Are seed oils bad for you? The evidence explained

By the DieticiansWorldwide.com editorial team · 17 October 2026 · Public health & global nutrition

What counts as a seed oil

"Seed oil" is a social media term rather than a scientific one. It usually means oils pressed or extracted from the seeds of plants, rather than from the flesh of a fruit such as an olive or avocado. The 2025 US Make Our Children Healthy Again Assessment names "soybean, corn, safflower, sunflower, cottonseed, and canola" as the refined seed oils it is concerned about (MAHA Assessment, May 2025, PDF). Canola is the North American name for a type of rapeseed oil. Grapeseed, rice bran and sesame oil are also seed oils.

What these oils share is a high proportion of polyunsaturated fat, especially linoleic acid, the main omega-6 fatty acid. Linoleic acid is an essential fatty acid: your body cannot make it, so you need some from food. Rapeseed oil is different from the others in having more monounsaturated fat and a useful amount of the plant omega-3, alpha-linolenic acid.

Why seed oils became controversial

The case against seed oils, as made online and in some policy documents, usually rests on four claims:

1. They are "industrial". Most are refined using heat and solvents, which critics say strips nutrients and creates harmful compounds.
2. They are "inflammatory". Linoleic acid can be converted in the body to arachidonic acid, a building block for some inflammatory signalling molecules.
3. They upset the omega-6 to omega-3 ratio. Modern diets contain far more omega-6 than omega-3, which some argue drives chronic disease.
4. They replaced traditional fats. Diets have shifted from butter and lard to vegetable oils over the same period that obesity and diabetes rose.

The MAHA Assessment repeats several of these points. It says US dietary fats shifted from butter and lard to "industrial fats from refined seed oils", that "industrial refining reduces micronutrients, such as vitamin E and phytosterols", and that these oils contribute to "an imbalanced omega-6/omega-3 ratio, a topic of ongoing research for its potential role in inflammation". The January 2026 Dietary Guidelines then listed "butter or beef tallow" as cooking options alongside olive oil, and the government's realfood.gov site describes real food as prepared "without added sugars, industrial oils, artificial flavors, or preservatives". Our explainer on the new food pyramid covers that guidance, and our guide to the MAHA report covers the wider programme.

These are testable claims, and each has been studied.

What the heart disease evidence shows

The main body of evidence

The American Heart Association's 2017 presidential advisory reviewed the trials and cohort studies on dietary fat. It concluded that "randomized controlled trials that lowered intake of dietary saturated fat and replaced it with polyunsaturated vegetable oil reduced CVD by ≈30%, similar to the reduction achieved by statin treatment", and that replacing saturated fat with refined carbohydrates and sugars did not reduce cardiovascular disease (Sacks and colleagues, Circulation 2017).

Cochrane reviews, which apply strict rules on trial quality, reach a more cautious version of the same answer. A 2020 review of 15 randomised trials with about 59,000 participants found that reducing saturated fat cut combined cardiovascular events by 21%, with moderate-quality evidence, but made little or no difference to deaths from all causes. Subgroup analysis did not show a significant difference between replacing saturated fat with polyunsaturated fat or with carbohydrate (Hooper and colleagues, Cochrane 2020). A 2018 Cochrane review focused on omega-6 fats found low-quality evidence that increasing them "may make little or no difference" to deaths or cardiovascular events (Hooper and colleagues, Cochrane 2018). Neither review found evidence that omega-6 fats cause harm.

Studies of blood levels are another line of evidence, because they do not rely on people remembering what they ate. A pooled analysis of 30 prospective studies in 13 countries, with 68,659 participants, found that people with higher linoleic acid levels in their blood or fat tissue had a lower risk of cardiovascular disease, cardiovascular death and ischaemic stroke (Marklund and colleagues, Circulation 2019).

The dissenting evidence

The other side of the debate rests largely on two old trials whose full data were recovered decades later by researchers at the US National Institutes of Health.

• Sydney Diet Heart Study (1966–73). Men who had recently had a heart attack were advised to replace saturated fat with safflower oil and safflower margarine. The recovered data showed higher death rates in the intervention group: 17.6% against 11.8% for all causes (Ramsden and colleagues, BMJ 2013).
• Minnesota Coronary Experiment (1968–73). In a nursing home and six state mental hospitals, replacing saturated fat with corn oil lowered cholesterol, but the recovered data did not show fewer deaths. The authors concluded that incomplete publication "has contributed to overestimation of the benefits" of replacing saturated fat with linoleic acid-rich oils (Ramsden and colleagues, BMJ 2016).

These analyses are often shared online as proof that seed oils are harmful. They are important, and they show why full trial data matter. But they have limits. The Minnesota study took place in institutions, and of its 9,423 participants only 2,355 were on the study diets for a year or more. The Sydney study tested advice in men who had already had a heart attack. And both are two trials among many. The larger reviews above include this evidence and still do not find harm.

Do seed oils cause inflammation?

This is the most common claim, and it has been tested directly. A systematic review of 15 randomised controlled trials in healthy people found that adding linoleic acid to the diet did not raise a wide range of inflammatory markers, including C-reactive protein, fibrinogen, cytokines and tumour necrosis factor-alpha. The authors concluded there was "virtually no evidence" from such trials that linoleic acid increases inflammatory markers (Johnson and Fritsche, Journal of the Academy of Nutrition and Dietetics 2012).

In April 2026, a scoping narrative review in Critical Reviews in Food Science and Nutrition revisited the whole controversy. It concluded that "data overwhelmingly support the safety and health benefits of seed oils and LA [linoleic acid]", that claims that linoleic acid "increases inflammation and oxidative stress, and therefore raises CVD risk, were found to be without foundation", and that concerns about harmful substances created during manufacture "were also found to lack support" (Nagra and colleagues, 2026). A narrative review is a weaker design than a systematic review with meta-analysis, because authors choose and weigh studies themselves, but its conclusions match those of the larger systematic reviews.

