Olive Oil vs Butter: A Weight Study
A big, careful study tracked more than 120,000 US adults for two decades and found that greater olive-oil use tracked with less long-term weight gain, while butter and margarine tracked with more. The sensible reading is quieter than the headline.
Published in early 2025 in the American Journal of Clinical Nutrition, this was one of the larger looks at olive oil and body weight to date. Researchers followed 121,119 US adults across three long-running cohorts for around two decades, comparing changes in what people ate with changes in what they weighed. Greater olive-oil intake was associated with less weight gain over time; butter, margarine and some other added fats were associated with more. It is genuinely encouraging. It is also, as ever, an association — and the difference matters.
What was actually measured
The headline figures are modest and worth stating plainly. Each extra half-tablespoon (about 7 g) of olive oil a day tracked with a small downward nudge in body weight; each equivalent increase in butter or margarine tracked the other way. Statistically, swapping butter for olive oil was the move most clearly linked with lower weight. These are averages across a vast group over many years, not a promise for any one person — but the direction was consistent across ages and body sizes, which is what gives a study like this its weight.
Why ‘observational’ is the key word
This kind of study watches how people already live; it does not assign them a diet. So it can show that olive-oil users gained less weight, but it cannot prove the oil itself did it. People who reach for olive oil tend to eat differently overall — more vegetables, fish and whole foods, fewer ultra-processed ones — and that whole pattern, not one bottle, may be doing much of the work. Good researchers adjust for what they can measure, but they cannot rule out everything. The honest summary is associated with, not causes.
The reasonable takeaway
So the useful reading is not “olive oil is slimming.” It is that, calorie for calorie, a better fat in place of a worse one fits a healthier way of eating — and olive oil is a very good fat to build that pattern around. It is still an energy-dense food at roughly 120 calories a tablespoon; drowning a salad in it will not help anyone lose weight. Olive oil is food, not medicine, and it earns its place at the table as part of a broadly Mediterranean way of eating rather than as a magic ingredient. This is general information, not medical advice.
| Study size | 121,119 US adults, three cohorts |
|---|---|
| Follow-up | Roughly two decades |
| Design | Observational (association, not proof) |
| Energy | About 120 kcal per tablespoon |
| Main fat | Monounsaturated (oleic acid) |
| Also present | Vitamin E; polyphenols (in extra virgin) |
| Best read as | A better fat swapped for a worse one |
What to do with this at home
- Use olive oil in place of butter or margarine, not on top of them — substitution is the point.
- Remember it is calorie-dense; measure it if weight is your goal rather than free-pouring.
- Favour extra virgin for its polyphenols and flavour, and use it raw where you can.
- Treat it as one good habit inside a whole way of eating, not a single fix.
Olive oil and weight: common questions
Does olive oil help you lose weight?
The study links greater olive-oil use with less long-term weight gain, especially when it replaces butter. But it is observational, so it shows association, not proof that the oil itself causes the effect. This is general information, not medical advice.
Why does the study not prove cause?
Because it observed how people already ate rather than assigning diets. Olive-oil users tend to eat differently overall, so their whole pattern — not just the oil — may explain much of the result.
Is olive oil low in calories?
No. At about 120 calories a tablespoon it is energy-dense. Its benefit here is as a better fat swapped for a worse one, not as a low-calorie food. Portion still matters.
Butter or olive oil, then?
The evidence leans toward using olive oil in place of butter for long-term weight and heart health. It is a substitution story, not a licence to add more fat overall.
Does the type of olive oil matter?
Extra virgin carries more polyphenols and flavour and is best used raw. For this weight association the key point is simply choosing olive oil over butter or margarine.
It is a big, careful study — and observational, which means it shows association, not that oil itself prevents weight gain. Olive-oil users tend to eat better across the board. The sensible reading is not ‘olive oil is slimming’ ; it is that a good fat in place of a worse one fits a healthier way of eating. Olive oil is food, not magic. Swap it for butter, keep an eye on the portion, and let it earn its place as part of the whole plate. This is general information, not medical advice.
Based on a 2025 study in the American Journal of Clinical Nutrition tracking 121,119 US adults across three cohorts; reported via News-Medical.