The 1983 UK guidelines and 1977 US guidelines aimed to cut coronary heart disease by recommending that the public limited overall fat consumption to 30% of total energy intake, and specifically warned that saturated fat intake should be limited to 10%.
But a fresh analysis of the evidence available at the time has suggested that there was a lack of any solid evidence from trials to back up the guidelines, and that the recommendations "should not have been introduced".
Writing in the BMJ journal Open Heart, a team of researchers from the UK and USA carried out a systematic review and meta-analysis of the trial data that would have been available to regulatory committees at the time – finding that the clinical evidence available did not back up the recommendations that were made.
However, several experts have warned that the new analysis, which focuses solely on data from randomised controlled trials (RCTs) does not depict the full picture, with Professor Christine Williams from the University of Reading suggesting that any suggestion that the recommendations were not based on good evidence is “misguided and potentially dangerous.”
Indeed, Professor Tom Sanders of King’s College London said the conclusion from the meta-analysis fails to take into account the totality of the evidence available at the time, while a linked editorial published by Rahul Bahl of the Royal Berkshire NHS Foundation Trust, issues a similar note of caution – suggesting that while there may be a disagreement in the interpretation of available data from RCTs, there is still reason to suggest a causal connection between fat consumption and coronary heart disease.
“In my view, the authors are wrong to suggest that advice to decrease total and saturated fat should not have been introduced,” commented Sanders.
A lack of evidence?
The fresh meta-analysis, led by Zoë Harcombe from the University of the West of Scotland, highlighted several limitations in the evidence available at the time guidelines were introduced, including the fact that no women were included, no trial tested the dietary recommendations, and no trial concluded that dietary guidelines should be drawn up.
"The results of the present meta-analysis support the hypothesis that the available [randomised controlled trials] did not support the introduction of dietary fat recommendations in order to reduce [coronary heart disease] risk or related mortality,” wrote Harcombe and her team.
The new review analysed data from six relevant trials, covering seven different dietary interventions, spanning an average of five years, and involving 2,467 men. All the trials had been published before 1983 and looked at the relationship between dietary fat, serum cholesterol, and the development of coronary heart disease.
According to Harcombe and her colleagues, there was no difference in deaths from all causes between the low fat diet and comparison groups, with 370 deaths in both, and no statistically significant difference in deaths from coronary heart disease, with 207 in the low fat diet groups and 216 in the comparison groups.
While reductions in serum cholesterol were found to be significantly greater in the low fat diet groups, this did not seem to have any impact on the death rates from all causes or from coronary heart disease, the analysis showed.
"Dietary advice not merely needs review; it should not have been introduced,” they concluded.
"It seems incomprehensible that dietary advice was introduced for 220 million Americans and 56 million UK citizens, given the contrary results from a small number of unhealthy men.”
Evidence based policy
According to Bahl though, some of the results reported in the new meta-analysis are controversial, and should be treated with caution – reiterating that while RCT data may be ‘mixed’, epidemiological and ecological evidence suggests a link between fat consumption and heart disease.
“The vast majority of dietary advice is based not upon RCTs but on epidemiological data. If data was required from RCTs there would indeed be very little advice,” said Professor Richard Mithen, acting director at the Institute of Food Research. “It’s true that epidemiology studies can be tricky to interpret, and that correlation does not always mean causation, but in reality it is very challenging to design and carry out randomised controlled trials that are big enough and powerful enough to determine effects that are relevant at the whole population scale.”
“Other reviews of RCTs have shown that the replacement of saturated fat with polyunsaturated fat carries cardiovascular benefits,” said Bahl. “Such disagreements between meta-analysts are not uncommon since small differences in the criteria used to include studies in different parts of the analysis can lead to large differences in reported results.”
He noted that even the most up-to-date review concluded that the evidence on which current dietary guidance is based was ‘very limited’ - but warned “this doesn't mean that the risk factor identified is not a true risk factor.”
“Public policies generally do not require RCT evidence, so to advocate their withdrawal here on the basis of the absence of such evidence seems unusual,” he stated.
According to Professor Kevin McConway from The Open University, the new report itself points out that evidence for the guidelines came from other sources as well, in particular from population studies.
“This new research tells us rather little about what the overall evidence base for the recommendations actually was. So it really doesn’t help us much in deciding whether or not the decisions made all those years ago were properly grounded in evidence,” he warned.
“More importantly, that’s not the relevant question for today. The relevant question for now is whether all the evidence available up to 2015 supports the dietary guidelines or not.”
However, Professor Williams noted that while the findings from the meta-analysis may be ‘misguided’, there are “justifiable concerns regarding evidence that saturated fats have been replaced in the diet by sugars”. Indeed, Williams suggested that this topic requires "a reasoned assessment of alternative approaches for the future".
Source: Open Heart
Published online, open access, doi: 10.1136/openhrt-2014-000196
“Evidence from randomised controlled trials did not support the introduction of dietary fat guidelines in 1977 and 1983: a systematic review and meta-analysis”
Authors: Zoë Harcombe, et al
Source: Open Heart
Published online, open access, doi: 10.1136/openhrt-2014-000229
“The evidence base for fat guidelines: a balanced diet”
Author: Rahul Bahl