Conclusion may have been underplayed

EDITOR—The paper by Hooper et al may fulfil the Cochrane review criteria for a meta-analysis of clinical trials, but it does not provide a systematic review of the issues relating to dietary fat intake and the prevention of cardiovascular disease.1

It is important to consider the overall appropriateness of each trial before its inclusion in a meta-analysis. Several of the trials included are small or of short duration. It is difficult to imagine how the risk of coronary heart disease that has accrued over a prolonged period may be reversed by dietary modification in less than two years. The diet and infarction trial (fat modification arm) contributed the greatest number of cardiovascular end points, yet there are at least two reasons why this trial was unlikely to show a beneficial effect. Limited dietary instruction was given, so there was little change in cholesterol concentrations. Since modification of fat quality produces a predictable average change in cholesterol, there was clearly limited compliance with dietary advice. Furthermore, the study was too short (two years) to have the expected reduction in clinical events and mortality.

The Veterans' Administration study, which contributed the second largest number of deaths, fulfilled the criteria for a good randomised, controlled, double blind clinical trial. But the experimental diet entailed a ratio of polyunsaturated to saturated fatty acids of 1.5, far greater than would today be considered desirable. Such a diet would be expected to reduce cholesterol and coronary heart disease morbidity and mortality—as indeed it did, but perhaps not total mortality, as was also the case. Is it appropriate to compare such an intervention with that used in the Oslo study, in which saturated fats were replaced by whole grain cereals, vegetables, fruit, and some unsaturated fatty acids and in which study cardiovascular events and total mortality …