Introduction
US public health dietary advice was announced by the Select Committee on Nutrition and Human Needs in 19771 and was followed by UK public health dietary advice issued by the National Advisory Committee on Nutritional Education in 1983.2 Dietary recommendations in both cases focused on reducing dietary fat intake, specifically to (i) reduce overall fat consumption to 30% of total energy intake and (ii) reduce saturated fat (SFA) consumption to 10% of total energy intake.
The recommendations were intended to address mortality from coronary heart disease (CHD). We recently published a systematic review and meta-analysis,3 which reported that evidence from randomised controlled trials (RCTs), available to the dietary guideline committees, did not support the introduced dietary fat recommendations. This systematic review and meta-analysis extends this work by re-examining the totality of RCT evidence, currently available, relating to the present dietary fat guidelines.
While no previous study had reviewed the evidence available to the 1977 and 1983 committees, a number of meta-analyses have reviewed RCT and/or epidemiological evidence available at their respective times of publication.4–11 None has found any significant result for dietary fat intervention and mortality: all-cause, cardiovascular disease (CVD) or CHD mortality.
A meta-analysis by Skeaff and Miller in 2009 included 28 US and European cohorts (6600 CHD deaths among 280 000 participants) and found no clear relationship between total or SFA intake and CHD events or deaths.4
In 2010, Siri-Tarino et al5 undertook a meta-analysis of 21 prospective cohort studies involving 347 747 participants, evaluating the association of SFA with CVD. They reported that there is no significant evidence for concluding that dietary SFA is associated with an increased risk of CHD or CVD.
Hooper et al6 ,7 examined RCT evidence in 2011 and 2015 and found no significant difference for total mortality or cardiovascular mortality resulting from modified dietary fat intake, reduced dietary fat intake or combined modified and reduced dietary fat intake.
Chowdhury et al's8 meta-analysis of RCTs and prospective cohort studies found no association of dietary SFA intake, nor of circulating SFAs, with CHD.
Schwingshackl and Hoffmann examined RCTs that reduced or modified dietary fat with regard to all-cause mortality, CVD mortality and CVD events, in participants with established CHD. They concluded that there was no evidence for benefit of reduced/modified fat diets in the secondary prevention of CHD.9
Mozaffarian et al10 reported evidence that consuming polyunsaturated fats in place of SFAs reduced CHD events, not mortality, in RCTs.
A number of these reviews have been challenged. Stamler posed questions following the Siri-Tarino et al publication.12 Chowdhury et al's8 meta-analysis received a number of letters of response, which led to the original article being amended. Mozaffarian et al's review was criticised13 for excluding two unfavourable trials14 ,15 and for including the non-randomised, cross-over trial excluded by other reviews.16 ,17
The most recent meta-analysis by Hooper et al7 suggested that reduction of SFA intake may result in a small but potentially important reduction in cardiovascular risk, not mortality.
There were two important findings of Harcombe et al:3 first, the evidence available to the dietary committees did not support the introduced guidelines; second, the evidence available had serious limitations and was inappropriate to use for population-wide recommendations. The six studies available in 198314 ,15 ,18–21 had reviewed 2467 men and no women. Five of the six studies were secondary prevention; one included primary and secondary prevention subjects.21
Based on these secondary findings, selection criteria for meta-analysis to inform population-wide recommendations should be restricted to RCTs, of sufficient size and duration, with primary prevention subjects, man and woman. There is only one study meeting these criteria,22 the Minnesota Coronary Survey, and the results of this were not significant.
As a meta-analysis cannot be undertaken on the one primary prevention, both-sex, study available, this follow-up study retains the selection criteria and thus limitations of Harcombe et al3 to re-examine dietary guidelines, for total and SFA, to assess their evidence base against the RCT evidence currently available.