The omega-6 to omega-3 ratio argument has a simpler answer. Because the trials above show no harm from linoleic acid, many nutrition scientists argue that the more useful fix is to raise omega-3 rather than cut omega-6. Eating oily fish regularly, plus walnuts, flaxseed or rapeseed oil, raises omega-3 without cutting out other fats; our guide to omega-3 supplements explains when supplements are and are not useful.

Refining, solvents and "industrial" oils

Most supermarket seed oils are refined. In a typical process, seeds are crushed, the oil is extracted with the help of a food-grade solvent, and the oil is then degummed, neutralised, bleached and deodorised to give a neutral taste, a light colour and a long shelf life. Cold-pressed and virgin oils skip most of these steps and keep more of their natural flavour and colour.

Two different questions get mixed up here. The first is whether refining removes useful compounds. It can: the MAHA Assessment's point that refining reduces vitamin E and phytosterols is reasonable, and it is one reason extra virgin olive oil, which is unrefined, is often preferred. The second is whether refining adds anything harmful. That is the claim the 2026 review examined, concluding that worries about harmful substances created during manufacture lack support.

So "industrial" describes how an oil is made, not what it does in the body. Butter and tallow are also produced at industrial scale; the health difference between fats comes mainly from their fatty acid make-up, which is where the evidence points.

Do seed oils make you gain weight?

Not more than any other fat. The NHS notes that all types of fat are high in energy: a gram of fat, "whether it's saturated or unsaturated", provides 9 kcal, against 4 kcal for carbohydrate and protein. Swapping sunflower oil for butter or tallow therefore does not cut calories, and pouring generous amounts of any oil into the pan adds up quickly.

What does link seed oils to weight gain is the food they come in. Fried takeaways, crisps, pastries and biscuits are energy dense and easy to overeat, and those foods would be just as easy to overeat if they were fried in lard. If weight is your concern, the amount of fat and the type of food matter far more than which oil is in the bottle. Measuring oil with a spoon rather than pouring it straight from the bottle is a simple habit that helps.

Seed oils, beef tallow and butter compared

Beef tallow searches have risen as people look for an alternative, so it helps to see what is actually in each fat. The table uses USDA FoodData Central values per 100 g of product (USDA FoodData Central). Butter contains water, so its fat figures are lower than those for pure fats.

The key difference is saturated fat. Tallow and butter are about half saturated fat; rapeseed oil is under a tenth. Saturated fat raises LDL cholesterol, a cause of atherosclerosis. In a randomised crossover trial in 47 healthy adults, swapping part of the habitual diet for butter raised total and LDL cholesterol more than the same amount of refined olive oil (Engel and Tholstrup, American Journal of Clinical Nutrition 2015). Preventive cardiologists writing for the American College of Cardiology in January 2026 said that "evidence from feeding trials and/or cohort studies does not support promotion of butter or beef tallow" (American College of Cardiology).

National guidance is consistent. The NHS advises that men should not eat more than 30 g and women not more than 20 g of saturated fat a day, and recommends replacing some saturated fat with unsaturated fat (NHS, fat: the facts). The World Health Organization's 2023 guideline keeps saturated fat at no more than 10% of energy and names polyunsaturated fats among the preferred replacements (WHO). Even the 2025–2030 US guidelines keep the 10% limit. On a 2,000-calorie day, 10% is about 22 g of saturated fat, which, using the table above, is the amount in roughly 45 g of tallow.

Where the critics have a point

The seed oil debate is often presented as all or nothing, but some concerns overlap with mainstream advice.

• Ultra-processed food. Much of the seed oil people eat comes inside crisps, fried takeaway food, biscuits and ready meals. Eating fewer of those products is good advice whatever you think about the oil. Our guide to ultra-processed foods explains the evidence. Interestingly, NOVA, the classification behind the ultra-processed food research, puts vegetable oils in group 2, processed culinary ingredients, alongside butter and lard, not with ultra-processed foods.
• Deep frying. The 2025–2030 US guidelines suggest swapping deep-fried cooking for baking, broiling, roasting, stir-frying or grilling, which most dietitians would agree with whichever fat you use.
• Food quality, not single nutrients. Brazil's national food guide advises using oils, fats, salt and sugar in small amounts to cook fresh food, rather than ranking individual fats; our Brazil country guide explains that approach.

How to choose and use cooking oils

A practical approach that fits the evidence and every major guideline:

1. Make olive or rapeseed oil your everyday oils. Both are low in saturated fat; rapeseed adds plant omega-3. Extra virgin olive oil is the central fat of the Mediterranean diet.
2. Use sunflower, soybean or corn oil when you need a neutral taste; they are not harmful in normal amounts.
3. Keep butter, ghee, tallow and coconut oil for flavour, in small amounts.
4. Cook more, fry less. Roasting, grilling and stir-frying use little oil.
5. Store oil in a cool, dark cupboard and use it within its best-before date.
6. Look at your whole diet. Vegetables, pulses, whole grains, nuts and fish matter more than any single fat; see our anti-inflammatory diet guide for a pattern built around them.

To check how your current eating compares with an evidence-based pattern, try the free healthy eating check. Food costs matter too: advice to replace everyday cooking oils with more expensive alternatives can be a real burden for households under pressure, a theme in our article on food insecurity. Some health systems now pair cooking skills with clinical care, which our guide to food as medicine covers. For the US policy background, see our United States country guide. If you have high cholesterol or heart disease and want a plan that fits your tastes, you can book a session with a registered dietitian.

Get the evidence-based nutrition newsletter