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Seed Oils. Health, Obesity and Inflammation. A Review of the Evidence.

Updated: Apr 4

seed oils health toxic obesity

Welcome to our long read on seed oils. In certain quarters, seed oils (also known as vegetable or plant 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 fattening, toxic and to be avoided at all costs? What effect do they have on chronic illness? What are the arguments, what does the evidence show us?

It is important to look at the totality of the evidence.

As ever, please talk to your doctor or medical practitioner most familiar with your medical history before implementing any changes in diet, exercise or lifestyle, especially if you are under treatment. Links to supporting studies and resources are found throughout the article and at the end of page.

 

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.


MUFAs  Monounsaturated Fatty Acids

There are various types of monounsaturated fats, with the most common being:

  • Oleic acid

  • Palmitoleic acids


Foods rich in MUFAs include avocado oil, olive oil, canola oil, peanut oil, safflower oil, and sesame oil.


Additionally, nuts such as almonds, cashews, hazelnuts, macadamia, and pistachios are high in MUFAs. They are also found in seeds such as pumpkin seeds, sunflower seeds, and flaxseeds, as well as in fatty fish like salmon, trout, and mackerel.


PUFAs Polyunsaturated Fatty Acids

There are two major classes of polyunsaturated fats: omega-6 and omega-3. These fatty acids are essential, meaning that the body cannot make them.


Omega-6 fatty acids are predominantly found in vegetable oils such as soybean, sunflower, corn, and cottonseed oils, as well as in nuts. Examples of omega-6 fatty acids include:


  • Linoleic acid (LA)

  • Arachidonic acid (ARA)

  • Gamma-linolenic acid (GLA)

  • Conjugated linoleic acid (CLA)

Omega-3 fatty acids include:


  • Alpha-linolenic acid (ALA), which is found in plants.

  • Eicosapentaenoic acid (EPA), Docosahexaenoic acid (DHA), and Docosapentaenoic acid (DPA). Primarily found in fatty fish such as salmon, mackerel, anchovy, and herring, as well as in some nuts and seeds.

 

The Arguments against Seed Oils


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, from plant or animal. Too much soybean oil causes weight gain, as does olive oil or lard. Over the last decades seed oil consumption has massively increased as has the consumption of processed foods. But are seed oils uniquely fattening or is this 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. In the West, since the 1960s, consumption of oils and corn sweeteners has dramatically increased. As have waistlines. 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 are 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 MUFAS and 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 (including many rats) and some recent studies suggest this, randomised control trials in humans have shown this not to be the case.

Meta-analyses on seed oils and inflammation

Wang Q, Liu R, Chang M, Zhang H, Jin Q, Wang X. Dietary oleic acid supplementation and blood inflammatory markers: a systematic review and meta-analysis of randomized controlled trials. Crit Rev Food Sci Nutr. 2022;62(9):2508-2525. doi: 10.1080/10408398.2020.1854673. Epub 2020 Dec 11. PMID: 33305589.


Kim HK, Kang EY, Go GW. Recent insights into dietary ω-6 fatty acid health implications using a systematic review. Food Sci Biotechnol. 2022 Aug 20;31(11):1365-1376. doi: 10.1007/s10068-022-01152-6. PMID: 36060573; PMCID: PMC9433510.


Calder PC, Campoy C, Eilander A, Fleith M, Forsyth S, Larsson PO, Schelkle B, Lohner S, Szommer A, van de Heijning BJM, Mensink RP. A systematic review of the effects of increasing arachidonic acid intake on PUFA status, metabolism and health-related outcomes in humans. Br J Nutr. 2019 Jun;121(11):1201-1214. doi: 10.1017/S0007114519000692. Epub 2019 May 27. PMID: 31130146.


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.


So what is going on here? Importantly regarding omega-3 intake, we know that sub-optimal amounts of dietary omega-3 fatty acids leads to poor health outcomes. Many people are not consuming enough omega-3. Therefore, it might not be so much the case that the omega-6:omega-3 ratio is too high, but rather that many people have a deficit of essential omega-3 that is driving poor health outcomes.


Once again the solution lies in the shopping basket or at the dining table. Consuming more omega-3s, say, from oily fish like sardines, salmon and mackerel, or walnuts, flaxseed and chia seed. Alternatively, taking an omega 3 (DHA, EPA) supplement, from fish or algal sources. And at the same time reducing seed oil rich foods - processed foods - will bring about health benefits.


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

Are seed oils inherently harmful to health, or, are they simply part of poor eating practices in modern times?
 

The Nutrition Transition


As societies transition from ancient to modern or rural to urban lifestyle, eating practices change. Typically, fibre is lost from the diet, sugar consumption increases, processed foods and modern cooking techniques such as deep frying become more commonplace. We can compare two different societies and cultures, the Tsimane tribe from Amazonian Bolivia and the US public.


