Olive oil

Extra-virgin olive oil is the backbone of the Mediterranean diet and the best-studied everyday oil, but its reputation runs ahead of its evidence. Its one big trial tested a whole diet and survived a retraction; the rest is observational, much of it low quality; and against other vegetable oils it shows no clear advantage. Most of its benefit comes from what it replaces.

Use extra-virgin olive oil as your everyday cooking and dressing oil, ideally a high-phenolic one, and use it in place of butter and other saturated fats. Canola, sunflower and safflower oils are sound alternatives.

What it is

Olive oil is pressed from olives. Most of its fat is monounsaturated, mainly oleic acid — fat with a single double bond in its chain, as opposed to saturated fat (none) and polyunsaturated fat (several). "Extra-virgin" is the least processed grade, and it also carries plant compounds called phenolics.

The big trial: fewer cardiovascular events, after a retraction

A Spanish trial called PREDIMED assigned 7,447 adults at high cardiovascular risk to a Mediterranean diet with extra olive oil, the same diet with extra nuts, or a low-fat control diet. Over a median of 4.8 years, both Mediterranean arms had about 30% fewer major cardiovascular events — heart attacks, strokes and cardiovascular deaths combined — with a hazard ratio of 0.69 for the olive-oil arm (95% confidence interval 0.53–0.91, the range where the true effect most plausibly lies) and 0.72 for the nut arm.^[1]

Those are corrected figures. The original 2013 paper was retracted in 2018 and republished after a statistical audit found randomisation problems affecting about 21% of participants (1,588 of 7,447), including one clinic that misused the randomisation table and whole households assigned to the same arm. The re-analysis found roughly the same benefit, so the conclusion survived.

The funding was public, from the Spanish health ministry's research institute and related bodies. The olive oil and nuts given to participants were donated by producers — Spanish olive-oil, almond and hazelnut firms and the California Walnut Commission — none of which, the paper states, had any role in the study's design, analysis or reporting. The trial tested a whole diet, not olive oil alone; more on it under PREDIMED.

Beyond PREDIMED, the evidence is observational

Pooled studies. An umbrella review of 31 meta-analyses linked the highest olive oil intakes with about 15% less cardiovascular disease and 16% less type 2 diabetes than the lowest. Cancer overall was exactly null. And the review rated 20 of its 31 underlying analyses as critically low quality, with only three high.^[2] The signals are consistent; the evidence beneath them is not strong.

What it replaces. Across 92,383 US adults followed for 28 years, swapping 10 g a day of butter, margarine, mayonnaise or dairy fat for olive oil tracked with 8% to 34% lower death rates, depending on the cause of death. But "no significant associations were observed when olive oil was compared with other vegetable oils combined."^[3]

The weakest of the good swaps. As a replacement for saturated fat, monounsaturated fat has less evidence than polyunsaturated fat. The World Health Organization's 2023 guideline makes swapping saturated fat for plant monounsaturated fat only a conditional recommendation, against a strong one for polyunsaturated fat.^[4] In 53 randomised trial comparisons in 1,690 adults, high-polyunsaturated diets beat high-monounsaturated ones on low-density lipoprotein (LDL) cholesterol by only about 3 mg/dL (under 0.1 mmol/L) — a real but small edge.^[5]

The phenolics, and why most bottles fall short

Extra-virgin olive oil's phenolics (oleocanthal, hydroxytyrosol, oleuropein) protect LDL cholesterol from oxidation, independently of the oil's fat content. The effect depends on dose, and European authorities allow the health claim only for oils carrying at least 5 mg of hydroxytyrosol and its relatives per 20 g of oil, eaten at 20 g a day. Most oil on the shelf doesn't reach that: an analytical survey found no more than about 10% of bottled oils had enough phenolics for the claim to apply.^[6]

One small trial is a curiosity rather than a guide. Healthy young women took 40 g a day of high-phenolic extra-virgin olive oil or sunflower oil for 8 weeks; within the olive-oil group LDL cholesterol rose slightly (+5.1 mg/dL, about 0.13 mmol/L) while fasting insulin fell.^[7] These were within-group changes with no head-to-head lipid comparison, and twelve people completed the olive-oil arm; it is not a reason to switch oils.

How it compares with other oils

Canola and soybean oil sit in the sound middle: balanced fat profiles, no harm signal in cohort data, fewer phenolics than extra-virgin olive oil. Their main polyunsaturated fat is covered under Seed oils.
Avocado and nut oils are other options. Avocado oil has no legal identity or grade standard in the US, and repeated analyses from one laboratory have found most retail samples rancid or cut with cheaper oils: 82% of bottles in 2020, around 70% of private-label bottles in 2023 and 89% of avocado-oil-labelled processed foods in 2026, some bottles labelled "pure" or "extra virgin" being almost entirely soybean oil.^[8]
Coconut oil is not a health food: it raises LDL cholesterol compared with non-tropical vegetable oils. See Dietary fats.

What to do

Make extra-virgin olive oil your default oil for cooking and dressing.
Use it instead of butter and other saturated fats. That swap, not the oil on its own, is where the evidence points.
Prefer a high-phenolic oil if you can find one; most bottles don't reach the level behind the health claim.
Don't treat it as unique. Canola, sunflower and safflower oils are sound alternatives; if you buy avocado oil, buy a brand you trust.