An examination of the randomised controlled trial and epidemiological evidence for the introduction of dietary fat recommendations in 1977 and 1983: a systematic review and meta-analysis

Introduction: National dietary guidelines were introduced in 1977 and 1983, by theUnited States (US) and United Kingdom (UK) governments respectively, with theintention of reducing deaths from coronary heart disease (CHD) by reducing ormodifying fat intake. The two specific guidelines recommended the consumption of nomore than 30% of total calories as dietary fat and no more than 10% of calories assaturated fat. To date, no analysis of the evidence base for these recommendations hasbeen undertaken.Methods: A systematic review and meta-analysis was undertaken of RCTs, publishedprior to 1983, which examined the relationship between dietary fat, serum cholesteroland mortality from CHD and all-causes. A systematic review and meta-analysis wasundertaken of RCTs, currently available, which examined the relationship betweendietary fat, serum cholesterol and mortality from CHD and all-causes.A systematic review was undertaken of epidemiological evidence published prior to1983, which examined the relationship between dietary fat, serum cholesterol andmortality from CHD. A systematic review and meta-analysis was undertaken ofepidemiological evidence currently available, which examined the relationship betweendietary fat, serum cholesterol and mortality from CHD.Results: For RCTs available at the time the dietary fat guidelines were introduced, therisk ratio (RR) for dietary fat interventions and all-cause mortality was 0.982 (95% CI,0.878 to 1.098). The RR for dietary fat interventions and CHD mortality was 0.951(95% CI, 0.784 to 1.155).For RCTs currently available, the RR for dietary fat interventions and all-causemortality was 0.991 (95% CI, 0.935 to 1.051). The RR for dietary fat interventions andCHD mortality was 0.976 (95% CI, 0.878 to 1.084).The reductions in mean serum cholesterol levels were significantly higher in theintervention groups (effect size > 1.0); this did not result in significant differences inCHD or all-cause mortality.The epidemiological evidence available at the time the dietary fat guidelines wereintroduced included one prospective cohort study that found an association betweenCHD mortality and saturated fat, but not total fat, intake. Five other studies found noassociation for total or saturated fat.5For epidemiological evidence currently available, the RR for total dietary fat and CHDdeaths was 1.06 (95% CI 0.97 to 1.16). The RR for saturated fat and CHD deaths was1.13 (95% CI 0.93 to 1.37).There were no statistically significant relationships from any of the meta-analysesundertaken.Only one RCT currently available examined dietary fat and CHD mortality in men andwomen without previous heart disease. All other RCTs lacked generalisabilty. Only oneprospective cohort study currently available examined dietary fat and CHD mortality inhealthy men and women. All other prospective cohort studies lacked generalisabilty.The specific dietary fat recommendations made were untested in any trial prior to beingintroduced.Conclusion: Studies examining the relationship between dietary fat and CHD mortalityhave largely been undertaken on men and often men with pre-existing heart disease. Theevidence for males with secondary disease was not conclusive. Dietary guidelines wereintroduced for whole populations despite this.