Ask nutrition researchers which eating pattern has the best evidence behind it and most will give the same answer: the Mediterranean diet. That is not because of olive-oil folklore. It is because of one very large Spanish trial that assigned thousands of people to eat that way and counted what happened next.
The trial
PREDIMED enrolled 7,447 adults aged 55 to 80, all at high cardiovascular risk but free of cardiovascular disease at the start — 57% were women. Participants were assigned to one of three groups: a Mediterranean diet supplemented with free extra-virgin olive oil, the same diet supplemented with free mixed nuts, or a control group advised to reduce dietary fat. Everyone got quarterly educational sessions, so the difference was the food on the table, not the attention.
After a median follow-up of 4.8 years, the trial was stopped early on a prespecified interim analysis: the gap was already clear. A primary-endpoint event — heart attack, stroke, or cardiovascular death — occurred in 96 people in the olive-oil group (3.8%), 83 in the nuts group (3.4%), and 109 in the control group (4.4%).
Adjusted for baseline characteristics, the hazard ratio was 0.69 for the olive-oil group and 0.72 for the nuts group compared with the control diet. In plain language: roughly a 30% lower rate of major cardiovascular events in both Mediterranean groups.
The honest footnote
This trial has a scar worth knowing about. The original 2013 report was withdrawn after the authors found protocol deviations — some household members enrolled without randomisation, some participants assigned without it at one site. The 2018 republication re-analysed everything without assuming everyone had been properly randomised, including propensity-score adjustments, and the result survived: the effect estimates barely moved, and held even after dropping the 1,588 participants whose assignments were suspect.
That episode is exactly why the finding carries weight now. It was challenged from the inside and came back standing.
What the cohorts add
Trials this size are rare — one exists, and it studied older high-risk Spaniards. The wider picture comes from cohort studies. A 2014 meta-analysis pooling the available cohorts found that each 2-point increase in Mediterranean-diet adherence score was associated with an 8% lower overall mortality (relative risk 0.92), a 10% lower cardiovascular risk (0.90), and a 4% lower cancer risk (0.96).
Cohorts cannot prove cause the way a trial does; people who eat this way differ in dozens of other ways. But the direction matches PREDIMED, and the gradient — more adherence, less risk — is what you expect from something real rather than something confounded.
What the diet actually was
Forget the restaurant version. The trial’s Mediterranean diet meant abundant vegetables, fruit, legumes, whole grains and fish, with olive oil as the main added fat and nuts as a staple — plus the free oil or nuts the study supplied. Nobody counted calories. The control group, told to cut fat, is roughly how official advice sounded for decades. The trial was, in effect, a contest between two eras of dietary guidance, and the newer one won.
What the data do not show
PREDIMED studied people at high cardiovascular risk in Spain, most of them older. It says nothing direct about twenty-somethings, about people outside Mediterranean food cultures, or about weight loss — this was never a weight-loss trial. It also cannot separate the oil from the nuts from the vegetables: the intervention was the whole pattern, so anyone selling one component as “the active ingredient” is freelancing beyond the evidence.
