top of page

Seed Oils are Toxic? A Review of the Evidence.

Updated: Dec 20, 2023


seed oils canola toxic

In the realm of health, nutrition and social media, one subject seems to elicit more toxicity than most: seed oils. In certain quarters, seed oils (also known a vegetable oils) are branded the enemy of health, the root of inflammation and obesity. They are "poison" and "toxic", canola oil especially. But scientific and dietary advice says that seed oils - affordable and neutral in taste - rich in monounsaturated (MUFA) and polyunsaturated fatty acids (PUFAs) are healthy.


Are seed oils toxic and to be avoided at all costs? What are the arguments, what does the evidence show us?

 

Why is this important?


Fat is an essential part of the diet and the balance of different types of fats in the diet is important for health. Irrespective of source, fat has 9 calories per gram.


We all need some, but not too much, fat in our diet. Replacing excess saturated fat with monounsaturated fatty acids (MUFAs) or polyunsaturated fatty acids (PUFAs) is beneficial for health - especially cardiovascular health - providing that not too much (in excess of caloric requirement) is consumed.


Omega-6 and omega-3 fatty acids are essential, meaning that the body cannot make them. There is another class of fatty acids, omega-9 that the body can make.


MUFAs

There are various types of monounsaturated fatty acids, the most common being oleic and palmitoliec acids.


Foods rich in MUFAs include: avocado oil, olive oil, canola oil, peanut oil, safflower oil, and sesame oil. Nuts such as almonds, cashews, hazelnuts, macadamia and pistachios are high in MUFAs.


PUFAs

There are two major classes of polyunsaturated fats: omega-6 and omega-3.


Omega-6 fatty acids are primarily found in vegetable oils (inlcuding soybean, sunflower, corn and cottonseed oils) and nuts. These include:


  • LA - Linoleic acid

  • ARA - Arachidonic acid

  • GLA - Gamma Linoleic acid

  • CLA - Conjugated Linoleic acid


Omega-3 fatty acids include:


  • ALA - found in plants.

  • EPA, DHA and DPA - primarily found in fatty fish (salmon, makerel, anchovy, herring); some nuts and seeds.


Saturated Fats

Saturated fats are found in all animal fats and even plants, for example: butter, lard and coconut oil. Saturated fats are solids at room temperature.


 

The Arguments against Seed Oils


Seed oils, rich in MUFAs and PUFAs, are decried by some as being “toxic” and “poison”. There are several lines of argument to be found.

“Seed oils make us fat.” Without a doubt, seed oils are high in calories. Just like any oil or fat, plant or animal. Too much soybean oil causes weight gain, as does olive oil or animal fat. Over the last decades seed oil consumption has massively increased as has the consumption of processed foods. But is this argument a seed oil issue or rather an eating practice issue?


If you are eating processed or fried foods, you will ingest more oil, fats and calories, be they omega 6 or saturated fats. Processed foods (often ultra-processed grains, mixed with oil, loaded with salt) are both unhealthy and popular. If your aim is to manage your weight, you need to manage your caloric intake, be the calories from oils, processed foods, fatty animal products or even whole foods.


Making better food choices when eating out or eating at home can all have a positive effect on diet and caloric intake. Reducing processed foods and fried foods are two areas to be mindful of.


“Seed oils are toxic, they chemically and heat processed.” Many seed oils, for example, canola (from rapeseed) are heat processed and deodorised. Of particular concern is a chemical called hexane used during the processing. Like many chemicals, hexane is allowed to be present in foods in very low quantities. Is hexane poisoning us? Is oil, using high heat and industrial processes, damaged before being bottled and sent to the supermarket? These are valid questions, we need to investigate the evidence.


Many substances that we consume are toxic, dependent on how much is consumed. In high concentrations hexane is neurotoxic. Much of the evidence we have relating to hexane exposure comes from industrial exposure, rather than from food consumption. Allowable limits on hexane in food are stipulated by health authorities. There does not appear to be any evidence of toxicity at quantities that might be consumed in the diet, unless one was consuming gallons of seed oils.


Worth to mention, seed oils are often used in commercial kitchens, for deep fat frying. These oils might be reused, reheated time and time again. Breaking down the oil into toxic components. A good argument for not eating fast food or, in the home kitchen, reheating or burning any oil for that matter.


