Olive oil or butter? What a 221,000-person study actually found

"Olive oil or butter?" is a standing kitchen argument. A large study published in March 2025 did not settle it as a taste question — but it did measure how the two fats relate to mortality within the same dataset. Most of the figures circulating in the press come from this study, and some of them are being reported wrongly.

Below we separate what the study says, what it cannot say, and what it means at the stove.

The study

Findings

• Those with the highest butter intake had a 15% higher risk of premature death than those with the lowest.
• Those with the highest plant-oil intake had a 16% lower risk.
• Replacing 10 grams a day of butter with plant-based oils was calculated to correspond to a 17% reduction in total and cancer mortality.
• The oils examined were soybean, canola and olive oil; all three were associated with lower total, cancer and cardiovascular mortality.

The mistake the press keeps making

In Turkish coverage the 15% from this study is often rendered as "people who use olive oil have a 15% lower risk of cardiovascular disease." That is wrong.

The 15% in the study is the higher mortality risk among those eating the most butter. The olive-oil-side figure is the 16% reduction, and the substitution calculation is 17%. Three separate numbers with three separate meanings; none is interchangeable with another.

What the study cannot say

This is an observational study. What people ate is self-reported, and nobody was instructed to eat anything. A causal sentence such as "butter shortens life" cannot be built from it. The sentence it supports is "higher butter consumption was associated with higher mortality."

The researchers state their own limitation clearly: participants were largely health professionals, which makes it harder to generalise to the wider population.

One further detail: the study treated olive oil in the same group as soybean and canola. So the finding is not "olive oil beats other plant oils"; it is "the picture improves when plant oils take butter's place."

The claim that olive oil is unaffected by heat

Another sentence recurs in the same coverage: that extra virgin olive oil produces no toxic compounds even at high temperatures. That is overstated.

Extra virgin olive oil is a stable fat thanks to its high oleic acid content and antioxidants — that much is true. But stable is not immune. In is olive oil suitable for frying we set out why smoke point is not the only criterion, and why how often the oil is reused and how long the heat is held matter as well.

The difference on the butter side is structural: butter is not pure fat but contains milk solids and water, and those solids brown at a relatively low temperature. In clarified butter, with the milk solids removed, that problem largely disappears — so which butter is meant also matters.

What it means in the kitchen

The study's most usable result is not a ban list but the substitution calculation: replacing 10 grams of butter a day with plant oil. Ten grams is roughly a teaspoon.

That does not mean removing butter from your life. It means making olive oil the main cooking fat and keeping butter for flavour — the same frame we set out in how much olive oil a day and olive oil and heart health.

A final reminder: studies of this kind measure dietary patterns, not single spoonfuls. Olive oil's effect is better read within the whole of the Mediterranean diet than on its own.

Sources

• Zhang Y, Wang D et al. Butter and plant-based oils intake and mortality in US women and men. JAMA Internal Medicine, 6 March 2025. doi:10.1001/jamainternmed.2025.0205
• Institutional announcement: Mass General Brigham
• Harvard Health: Want to live longer? Replace butter with plant-based oil
• On smoke point, frying and daily amounts: zeytin.net