Seed oils – what did trials conclude?

Summary

* Continuing our mini-series on seed oils, this note summarises the randomised controlled trial (RCT) evidence for studies that reduced and/or modified fat intake. Fat intake modification aimed to reduce saturated fat and to increase polyunsaturated fat. This was achieved in most trials with seed oil interventions. The purpose of these studies was to test the diet-heart hypothesis, i.e., did a reduction in total fat and/or saturated fat reduce heart disease (and reduce cholesterol, as a blood marker).

* This topic is my main area of expertise. My PhD examined RCTs and population studies, which tested the diet-heart-cholesterol hypothesis. This note focuses on RCTs, because they provide superior evidence and also because...

* ...this mini-series was inspired by Professor Sarah Berry, the chief scientist at an organisation called ZOE, claiming that seed oils reduce cardiovascular disease. In podcasts, Berry singled out and dismissed one RCT – the Sydney Diet Heart Study (SDHS) – which found against seed oils.

* This note rebuts the dismissal of the SDHS. It then presents the summary findings of the 10 RCTs in this field (those long enough and large enough to have cholesterol, heart deaths and all-cause deaths measured and reported). It pools together all the trials to show what the totality of the evidence found. It also presents the conclusions from the seed oil studies – none of which found in favour of seed oils and a number of which issued cautions.