“We eat too many PUFAs - and not enough omega-3 fats - this causes inflammation” There are concerns over the ratio of oils in the diet, too much omega-6 vs omega-3 creating chronic inflammation. Specifically that consuming linoleic and arachidonic acids increase inflammation in the body. Whilst early trials on rats showed this to be the case, multiple human randomised control trials have shown this not to be true.


We do know that sub-optimal amounts of dietary omega-3 fatty acids leads to poor health outcomes. This needs to be addressed. Once again the solution lies in the shopping basket or at the dining table. Reducing the amount of processed oils in the diet and eating more omega-3s, say, oily fish like sardines, salmon and mackerel, or walnuts, flaxseed and chia seed. Perhaps taking an omega 3 (DHA, EPA) supplement, from fish or algae. Note, fish do not create omega-3 fats, they come from algae at the bottom of the food chain and are concentrated as the move up the chain.


For further reading, please see the list of related studies at bottom of page.

 

Where Does the Evidence Lead Us?


When we look at the evidence, across multiple trials and studies on human health outcomes (as opposed to studies on mice or in vitro) there is little evidence to show that seed oils are harmful. As is often the case in nutrition science, some small scale studies show conflicting results so it is worthwhile to see what the totality of the evidence shows.


Below we review key studies on the consumption of seed oils and various chronic illness outcomes. We will quote directly from the studies and provide the sources immediately after each quote.


Cardiovascular Disease

PUFAs are abundant in many fat-rich foods, especially vegetable oils and fish, and have attracted wide attention due to their important physiological functions in the human body. Our study found that dietary intake of PUFAs such as ODTA, ALA, DPA, and ODA were significantly associated with different CVDs. Higher level of dietary ALA intake was related to lower risks of all-cause mortality, CVD-specific mortality, and other cause-specific mortality. In addition, AA and DPA were also seemed to be benefit to cardiovascular health.


ALA is one of the most common essential PUFAs available in plant sources, which has been given much attention to the health benefits of it. A systematic review and meta-analysis basing on 41 prospective cohort studies, containing 1,197,564 participants showed that dietary ALA intake was linked decreased risks of all-causes mortality and CVD-specific mortality. A systematic assessment study also indicated increasing ALA slightly lowered risk of cardiovascular events. Similarly, in the current study, higher dietary ALA intake was associated with both lower risk of all-cause mortality and CVD-specific mortality.” Source: Zhong N, Han P, Wang Y, Zheng C. Associations of polyunsaturated fatty acids with cardiovascular disease and mortality: a study of NHANES database in 2003-2018. BMC Endocr Disord. 2023 Aug 29;23(1):185. doi: 10.1186/s12902-023-01412-4. PMID: 37644429; PMCID: PMC10464142.


“Increasing polyunsaturated fatty acid (PUFA) intake probably makes little or no difference (neither benefit nor harm) to all‐cause mortality and probably slightly reduces the risk of coronary heart disease events and cardiovascular disease events (all moderate‐quality evidence). Increased PUFA intake may slightly reduce risk of coronary heart disease mortality and stroke (although for stroke the confidence intervals include important harm), but may have little or no effect on cardiovascular disease mortality (all low‐quality evidence). Increasing PUFA does reduce total cholesterol, probably reduces triglyceride, probably has little or no effect on high‐density lipoprotein (HDL) or low‐density lipoprotein (LDL) and probably increases body weight.


This suggests that increasing PUFA intake may have beneficial effects on risk of cardiovascular disease events, coronary heart disease mortality, coronary heart disease events and stroke. The mechanism may be via reduction of total cholesterol and triglyceride. However increasing PUFA will probably lead to slight body weight increase.” Source: Abdelhamid AS, Martin N, Bridges C, Brainard JS, Wang X, Brown TJ, Hanson S, Jimoh OF, Ajabnoor SM, Deane KH, Song F, Hooper L. Polyunsaturated fatty acids for the primary and secondary prevention of cardiovascular disease. Cochrane Database Syst Rev. 2018 Jul 18;7(7):CD012345. doi: 10.1002/14651858.CD012345.pub2. Update in: Cochrane Database Syst Rev. 2018 Nov 27;11:CD012345. PMID: 30019767; PMCID: PMC6513571.

