Do Seed Oils Make You Fat? Separating Fact from Fiction

Alastair Hunt
May 3
9 min read

Updated: Aug 1

Part 2 of the Seed Oils Series

All fats are calorie dense, and seed oils are no exception. At approximately 9 calories per gram, fat provides more than double the energy per gram compared to carbohydrates or proteins. Whether fat comes from olive oil, butter, sunflower oil or tallow, overconsumption of calories - regardless of source - results in weight gain when it exceeds energy expenditure.

Seed oils are often implicated in discussions about obesity because of their widespread use in processed and ultra-processed foods. Products like crisps, fried foods, fast food burgers, pastries and ready meals often contain significant amounts of seed oils. However, it is important to separate cause from context. Seed oils in these foods are part of a broader nutritional pattern that includes high intakes of refined carbohydrates, added sugars and low fibre - a combination strongly associated with overeating and weight gain.

In the US, while refined sugar consumption has decreased over time, both seed oil (cooking oil, below) and sugar in other forms has increased. Read more on the research from Pew Research here.

The average US citizen (like many others) now faces a triple whammy of highly calorific oils/fats, sugars/sweeteners and processed carbohydrates that lead to weight gain and ill health.

Related to this topic, there is sometimes an argument that the US Dietary Recommendations have caused the obesity crisis although the data shows that US residents simply have not or not been able to follow them.

The CDC website states that "In 2019, 12.3% and 10.0% of surveyed adults met fruit and vegetable intake recommendations, respectively. Meeting fruit intake recommendations was highest among Hispanic adults (16.4%) and lowest among males (10.1%). Meeting vegetable intake recommendations was highest among adults aged ≥51 years (12.5%) and lowest among adults with low income (6.8%)." Source: link here

Modern studies increasingly highlight that ultra-processed foods - defined as formulations made mostly from substances extracted from foods plus additives - disrupt normal appetite control. A landmark 2019 randomised trial from the National Institutes of Health (Hall et al.) demonstrated that when participants were fed ultra-processed foods ad libitum, they consumed approximately 500 more calories per day and gained weight, compared to when they ate minimally processed foods. This occurred despite meals being matched for calories and macronutrients. Seed oils were present in the ultra-processed diet but were not identified as a primary cause of the increased intake.

See links to this and other supporting studies at Bottom of Page.

In isolation, consumption of seed oils does not inherently cause weight gain. Many intervention studies where diets high in polyunsaturated fats (from oils, nuts, seeds) replaced diets rich in saturated fats showed either weight neutrality or slight weight reductions. Meta-analyses confirm that higher unsaturated fat diets can improve cardiometabolic health without promoting obesity, especially when paired with higher intakes of whole plant foods.

Understanding energy balance remains key. The body gains fat when energy intake exceeds expenditure, not when a particular nutrient is consumed. Energy-dense foods (fats) require mindful portion control, but demonising a single type of oil misses the wider dietary environment driving modern obesity trends.

The causes of obesity are multifactorial. Genetics, hormonal factors, medications, sleep, stress and environmental conditions all influence body weight. Always discuss weight management strategies with a qualified health professional, particularly if managing a medical condition.

When it come to weight management, seed oils contribute calories but context - processed foods, overeating and dietary patterns - matters far more. For more studies on this subject area, scroll to bottom of page. 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.

Next... Part 3: Do Seed Oils Cause Inflammation? What the Evidence Really Says

Final Thoughts

While seed oils can be a divisive issue online, we are relatively agnostic when it come to diet and eating patterns. If you enjoy cooking with olive oil, a dab of butter, avocado oil or (as a treat) duck fat, we do too. In the same regard, we are not concerned about a splash of canola in our stir fry and we try to avoid ultra-processed foods as part of our daily eating.

Changing health for the better is about progress, not perfection, and even small changes can make a meaningful difference over time. For most people, improving health is about finding motivation and prioritising self-care - with an ultimate goal of taking action. If you want to take effective and targeted steps that fit into your unique lifestyle and circumstances, The Whole Health Practice is here to help.

Whether your interest is healthspan and longevity, to beat chronic illness or to enhance your mental health and well-being, our consultations and programs deliver results that are tailored to your needs.

