Chapter 3 – A systematic review and meta-analysis of the randomised controlled trial evidence available at the time of the introduction of dietary fat recommendations

Introduction

US public health dietary advice was announced by the Select Committee on Nutrition and Human needs in 1977 [15] and was followed by UK public health dietary advice issued by the National Advisory Committee on Nutritional Education in 1983 [16]. 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 consumption to 10% of total energy intake.

The recommendations were an attempt to address coronary heart disease (CHD) mortality. Both documents acknowledged that the evidence was not conclusive. Hegsted's introduction to the Dietary Goals for The United States noted "there will undoubtedly be many people who will say we have not proven our point "(p.8) [15]. The UK publication referred to "a strong consensus of opinion" (p.24) [16].

The evidence available to dietary committees at that time comprised epidemiological studies and randomised controlled trials (RCTs). The most comprehensive population study undertaken was The Seven Countries Study by Keys et al [19]. This reported that CHD "tended to be related" to serum cholesterol values and these in turn "tended to be related" to the proportion of calories provided by saturated fats in the diet (p.I-193-4) [51]. Keys acknowledged that epidemiological studies could reveal relationships, not causation [19]. RCTs provide the best evidence [25 133].

While the UK nutritional guidelines [16] made reference to The Seven Countries Study, the US committee document [15] did not. Neither publication made reference to any of the RCTs available at that time. However, the US Committee report reported data from the non-randomised, cross-over trial, the Finnish Mental Hospital Study [32 33].

Although a number of reviews of RCTs have been undertaken, no review has examined the RCT evidence available at the time dietary fat guidelines were introduced [40-42 134]. Furthermore, these guidelines have not been changed since they were announced, correspondingly, the validity of their evidence based remains current.

This chapter will report on a systematic review and meta-analysis to assess if the published RCTs available to the dietary committees supported their recommendations on dietary fat. With this in mind, the hypothesis is that RCT evidence available to the dietary committees at the time of issuing recommendations did not support the contention that reducing dietary fat intake would contribute to a reduction in CHD risk or related mortality.

Methods

A systematic review and meta-analysis was conducted in accordance with the PRISMA guidelines [3].