 
Type 2 Diabetes

“In the present systematic review and meta-analysis of prospective cohort studies, we found an inverse association between dietary intake and biomarkers of linoleic acid (LA) and the risk of type 2 diabetes (T2DM). The magnitude of associations, based on the dose-response analysis, revealed that each 5% increment in energy from LA intake was associated with a 10% lower risk of T2DM, while a 15% reduction in diabetes risk was observed per SD increase in biomarker levels of LA. There was also evidence of a nonlinear association between LA intake and diabetes, with the lowest risk at highest intakes. The association with biomarker levels of LA was robust, as it persisted in sensitivity analyses; however, this was not the case with dietary LA intakes because with exclusion of one study at a time, the association with dietary LA became nonsignificant. This is the first meta-analysis to investigate both dietary intake and biomarkers of LA in relation to the incidence of T2DM.


The present meta-analysis of 31 prospective cohort studies showed that high intake of dietary LA and elevated concentrations of LA in the body were both significantly associated with a lower risk of T2DM. In addition, the association between LA and reduced diabetes risk was significant at an intake of 5.5–7.0% of energy from LA. Although further studies are needed before these findings can be considered conclusive, these findings suggest that dietary recommendations to increase LA intake may reduce the risk of diabetes.Mousavi SM, Jalilpiran Y, Karimi E, Aune D, Larijani B, Mozaffarian D, Willett WC, Esmaillzadeh A. Source: Dietary Intake of Linoleic Acid, Its Concentrations, and the Risk of Type 2 Diabetes: A Systematic Review and Dose-Response Meta-analysis of Prospective Cohort Studies. Diabetes Care. 2021 Sep;44(9):2173-2181. doi: 10.2337/dc21-0438. Epub 2021 Aug 20. PMID: 34417277.


“These findings help inform dietary guidance on macronutrients to influence metabolic health. Currently, major organizations recommend that saturated fatty acids (SFA) be replaced with MUFA or PUFA, largely to improve lipid profiles rather than glucose-insulin metrics, for the primary and secondary prevention of diabetes. Our investigation of trials with relatively short average duration (28 d) suggests that consuming more unsaturated fats (MUFA, PUFA) in place of either carbohydrate or SFA may improve HbA1C and HOMA-IR; and that focusing on PUFA in particular may have additional benefits on insulin secretion capacity. The comparatively similar effects of SFA versus carbohydrate on glucose-insulin homeostasis are consistent with their similar overall associations with both incident diabetes and cardiovascular events. Translated to foods, these finding support increased consumption of vegetable oils and spreads, nuts, fish, and vegetables rich in unsaturated fats (e.g., avocado), in place of either animal fats or refined grains, starches, and sugars.


In conclusion, this systematic review and meta-analysis provides novel quantitative evidence for effects of major dietary fats and carbohydrate on glucose-insulin homeostasis. The results support guidelines to increase MUFA intake to improve glycaemia and insulin resistance, with possibly stronger effects among patients with type 2 diabetes, and to increase PUFA intake in the general population to improve long-term glycaemic control, insulin resistance, and insulin secretion capacity, in place of saturated fatty acids or carbohydrate. These findings help inform public health and clinical dietary guidelines to improve metabolic health.” Source: Imamura F, Micha R, Wu JH, de Oliveira Otto MC, Otite FO, Abioye AI, Mozaffarian D. Effects of Saturated Fat, Polyunsaturated Fat, Monounsaturated Fat, and Carbohydrate on Glucose-Insulin Homeostasis: A Systematic Review and Meta-analysis of Randomised Controlled Feeding Trials. PLoS Med. 2016 Jul 19;13(7):e1002087. doi: 10.1371/journal.pmed.1002087. PMID: 27434027; PMCID: PMC4951141.

 

So where does this leave us? Are seed oils “toxic” or "poisonous"? A large body of research, in humans and our health outcomes, shows that seed oils containing MUFAs and PUFAs lower cholesterol, improve cardiovascular heart health and contribute to better blood sugar control. There is no evidence of long-term harm from the extraction process.


  • If you prefer to avoid seed soils, by all means do so. For home cooking use olive oil, rich in polyphenols, or avocado oil.

  • Minimise consumption of processed and fast foods; focus on a whole food diet.

  • For general health and weight management, don’t consume any oil or fat in quantity.

  • When frying at home, choose an oil with a high smoke point. Consider avocado, peanut, canola, sunflower or sesame oil. Avoid burning (smoking) or reusing oil when you cook.

