Seed oils do not need to be avoided as a category. Replacing butter with unsaturated oils can lower LDL cholesterol. That does not make every oil, cooking method or meal equivalent. 1

What the 2026 evidence actually says

“Seed oil” is not one fatty acid. Standard sunflower, soybean and corn oils are rich in polyunsaturated fats. Canola is predominantly monounsaturated, while high-oleic sunflower differs from standard sunflower. 1

Linoleic acid is the omega-6 fat at the centre of this debate. To interpret the evidence, separate cholesterol measurements, clinical events and observational associations. They answer different questions.

For cholesterol, the replacement matters. Swapping saturated fats for unsaturated oils generally lowers LDL. This is not evidence that adding extra oil to an unchanged diet provides the same benefit. 1

For cardiovascular events, the picture is more qualified. A review in the February 2026 Annals of Internal Medicine included 17 trials and 66,337 participants. It examined reducing or replacing saturated fat. 2

The authors estimated smaller absolute benefits over five years for lower-risk people and potentially worthwhile benefits for higher-risk people.

Replacing saturated fat with polyunsaturated fat showed a stronger signal for nonfatal heart attack. 2

However, several overall estimates included no effect. Trials differed in dietary changes and adherence. These results neither guarantee protection nor establish that linoleic acid alone prevents heart disease. 2

Cohorts provide complementary evidence. Pooled prospective studies associate higher linoleic-acid biomarkers with lower cardiovascular risk. They do not randomly assign people to different oils. 3

A separate pooled analysis of 39,740 adults associated higher linoleic-acid biomarkers with lower type 2 diabetes incidence. Metabolism, other dietary habits and residual confounding limit causal conclusions. 4

Linoleic acid and inflammation: the claim and the data

The familiar argument runs from linoleic acid to arachidonic acid, then to inflammatory signalling molecules called eicosanoids. The pathway exists, but its presence does not establish chronic inflammation from eating seed oils. 4

Human studies do not show a simple relationship where more dietary linoleic acid always produces more circulating arachidonic acid. Arachidonic acid also contributes to molecules involved in resolving inflammation. 4

A 2017 meta-analysis of 30 randomised trials, involving 1,377 participants, found no significant overall increase in CRP or IL-6 when linoleic-acid intake increased. These are commonly studied inflammatory markers. 5

There was an exploratory signal for higher CRP with larger increases in intake. Some markers had few contributing studies. The conclusion is no consistent inflammatory effect, not proof of no effect at every dose or in every person. 5

Newer evidence adds nuance. A June 2026 trial compared diets targeting 2.5% versus 10% of energy from linoleic acid for 12 weeks. Fifty-two of 80 randomised participants completed it. 6

The higher-linoleic-acid diet lowered circulating EPA, an omega-3 fatty acid, without increasing arachidonic acid. It also shifted lipid-mediator production in blood stimulated outside the body. 6

That is relevant mechanistic evidence, but not a demonstration of chronic inflammatory disease or more heart attacks. The small sample and attrition matter. Stable CRP does not mean every biochemical pathway is unchanged. 5 6

Oxidation and processing: what changes with heating

Fresh oil and oil exposed to prolonged frying are chemically different. High temperatures, oxygen, food moisture and residues promote deterioration, including the formation of polar compounds used to assess oil quality. 7

The rate depends on the oil, temperature, food and heating conditions. There is no universal rule that an oil becomes unsafe after six uses, or that fewer uses guarantee safety. 7

For example, using fresh oil for a brief stir-fry is not the same exposure as holding oil hot through repeated batches of battered food. That distinction matters without assigning either meal a precise disease risk. 7

Refining is also not the same as deterioration. It changes flavour and removes some antioxidants, but refined oils can suit higher-temperature cooking. “Processed” alone is not a useful verdict on an oil. 8

Chemical deterioration deserves sensible handling precautions. It does not establish that oxidation matters more than fatty-acid composition, or cancel the evidence about replacing saturated fat.

A practical NZ kitchen read

Olive oil is a reasonable default. It is well studied for cardiovascular outcomes, but that does not prove it is superior to every other unsaturated oil. 1

Extra virgin olive oil can be used for cooking, not just dressing salads. Canola and high-oleic sunflower are practical alternatives. Avocado oil is optional, not a necessary upgrade. 8

Butter can add flavour, but replacing it with unsaturated oil is the more evidence-aligned choice for lowering LDL. 1

Keep bottles closed in a cool, dark cupboard away from the stove. Dark glass or tins help protect oil from light. Avoid buying more than you will use. 8

Discard rancid oil. Unusual colour, persistent foaming or smoking during frying also warrant replacement. Smell alone is not a complete check of deterioration. 7

Keep the food pattern in view

Kūmara roasted in canola and takeaway fries are not interchangeable meals. Oil choice belongs alongside the food, preparation and frequency, rather than becoming the sole measure of whether a meal is healthy. 8

Start by asking what the oil replaces and what you usually eat with it. Do not assume a biscuit becomes a nutritious staple because it contains olive oil, or reject vegetables because they were cooked in canola.

Coaching observations cannot establish which ingredient caused a blood-test change when food intake, activity, weight and other behaviours changed together. That requires a more controlled comparison.

What to do this week

  • Choose one cooking oil that suits your budget and meals. You do not need separate “cold-use” and “pan-use” olive oils. 8
  • Check how you heat and store it, rather than counting supposedly safe reuses. 7
  • Look at your usual meals before replacing every bottle in the pantry.

If you have high cholesterol, known cardiovascular disease or a family history of early heart disease, discuss your overall risk and dietary changes with your GP or a registered dietitian. 9

A routine lipid assessment usually does not require fasting, although there are exceptions. Let your clinician advise on preparation and follow-up rather than arranging repeated tests around individual oil changes. 9

Hs-CRP is not part of standard NZ cardiovascular-risk assessment. Blood lipids and blood pressure generally deserve priority over requesting an inflammation test to judge your cooking oil. 1

For the wider assessment, see our NZ metabolic health guide.