (Flaxseed oil)

  • SFA: ~10%
  • MUFA: ~15%
  • n-6 PUFA: ~15%
  • n-3 PUFA: ~60%

16. Canola/rapeseed oil

  • SFA: ~5%
  • MUFA: ~60%
  • n-6 PUFA: ~20%
  • n-3 PUFA: ~10%

Seed oils have been praised as "heart healthy" due to their high MUFA/PUFA content, as they have been shown to decrease heart disease risk.3 However, these oils are accused of being the cause of heart disease by some bloggers. Claims like these assume that seed oils are overloading our bodies with a certain type of fat called Omega-6 fatty acids.

What are Omega-6 fatty acids?

Omega-6 (linolenic acid) and Omega-3 (linoleic acid) are PUFA's that are both considered essential nutrients. This means that our bodies cannot make these fatty acids, so we need to eat them.4 While we need both, how much we eat of one fatty acid compared to the other is important. Studies suggest that we need about four times as much Omega-6 in our diet compared to Omega-3.5 Some researchers think that an average "American" or Western diet can supply up to 20 times as much Omega-6 compared to Omega-3.6 This means that we could be eating much more Omega-6 than we really need to.

Are seed oils inflammatory?

Consuming high amounts of Omega-6 and low amounts of Omega-3 is associated with inflammation in the body.7 Inflammation is our body's natural defense when we get sick or injured, and is needed to help us heal.8 However, when inflammation is occurring in our bodies for long periods of time (chronic or "systemic" inflammation) it can increase heart disease risk.9 this is the basis behind why some are claiming that seed oils are a root cause of heart disease. However,  we don't have strong evidence to suggest that increasing Omega-6 intake significantly increases inflammation in the body.10 Studies show that what does increase inflammation is when one consumes too many Omega-6 alongside too little Omega-3, which is common within an "American" diet.6,7However, calling Omega-6 inflammatory on its own is misleading.

Does Omega-6 turn into harmful compounds when heated?

Omega-6 is an unstable compound, meaning that it easily breaks down and changes into harmful compounds (metabolites) after being heated or exposed to oxygen for a long time.11 Studies show that these metabolites could damage our cells, which could contribute to the development of heart disease.12 However, many of these studies were completed on mice or isolated cells - not real human bodies. There currently aren't any human studies which directly link seed oils to the same damaging effects. Researchers still don't fully understand the impact of these metabolites on the human body, or how much of these we are truly eating within seed oils. What research shows is that people who had higher amounts of seed oils within their fat cells ended up living longer than people who had less.13 If eating seed oils was truly damaging to our health, these studies should show the opposite.

Practical recommendations:

Increase Omega-3 intake: Not only would this help to close the gap between high Omega-6 and low Omega-3 consumption, but we have strong evidence that suggests increasing Omega-3 intake largely reduces heart disease risk.14 We currently do not have strong evidence to suggest that avoiding seed oils or Omegs-6 would be beneficial. Remember, it's still an essential nutrient.

Some sources of Omega-3 fatty acids include:15
 

  • Fatty fish (salmon, mackerel, sardines)
  • Seeds (chia, flax, hemp)
  • Nuts (walnuts, almonds, hazelnuts, peanuts, macadamia)
  • Seaweed and algae
  • Wheat germ
  • Egg yolks
  • Milk with Omegs-3 added
  • Supplements (such as fish oil or algae-based)

Resources

  1. Xiang F, Ding C, Wang M, et al. Vegetable oils: Classification, quality analysis, nutritional value and lipidomics applications. Food Chemistry. 2024;439:138059. doi: 10.1016/j.foodchem.2023.138059
  2. Rosqvist F, Niinistö S. Fats and oils – a scoping review for Nordic Nutrition Recommendations 2023. Food & Nutrition Research. 2024;68:10487. doi: 10.29219/fnr.v68.10487
  3. Hooper L, Martin N, Jimoh OF, Kirk C, Foster E, Abdelhamid AS. Reduction in saturated fat intake for cardiovascular disease. Cochrane Database of Systematic Reviews. 2020;8:1465-1858. doi: 10.1002/14651858.CD011737.pub3
  4. Jones PJH, Rideout T. Lipids, sterols, and their metabolites. In: Ross AC, Caballero B, Cousins RJ, Tucker KL, Ziegler TR, eds. Modern Nutrition in Health and Disease. 11th ed. Baltimore, MD: Lippincott Williams & Wilkins; 2014.
  5. Simopoulos AP. The importance of the omega-6/omega-3 fatty acid ratio in cardiovascular disease and other chronic diseases. Exp Biol Med (Maywood). 2008;233(6):674-688. doi:10.3181/0711-MR-311
  6. Simopoulos AP, DiNicolantonio JJ. The importance of a balanced ω-6 to ω-3 ratio in the prevention and management of obesity. Open Heart. 2016;3(2):e000385. Published 2016 Sep 20. doi:10.1136/openhrt-2015-000385
  7. DiNicolantonio JJ, O'Keefe J. The Importance of Maintaining a Low Omega-6/Omega-3 Ratio for Reducing the Risk of Autoimmune Diseases, Asthma, and Allergies. Mo Med. 2021;118(5):453-459.
  8. Pahwa R, Goyal A, Jialal I. Chronic Inflammation. [Updated 2023 Aug 7]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK493173/
  9. Inflammation and Heart Disease. American Heart Association. Updated January 11, 2024. Accessed September 14, 2024. https://www.heart.org/en/health-topics/consumer-healthcare/what-is-cardiovascular-disease/inflammation-and-heart-disease
  10. Johnson GH, Fritsche K. Effect of dietary linoleic acid on markers of inflammation in healthy persons: a systematic review of randomized controlled trials. J Acad Nutr Diet. 2012;112(7):1029-1041.e10415. doi:10.1016/j.jand.2012.03.029
  11. Szabo Z, Marosvölgyi T, Szabo E, et al. Effects of Repeated Heating on Fatty Acid Composition of Plant-Based Cooking Oils. Foods. 2022;11(2):192. Published 2022 Jan 12. doi:10.3390/foods11020192
  12. Mercola J, D'Adamo CR. Linoleic Acid: A Narrative Review of the Effects of Increased Intake in the Standard American Diet and Associations with Chronic Disease. Nutrients. 2023;15(14):3129. Published 2023 Jul 13. doi:10.3390/nu15143129
  13. Iggman D, Ärnlöv J, Cederholm T, Risérus U. Association of Adipose Tissue Fatty Acids With Cardiovascular and All-Cause Mortality in Elderly Men. JAMA Cardiol. 2016;1(7):745–753. doi:10.1001/jamacardio.2016.2259
  14. Khan SU, Lone AN, Khan MS, et al. Effect of omega-3 fatty acids on cardiovascular outcomes: A systematic review and meta-analysis. EClinicalMedicine. 2021;38:100997. Published 2021 Jul 8. doi:10.1016/j.eclinm.2021.100997
  15. Omega-3 Fatty Acids. National Institutes of Health. Updated July 18, 2022. Accessed September 14, 2024. https://ods.od.nih.gov/factsheets/Omega3FattyAcids-Consumer/#:~:text=Fish%20and%20other%20seafood%20(especially,soybean%20oil%2C%20and%20canola%20oil)

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