  • Include nuts, seeds in your diet. These sources of oils are rich in health promoting polyphenols, antioxidant compounds, micronutrients and fibre.

  • Include omega 3 rich fatty fish into your diet, consider using an omega 3 supplement.


And finally, maintain a healthy weight, exercise and destress. These are the pillars of health.


Stay Healthy,


Alastair

 

Achieve Your Health Goals


Your health, physical – mental – social - is complex and affected by multiple factors within and outside of your control. Our consults and programmes address the whole person, the root causes of ill health and maximising your health, performance & vitality.


  • Take the first step. Contact us to arrange an introductory call, to discuss how we can support your journey to health. We are based in Singapore and work with clients globally.

  • Book a Whole Health Consult to assess, identify and prioritise key factors (known and unknown) that affect your health. And receive personalised recommendations on how to address them.

  • Want to put recommendations into action? Learn more about our programmes for individuals or teams.

 

Related Studies


Kapoor B, Kapoor D, Gautam S, Singh R, Bhardwaj S. Dietary Polyunsaturated Fatty Acids (PUFAs): Uses and Potential Health Benefits. Curr Nutr Rep. 2021 Sep;10(3):232-242. doi: 10.1007/s13668-021-00363-3. Epub 2021 Jul 13. PMID: 34255301.


Hooper L, Martin N, Jimoh OF, Kirk C, Foster E, Abdelhamid AS. Reduction in saturated fat intake for cardiovascular disease. Cochrane Database Syst Rev. 2020 Aug 21;8(8):CD011737. doi: 10.1002/14651858.CD011737.pub3. PMID: 32827219; PMCID: PMC8092457.


Marklund M, Wu JHY, Imamura F, Del Gobbo LC, Fretts A, de Goede J, Shi P, Tintle N, Wennberg M, Aslibekyan S, Chen TA, de Oliveira Otto MC, Hirakawa Y, Eriksen HH, Kröger J, Laguzzi F, Lankinen M, Murphy RA, Prem K, Samieri C, Virtanen J, Wood AC, Wong K, Yang WS, Zhou X, Baylin A, Boer JMA, Brouwer IA, Campos H, Chaves PHM, Chien KL, de Faire U, Djoussé L, Eiriksdottir G, El-Abbadi N, Forouhi NG, Michael Gaziano J, Geleijnse JM, Gigante B, Giles G, Guallar E, Gudnason V, Harris T, Harris WS, Helmer C, Hellenius ML, Hodge A, Hu FB, Jacques PF, Jansson JH, Kalsbeek A, Khaw KT, Koh WP, Laakso M, Leander K, Lin HJ, Lind L, Luben R, Luo J, McKnight B, Mursu J, Ninomiya T, Overvad K, Psaty BM, Rimm E, Schulze MB, Siscovick D, Skjelbo Nielsen M, Smith AV, Steffen BT, Steffen L, Sun Q, Sundström J, Tsai MY, Tunstall-Pedoe H, Uusitupa MIJ, van Dam RM, Veenstra J, Monique Verschuren WM, Wareham N, Willett W, Woodward M, Yuan JM, Micha R, Lemaitre RN, Mozaffarian D, Risérus U; Cohorts for Heart and Aging Research in Genomic Epidemiology (CHARGE) Fatty Acids and Outcomes Research Consortium (FORCE). Biomarkers of Dietary Omega-6 Fatty Acids and Incident Cardiovascular Disease and Mortality. Circulation. 2019 May 21;139(21):2422-2436. doi: 10.1161/CIRCULATIONAHA.118.038908. PMID: 30971107; PMCID: PMC6582360.


Virtanen JK, Wu JHY, Voutilainen S, Mursu J, Tuomainen TP. Serum n-6 polyunsaturated fatty acids and risk of death: the Kuopio Ischaemic Heart Disease Risk Factor Study. Am J Clin Nutr. 2018 Mar 1;107(3):427-435. doi: 10.1093/ajcn/nqx063. PMID: 29566193.


