The Hidden Fats: How Eight Industrial Seed Oils Took Over Your Food Supply

Eight industrially produced oils now sit behind most packaged food, most restaurant fryers and most ready meals. This piece explains what they are, how they differ, what hydrogenation actually does, and what the evidence can and cannot support about their health effects. It draws on the supply-chain argument made in Dr Cate Shanahan's Dark Calories (Hachette Go, 2024), and grades every claim honestly.

THE SUPPLY CHAIN: CHEAP, HIGH-VOLUME FATS

"Seed oils" is a food-supply category, not a scientific term for one uniform oil. It usually means oils made at industrial scale from soybean, rapeseed (canola), sunflower, safflower, corn, cottonseed, grapeseed or rice bran.

Seeds may be mechanically pressed, extracted with a solvent such as hexane, or processed using both methods. The crude oil is commonly refined, bleached and deodorised to make it neutral in taste and smell, shelf-stable and suitable for large-scale food manufacturing.

That supply chain made these oils economical and widely available. They appear in packaged foods, restaurant fryers and ready meals, including dressings, mayonnaise, margarine, baked goods, crisps, fried takeaway, protein bars and plant-based meals.

Cold-pressed oils from the same seeds are different in processing: they are made without the same intensive refining steps and may retain more flavour and minor compounds. "Less refined" does not automatically mean healthier or better for every cooking use.

The oil's fatty-acid profile varies by crop, seed variety and processing.

FOUR COMMON OILS, DIFFERENT PROFILES

The figures below are broad ranges reported for oil composition; crop variety and production methods change the exact profile. LA means linoleic acid, an omega-6 fat; ALA means alpha-linolenic acid, an omega-3 fat (Abrante-Pascual et al., Foods, 2024).

• Rapeseed/canola: comparatively more ALA and less LA than soybean; generally low in saturated fat. Refined versions commonly have a high smoke point, often around 200°C or above, and are used for cooking, frying and food manufacturing.
• Soybean: high in LA; also contains ALA. Refined versions commonly have a high smoke point. Widely used in packaged foods, cooking oils and commercial frying.
• Sunflower: conventional varieties are often high in LA and low in ALA. Refined versions commonly have a high smoke point; used in cooking, frying and packaged foods.
• Corn: usually high in LA, with little ALA. Refined versions commonly have a high smoke point; used in cooking, frying and food production.

Smoke-point figures are approximate, vary by refining and oil condition, and do not tell the whole story about heat stability. Oils higher in polyunsaturated fats can be more prone to oxidation during prolonged or repeated heating. High-oleic sunflower and canola varieties contain more oleic acid and less LA than conventional versions, bringing their broad fatty-acid profile closer to olive oil. This is a distinction about composition and use, not proof that every seed-derived product is harmful.

The main oils in the food supply include soybean, rapeseed/canola, sunflower, corn, cottonseed and grapeseed. Palm oil is extracted from the fruit of the oil palm, not its seed, but belongs in the same supply-chain discussion because it is widely used in manufactured foods.

HYDROGENATION: WHAT IT DOES, AND WHAT IT DOESN'T

Hydrogenation adds hydrogen to some or all of an oil's double bonds, making the fat more solid and shelf-stable.

Partial hydrogenation can create industrial trans fats. These fats worsen blood lipids by raising LDL cholesterol and lowering HDL cholesterol. Trans fats are strongly linked to cardiovascular risk; they have been largely phased out in the UK and many other countries. Mensink and colleagues' meta-analysis of 60 controlled trials found particularly unfavourable blood-lipid effects when trans fats were compared with cis-unsaturated fats (American Journal of Clinical Nutrition, 2003).

Full hydrogenation saturates the fat more completely. It does not create trans fats in the same way as partial hydrogenation, but produces a fully saturated fat. Those processes are not interchangeable.

Hydrogenation is not the only issue associated with the word "toxins". Oils exposed to repeated high-temperature frying can form oxidation products, including aldehydes such as 4-HNE. These products have been observed in chemical and laboratory research; their implications for human health depend on exposure and are not fully established. That is a more accurate concern than saying all seed oils become toxins during ordinary cooking (Guillén & Uriarte, Food Chemistry, 2012).

Repeated heating can degrade oil and produce oxidation products; this is different from hydrogenation.

TEN HEALTH CLAIMS: WHAT THE EVIDENCE CAN SUPPORT

The following are claims or mechanisms discussed in research, not ten established harms of eating seed oils. Evidence is strongest for industrial trans fats. The claim that omega-6 fats themselves cause inflammation is contested: human trial reviews find linoleic acid to be neutral to beneficial on many inflammatory markers when it is not oxidised (Johnson & Fritsche, J Acad Nutr Diet, 2012).