Our foundational Whole Health Consult identifies and prioritises the key factors - known and unknown - that affect health and wellbeing. It provides targeted recommendations tailored to you, the individual, and your unique lifestyle.

Stay Healthy,

Alastair

Join us, or scroll down to contact us and learn more about our services:

LinkedIn, FaceBook, Instagram, Community, Articles and More

Achieve your Health Goals

Our 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.

Whole Health Consult

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

Coaching Programmes

Related Studies

Mohtashamian A, Mahabady M, Bagheri F, Barghchi H, Aminianfar A. Effects of canola oil on body weight and composition in adults: an updated systematic review and meta-analysis of 32 randomized controlled trials. Nutr J. 2025 Apr 8;24(1):55. doi: 10.1186/s12937-025-01117-5. PMID: 40200305; PMCID: PMC11980287.

"In the present study, supplementation with canola oil (CO) did not significantly alter the BW. However, based on the result from the subgroup analysis, CO supplementation significantly decreased body weight in parallel design studies, diabetic patients, people more than 50 years old, and studies with consumption of more than 30 gr canola per day. Unlike our results, a previously published meta-analysis demonstrated that CO supplementation could decrease BW in all participants. Based on our results, it seems there is a dose-dependent response to the consumption of CO. It seems that the weight loss effect of CO will appear in case of consumption of more than 30 g per day, in which we didn’t see any significant effect from CO supplementation in people who consumed less than 30 g of CO per day. In addition, diabetic patients and older people (> 50y) might take more advantage of supplementation with CO. Based on evidence saturated fatty acids are more fattening compared to unsaturated fatty acids. The type of dietary fatty acids and the appropriate omega-3 to omega-6 ratio are also effective in the amount of fat deposition in the body. It is noteworthy that CO is a rich source of essential unsaturated fatty acids such as omega-3 and −6 and also has a suitable ratio of omega-3 to omega-6 (1:2), which could explain its anti-obesity effects. In addition, special fatty acids such as MCTs (which are high in CO) could induce satiety more than long-chain fatty acids."

Raeisi-Dehkordi H, Amiri M, Humphries KH, Salehi-Abargouei A. The Effect of Canola Oil on Body Weight and Composition: A Systematic Review and Meta-Analysis of Randomized Controlled Clinical Trials. Adv Nutr. 2019 May 1;10(3):419-432. doi: 10.1093/advances/nmy108. PMID: 30809634; PMCID: PMC6520036.

"This systematic review and meta-analysis of controlled clinical trials revealed that canola oil intake significantly decreases body weight; however, canola oil intake did not significantly affect BMI, waist circumference, body fat, WHR, hip circumference, lean body mass, or android-to-gynoid ratio. Subgroup analyses affirmed that canola oil intake might decrease body weight in female participants but not male subjects; also its reducing effect was observed in nonfeeding trials, studies with a parallel design, studies which compared canola oil with SFAs, and those subjects with hyperlipidemia or T2DM. Furthermore, subgroup analysis showed that waist circumference was reduced when canola oil was compared with the usual diet and in studies that lasted for ≤4 wk."

Hall KD, Ayuketah A, Brychta R, Cai H, Cassimatis T, Chen KY, Chung ST, Costa E, Courville A, Darcey V, Fletcher LA, Forde CG, Gharib AM, Guo J, Howard R, Joseph PV, McGehee S, Ouwerkerk R, Raisinger K, Rozga I, Stagliano M, Walter M, Walter PJ, Yang S, Zhou M. Ultra-Processed Diets Cause Excess Calorie Intake and Weight Gain: An Inpatient Randomized Controlled Trial of Ad Libitum Food Intake. Cell Metab. 2019 Jul 2;30(1):67-77.e3. doi: 10.1016/j.cmet.2019.05.008. Epub 2019 May 16. Erratum in: Cell Metab. 2019 Jul 2;30(1):226. doi: 10.1016/j.cmet.2019.05.020. Erratum in: Cell Metab. 2020 Oct 6;32(4):690. doi: 10.1016/j.cmet.2020.08.014. PMID: 31105044; PMCID: PMC7946062.