Wu JHY, Marklund M, Imamura F, Tintle N, Ardisson Korat AV, de Goede J, Zhou X, Yang WS, de Oliveira Otto MC, Kröger J, Qureshi W, Virtanen JK, Bassett JK, Frazier-Wood AC, Lankinen M, Murphy RA, Rajaobelina K, Del Gobbo LC, Forouhi NG, Luben R, Khaw KT, Wareham N, Kalsbeek A, Veenstra J, Luo J, Hu FB, Lin HJ, Siscovick DS, Boeing H, Chen TA, Steffen B, Steffen LM, Hodge A, Eriksdottir G, Smith AV, Gudnason V, Harris TB, Brouwer IA, Berr C, Helmer C, Samieri C, Laakso M, Tsai MY, Giles GG, Nurmi T, Wagenknecht L, Schulze MB, Lemaitre RN, Chien KL, Soedamah-Muthu SS, Geleijnse JM, Sun Q, Harris WS, Lind L, Ärnlöv J, Riserus U, Micha R, Mozaffarian D; Cohorts for Heart and Aging Research in Genomic Epidemiology (CHARGE) Fatty Acids and Outcomes Research Consortium (FORCE). Omega-6 fatty acid biomarkers and incident type 2 diabetes: pooled analysis of individual-level data for 39 740 adults from 20 pros Lancet Diabetes Endocrinol. 2017 Dec;5(12):965-974. doi: 10.1016/S2213-8587(17)30307-8. Epub 2017 Oct 12. PMID: 29032079; PMCID: PMC6029721.


Su H, Liu R, Chang M, Huang J, Wang X. Dietary linoleic acid intake and blood inflammatory markers: a systematic review and meta-analysis of randomized controlled trials. Food Funct. 2017 Sep 20;8(9):3091-3103. doi: 10.1039/c7fo00433h. PMID: 28752873.


Wu JH, Lemaitre RN, King IB, Song X, Psaty BM, Siscovick DS, Mozaffarian D. Circulating omega-6 polyunsaturated fatty acids and total and cause-specific mortality: the Cardiovascular Health Study. Circulation. 2014 Oct 7;130(15):1245-53. doi: 10.1161/CIRCULATIONAHA.114.011590. Epub 2014 Aug 14. PMID: 25124495; PMCID: PMC4189990.


Krishnan S, Cooper JA. Effect of dietary fatty acid composition on substrate utilization and body weight maintenance in humans. Eur J Nutr. 2014 Apr;53(3):691-710. doi: 10.1007/s00394-013-0638-z. Epub 2013 Dec 22. PMID: 24363161.


Farvid MS, Ding M, Pan A, Sun Q, Chiuve SE, Steffen LM, Willett WC, Hu FB. Dietary linoleic acid and risk of coronary heart disease: a systematic review and meta-analysis of prospective cohort studies. Circulation. 2014 Oct 28;130(18):1568-78. doi: 10.1161/CIRCULATIONAHA.114.010236. Epub 2014 Aug 26. PMID: 25161045; PMCID: PMC4334131.


Bjermo H, Iggman D, Kullberg J, Dahlman I, Johansson L, Persson L, Berglund J, Pulkki K, Basu S, Uusitupa M, Rudling M, Arner P, Cederholm T, Ahlström H, Risérus U. Effects of n-6 PUFAs compared with SFAs on liver fat, lipoproteins, and inflammation in abdominal obesity: a randomized controlled trial. Am J Clin Nutr. 2012 May;95(5):1003-12. doi: 10.3945/ajcn.111.030114. Epub 2012 Apr 4. PMID: 22492369.


Rett BS, Whelan J. Increasing dietary linoleic acid does not increase tissue arachidonic acid content in adults consuming Western-type diets: a systematic review. Nutr Metab (Lond). 2011 Jun 10;8:36. doi: 10.1186/1743-7075-8-36. PMID: 21663641; PMCID: PMC3132704.


Mozaffarian D, Micha R, Wallace S. Effects on coronary heart disease of increasing polyunsaturated fat in place of saturated fat: a systematic review and meta-analysis of randomized controlled trials. PLoS Med. 2010 Mar 23;7(3):e1000252. doi: 10.1371/journal.pmed.1000252. PMID: 20351774; PMCID: PMC2843598.


Harris WS. The omega-6/omega-3 ratio and cardiovascular disease risk: uses and abuses. Curr Atheroscler Rep. 2006 Nov;8(6):453-9. doi: 10.1007/s11883-006-0019-7. PMID: 17045070.


0 comments
bottom of page