Are Seed Oils Bad for You? What Your Blood Test Actually Shows

1. What seed oils actually are

“Seed oils” is an informal label for the vegetable oils pressed or extracted from seeds rather than fruits. The common ones are sunflower, rapeseed (sold as canola in North America), soybean, corn, grapeseed, cottonseed and safflower. What unites them is that they are high in polyunsaturated fat, and in particular in linoleic acid, the main dietary omega-6 fatty acid.

Linoleic acid is the molecule at the centre of the whole argument. The human body cannot make it, so a certain amount is essential and has to come from food. The contested question is not whether we need some, but whether the larger amounts that come with a modern diet are harmful. The online case rests on two specific accusations.

It is worth separating the oil from the food it usually arrives in. Linoleic acid in a home salad dressing and linoleic acid deep inside a packet of crisps are chemically the same, but the company it keeps is very different. Holding that distinction in mind makes the evidence far easier to read.

2. The case against them, in three claims

Strip away the volume and the seed-oil case comes down to three assertions. Two are testable against decades of research; the third is the one that survives scrutiny.

• “They cause inflammation.” The idea that omega-6 linoleic acid is metabolised into compounds that drive chronic inflammation throughout the body.
• “They are bad for your heart.” The claim that, being industrial and high in omega-6, they raise cardiovascular risk.
• “They oxidise and they hide in junk food.” The concern that repeatedly heated oils form harmful compounds, and that their main role in the diet is to make ultra-processed food cheap and palatable.

Take them in turn. The first two are where the fad and the evidence part company most sharply.

3. Claim one: do seed oils cause inflammation?

This is the load-bearing claim of the entire movement, and it is the one the human evidence supports least. The theory is that linoleic acid converts to arachidonic acid, a precursor to pro-inflammatory signalling molecules. On a whiteboard that looks alarming. In actual people eating actual food, the conversion is tightly regulated and the predicted effect largely fails to appear.

A systematic review of randomised controlled trials in the Journal of the Academy of Nutrition and Dietetics (2012) examined exactly this. It found that, in healthy adults, increasing dietary linoleic acid did not raise concentrations of the major inflammatory markers, including C-reactive protein. Some of the epidemiology pointed the other way, associating higher linoleic acid with lower inflammation.

The picture has only firmed up since. A 2026 clinician's review of cardiovascular nutrition controversies in JACC: Advances concluded that seed oils provide cardiometabolic benefits “without evidence of proinflammatory concerns in human studies”, noting that canola and sunflower oil reduced inflammation with effects similar to olive oil. Both the British Heart Foundation and HEART UK reach the same conclusion in plain language: the inflammation claim is not supported by the body of evidence.

4. Claim two: what seed oils do to your cholesterol

Here the evidence runs in the opposite direction to the fad. When polyunsaturated fats from seed oils replace saturated fats in the diet, LDL cholesterol tends to fall. That is one of the most reproducible findings in nutrition science, and it is the basis for the long-standing advice from the NHS to swap saturated fats for unsaturated ones.

The 2026 JACC: Advances review put numbers on it, reporting that canola oil lowered LDL cholesterol by roughly 6.4 mg/dL versus baseline, and summarising the wider literature bluntly: higher intake of omega-6 fats is associated with lower risk of heart disease, stroke, diabetes and death from all causes. A meta-analysis of prospective cohorts found that higher dietary linoleic acid was associated with a lower risk of coronary heart disease (Circulation, 2014).

This is the heart of the disconnect. The popular narrative treats seed oils as a cardiovascular threat; the cardiology literature treats them, in the context of replacing saturated fat, as part of the solution. None of which means the oils are magic, or that you should drink them. It means the specific charge that they wreck your lipids points the wrong way.

5. Where the worry has a grain of truth

A fad this sticky usually has a real grievance buried inside it, and seed oils are no exception. Two points are worth taking seriously, even as the headline claims fall apart.

Oxidation when abused. Polyunsaturated fats are chemically less stable than saturated ones, and repeatedly reheating oil (the deep-fat fryer left on all day) does generate oxidation products you would rather not eat in quantity. This is an argument about how oil is handled, not about the molecule itself. Fresh oil used at home and a fryer running for a week are not the same exposure.

The company they keep. The honest concern is that most seed oil in the average diet arrives inside ultra-processed food: crisps, biscuits, fried takeaways, packaged snacks. Those foods are worth eating less of, but the reason is the whole package of refined starch, salt, sugar and calorie density, not the oil acting as a uniquely toxic ingredient. Blaming the oil lets the rest of the recipe off the hook.

So the practical takeaway is not “seed oils are poison”. It is the far less viral “cook with fresh oil and eat less junk”, which was sound advice long before the trend arrived.

6. The honest verdict

On the weight of current evidence, seed oils used as ordinary cooking fats are not harmful to heart health, and in place of saturated fat they look beneficial for cholesterol. The inflammation claim does not survive human trials. The legitimate caution is about deep-fried, ultra-processed food and abused frying oil, which is a reason to eat better overall, not a reason to fear a bottle of rapeseed oil in your kitchen.

But here is the part the online debate misses entirely. Population averages are not you. Your genetics, your baseline lipids, your insulin sensitivity and the rest of your diet all shape how you personally respond to any fat. The way out of the argument is not a louder opinion. It is data about your own body.

7. Don't argue, measure: the four markers that matter

If you want to know what your diet, seed oils included, is doing to your cardiovascular risk, four blood markers tell you far more than any podcast ever could. These are the numbers that actually move with what you eat.

• ApoB. A count of the actual atherogenic particles in your blood, and arguably the single best lipid marker of cardiovascular risk. See our ApoB blood test guide.
• LDL cholesterol. The marker seed oils are claimed to harm and that the evidence says they tend to lower. Our cholesterol blood test guide explains the full lipid panel.
• Triglycerides. More responsive to refined carbohydrate and alcohol than to cooking oil, which is itself instructive. See the triglycerides guide.
• hs-CRP. A sensitive marker of low-grade inflammation, and the exact number the seed-oil theory predicts should rise. Our CRP blood test guide and inflammation guide cover how to read it.

Helvy's Advanced Heart Health panel (£159) measures all four, ApoB, LDL, triglycerides and hs-CRP, alongside Lp(a), HbA1c and the full cholesterol breakdown. It is a home finger-prick test processed by UKAS-accredited UK laboratories, with results in around 5 working days. Our heart health blood test guide walks through what each marker means.

The smart way to use it is a before-and-after. Test now, change one thing in your diet for eight to twelve weeks, and test again. That is a personal controlled experiment, and it beats any amount of online certainty.

8. How to read your numbers

A few principles keep the interpretation honest. A blood test measures biomarkers and offers wellness insight; it does not diagnose heart disease, and any result that concerns you is a conversation to have with a qualified clinician.

• Trends beat single readings. One result is a snapshot. The signal is in how your ApoB and LDL move when you change your diet, not in a single number on a single day.
• hs-CRP is sensitive but non-specific. It rises with any recent infection, injury or hard training session, so retest when you are well and rested before drawing conclusions about your everyday baseline.
• Look at the panel together. A favourable lipid profile with a quiet hs-CRP tells a very different story than high ApoB alongside raised inflammation. The markers are most useful read as a set.

If you would rather assemble your own marker list, the build-my-test tool walks you through it.

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