The Tsimane

Anthropologists from UC Santa Barbara studied this in depth as part of the Tsimane Health and Life History Project, studying the diet of a hunter-gatherer Amazonian tribe (the Tsimane) in transition, as they increasingly accessed processed foods and modern eating practices. Over five years they saw the Tsimane’s total energy intake increase significantly, particularly the consumption of food additives:


  • Lard (animal fat)

  • Oils

  • Sugar


To quote from the study "Most dramatically, per-capita consumption of refined sugar and oil increased by 15.8 g/d and 4.9 mL/d over the 5.5-y study period." In caloric terms this equates to an additional 63.2 calories per day from sugar and 40.6 calories per day from oil.


Read an interview with one of the study's authors, Michael Gurven, here, and this is the study: Kraft TS, Stieglitz J, Trumble BC, Martin M, Kaplan H, Gurven M. Nutrition transition in 2 lowland Bolivian subsistence populations. Am J Clin Nutr. 2018 Dec 1;108(6):1183-1195. doi: 10.1093/ajcn/nqy250. PMID: 30383188; PMCID: PMC6290367.


The US Public

Looking to a society that has transitioned, we can see greater shifts in eating patterns. In the US, while refined sugar consumption has gone down, both seed oil and corn sweeteners (also 'sugar') have increased in parallel, as have 'corn products' - likely refined flour, effectively... sugar. Read more on the research from Pew Research here.



The average US citizen faces a triple whammy of high calorific oils/fats, sugars/sweeteners and processed carbohydrates that lead to weight gain and ill health. While beef consumption may have declined by a third, chicken (by no means a health food given modern rearing practices) has doubled, as has cheese consumption.


Are seed oils the root of America's obesity problem? Only partially. Perhaps America's processed food eating practices are more to blame.

 

Where Does the Evidence - in Humans - Lead Us?


When we look at the evidence below, across multiple trials and studies on human health outcomes (as opposed to short timeframe studies on mice or in vitro) there is little evidence to show that seed oils are harmful.


That is not to say that all studies are 100% consistent, studies can show different results so it is worthwhile to see what the totality of the evidence shows. We have focused on using systematic reviews and meta-analyses (studies that combine multiple studies) and where (but not always) possible, randomised control trials to reduce research bias.


As the science has evolved rapidly in this area, it is also worthwhile to focus on recent evidence.


Below we review key studies on the consumption of seed oils and various chronic illness outcomes. We quote directly from the studies and include more than one to provide a broader perspective. It's a bit of a read. Sources are provided immediately after each quote.


Condition - Chronic Illness

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.

Cognitive Health

Type 2 Diabetes

Weight Gain

 

In Summary


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, contribute to better blood sugar control and even lead to better weight control than diets high in saturated fats. We can also see that overconsumption of fats, oils - from whatever the source, seed oil or not - is inherently fattening and harmful.


Seed oils don't need to be demonised, just used with care and recognised where they can be found. So what can we do when it comes to oils, from plant or animal?


  • Removing processed and fried foods leads to better weight and health outcomes. Minimise consumption of processed and fast foods; chips, crisps, buns, cakes and starchy delights. Save these for an occasional treat!

  • Focus on a whole food (or 'minimally processed' food) diet. You can be plant forward without giving up meat or fish.

  • For general health and weight management, don’t consume any oil or fat in quantity. Using a seed oil to fry or a small knob of butter to flavour your vegetables is not going to harm you.

  • 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.

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


Ultimately it would appear to be wrong to demonise seed oils and more appropriate to focus on our broader eating patterns. To be more aware of 'hidden' fats and oils, to remove or minimise processed (and especially ultra processed) foods and concentrate on enjoying whole foods in our daily eating choices.


Stay Healthy,


Alastair

 

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Related Studies


Lee SH, Zhao L, Park S, Moore LV, Hamner HC, Galuska DA, Blanck HM. High Added Sugars Intake among US Adults: Characteristics, Eating Occasions, and Top Sources, 2015-2018. Nutrients. 2023 Jan 4;15(2):265. doi: 10.3390/nu15020265. PMID: 36678136; PMCID: PMC9867287.


Wang Q, Zhang H, Jin Q, Wang X. Effects of Dietary Linoleic Acid on Blood Lipid Profiles: A Systematic Review and Meta-Analysis of 40 Randomized Controlled Trials. Foods. 2023 May 25;12(11):2129. doi: 10.3390/foods12112129. PMID: 37297374; PMCID: PMC10253160.


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.


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.


Zhao JV, Schooling CM. Effect of linoleic acid on ischemic heart disease and its risk factors: a Mendelian randomization study. BMC Med. 2019 Mar 14;17(1):61. doi: 10.1186/s12916-019-1293-x. PMID: 30866921; PMCID: PMC6417131.


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.


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