"This systematic review and meta-analysis of controlled clinical trials revealed that canola oil (CO) intake significantly decreases body weight; however, CO intake did not significantly affect BMI, waist circumference, body fat, waist-to-hip ratio, hip circumference, lean body mass, or android-to-gynoid ratio. Subgroup analyses affirmed that CO intake might decrease body weight in female participants but not male subjects; also its reducing effect was observed in nonfeeding trials, studies with a parallel design, studies which compared CO with saturated fatty acids, and those subjects with hyperlipidemia or type 2 diabetes mellitus. Furthermore, subgroup analysis showed that waist circumference was reduced when CO was compared with the usual diet and in studies that lasted for ≤4 wk."

Brayner B, Kaur G, Keske MA, Perez-Cornago A, Piernas C, Livingstone KM. Dietary Patterns Characterized by Fat Type in Association with Obesity and Type 2 Diabetes: A Longitudinal Study of UK Biobank Participants. J Nutr. 2021 Nov 2;151(11):3570-3578. doi: 10.1093/jn/nxab275. PMID: 34522964.

"This study examined the longitudinal associations between dietary patterns characterized by fat type and incidence of obesity, abdominal obesity, and type 2 diabetes (T2D) in adults participating in the UK Biobank study. Our main findings were that the incidence of obesity and abdominal obesity after 6.3 y was slightly higher in participants consuming a dietary pattern positively correlated with saturated fatty acid intake (DP2) and negatively correlated with PUFA intake. However, neither of the identified dietary patterns were associated with incidence of T2D. Given the importance of considering nutrient intake within the context of the foods consumed, these findings support dietary guidelines for obesity prevention that consider the type of dietary fat as part of an overall dietary pattern."

"This systematic review and meta-analysis of controlled clinical trials revealed that Canola Oil (CO) intake significantly decreases BW; however, CO intake did not significantly affect BMI, waist circumference, body fat, WHR, HC, lean body mass, or A:G. Subgroup analyses affirmed that CO intake might decrease bodyweight in female participants but not male subjects; also its reducing effect was observed in nonfeeding trials, studies with a parallel design, studies which compared CO with saturated fatty acids, and those subjects with hyperlipidemia or type 2 diabetes. Furthermore, subgroup analysis showed that waist circumference, was reduced when CO was compared with the usual diet and in studies that lasted for ≤4 wk.

In conclusion, the current systematic review and meta-analysis demonstrated that CO consumption leads to a modest but significant reduction in BW, particularly when compared with SFAs. However, no significant effect was seen on the other body composition indexes. Additional, well-designed clinical trials of the effect of CO consumption on BW and body composition are still required to confirm these results."

Liu X, Li Y, Tobias DK, Wang DD, Manson JE, Willett WC, Hu FB. Changes in Types of Dietary Fats Influence Long-term Weight Change in US Women and Men. J Nutr. 2018 Nov 1;148(11):1821-1829. doi: 10.1093/jn/nxy183. PMID: 30247611; PMCID: PMC6209808.

"In 3 large prospective cohorts of US men and women, we found that types of dietary fats have diverging effects on long-term weight change. In particular, increasing consumption of n–6 and n–3 PUFAs as well as plant-based MUFA at the expense of carbohydrate was associated with less weight gain, whereas increasing intake of SFA and trans-fat was associated with greater weight gain."

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.

"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 SFA or carbohydrate. These findings help inform public health and clinical dietary guidelines to improve metabolic health."

Cardel M, Lemas DJ, Jackson KH, Friedman JE, Fernández JR. Higher Intake of PUFAs Is Associated with Lower Total and Visceral Adiposity and Higher Lean Mass in a Racially Diverse Sample of Children. J Nutr. 2015 Sep;145(9):2146-52. doi: 10.3945/jn.115.212365. Epub 2015 Aug 12. PMID: 26269238; PMCID: PMC4548162.

Rosqvist F, Iggman D, Kullberg J, Cedernaes J, Johansson HE, Larsson A, Johansson L, Ahlström H, Arner P, Dahlman I, Risérus U. Overfeeding polyunsaturated and saturated fat causes distinct effects on liver and visceral fat accumulation in humans. Diabetes. 2014 Jul;63(7):2356-68. doi: 10.2337/db13-1622. Epub 2014 Feb 18. PMID: 24550191.