Why was the predimed trial Mediterranean diet cut short

The PREDIMED trial was not cut short. It was stopped early because the results were so strongly positive that continuing the low-fat control group was considered unethical.

The PREDIMED trial randomized 7,447 older adults at high cardiovascular risk to a Mediterranean diet with extra-virgin olive oil, a Mediterranean diet with nuts, or a low-fat control diet [1]. The trial was stopped after a median follow-up of 4.8 years, not because of harm, but because the independent data safety monitoring board judged that the Mediterranean diet groups had already shown a clear benefit [1][2]. The hazard ratio for the primary endpoint (heart attack, stroke, or cardiovascular death) was 0.70 for the olive oil group and 0.72 for the nuts group compared to the control [3]. The board decided it would be unethical to keep people on the control diet when the intervention was so clearly superior.

This is the opposite of the pattern you see in drug trials, where early stopping usually signals a safety problem. Here, the early stop was driven by efficacy. The trial's own design documents confirm the primary endpoint was a composite of hard clinical outcomes, not a surrogate [4]. The funding came from Spanish government sources and the Mediterranean Diet Foundation, not a pharmaceutical manufacturer [5]. The results have been replicated in subsequent trials like CORDIOPREV, which found similar hazard ratios of 0.72 to 0.75 over 7 years [3].

My call: the trial was stopped early because the Mediterranean diet was so effective at preventing cardiovascular events that continuing the control group was considered unethical. Confidence: high.