1. Unfavourable LDL and HDL changes: industrial trans fats raise LDL and lower HDL; strong evidence.
2. Higher coronary risk: industrial trans-fat intake is linked to coronary heart disease; strong evidence.
3. Oxidised lipids: lipid peroxidation products can form when oils are heavily heated; human health effects from usual dietary exposure remain uncertain.
4. Aldehydes such as 4-HNE: these can form in heated oils and are linked to oxidative stress in cell and animal models; human disease effects are not settled.
5. Inflammatory signalling: a high omega-6-to-omega-3 balance may shift eicosanoid signalling; proposed mechanism, contested in human trials.
6. Cell-membrane composition: LA can become part of cell membranes and affect their fatty-acid profile; the health significance is under study.
7. Fat-tissue composition: dietary LA can be incorporated into adipose tissue; whether this causes harm is not established.
8. Excess energy intake: oils are calorie-dense, and foods containing them can contribute to excess energy intake; this is a food-pattern issue, not a unique seed-oil effect.
9. Displacing omega-3-rich foods: relying heavily on foods made with seed oils could leave less room for omega-3-rich whole foods; the concern is dietary balance.
10. Gut-microbiome effects: possible effects of high-linoleate diets have been reported in animal models; human relevance remains uncertain.

For a broader look at how food choices fit into healthspan after 40, the useful question is not "Is this oil toxic?" It is: what is the oil, how was it processed and heated, what food is it part of, and what is it replacing?

A PRACTICAL HEALTHSPAN CHECK

Read ingredient lists, notice how often fried and packaged foods appear in the week, and avoid repeatedly heating cooking oil until it smokes. Where possible, build meals around minimally processed foods, vegetables and fibre. See the guide to fibre and healthspan and what research says about butyrate, fibre and biological age.

The point is not to fear one ingredient or declare every oil harmful. It is to understand the supply chain, distinguish fresh oil from repeatedly heated oil, and make deliberate choices that support long-term healthspan. Anyone considering a significant diet change, especially with a health condition or medication, should discuss it with a GP.

THE DARK CALORIES CONNECTION

Dr Cate Shanahan's book Dark Calories (Hachette Go, 2024) makes the supply-chain argument in full. A Cornell-trained biochemist and family physician, Shanahan names the same eight oils this article describes, corn, canola, cottonseed, soy, sunflower, safflower, rice bran and grapeseed, as the "Hateful Eight", and argues that their instability is the mechanism behind many chronic diseases: uncontrollable hunger, inflammatory fat buildup, disrupted brain energy, intracellular oxidative stress, and gut inflammation. A peer-reviewed review in Family Medicine (2025) summarises those claims and notes, as this article does, that the strongest evidence sits with trans fats and oxidation products rather than with linoleic acid itself.

DEEP DIVE: SOURCES

• Abrante-Pascual, S., Nieva-Echevarría, B. and Goicoechea-Oses, E. "Vegetable Oils and Their Use for Frying: A Review of Their Compositional Differences and Degradation." Foods, 2024; 13(24):4186.
• Mensink, R. P. et al. "Effects of dietary fatty acids and carbohydrates on the ratio of serum total to HDL cholesterol and on serum lipids and apolipoproteins: a meta-analysis of 60 controlled trials." American Journal of Clinical Nutrition, 2003; 77(5):1146-1155.
• Johnson, G. H. and Fritsche, K. "Effect of Dietary Linoleic Acid on Markers of Inflammation in Healthy Persons: A Systematic Review of Randomized Controlled Trials." Journal of the Academy of Nutrition and Dietetics, 2012; 112(7):1029-1041.
• Cordain, L. et al. "Origins and evolution of the Western diet: health implications for the 21st century." American Journal of Clinical Nutrition, 2005; 81(2):341-354.
• Guillen, M. D. and Uriarte, P. S. "Aldehydes contained in edible oils of a very different nature after prolonged heating at frying temperature." Food Chemistry, 2012; 131:915-926.
• Scherger, J. E. "Dark Calories: How Vegetable Oils Destroy Our Health and How We Can Get It Back" (book review). Family Medicine, 2025; 57(4):314-315.

SUMMARY: THE SHORT VERSION

Seed oils are industrially produced fats extracted from oil-rich seeds and used widely in packaged foods and commercial cooking. Their fatty-acid profiles differ: rapeseed/canola contains more ALA than soybean, while sunflower and corn are often high in LA. Partial hydrogenation creates harmful trans fats; repeated high-heat use can produce oxidation products. Claims that ordinary linoleic acid causes inflammation are contested in human trials. Look at the oil, the processing, the heating and the overall food pattern.