Dietary guidelines were introduced in the US in 1977 (ref 1) and in the UK in 1983 (ref 2). The dietary recommendations in both cases were to reduce 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 made in the belief that dietary fat generally, and saturated fat particularly, caused coronary heart disease (CHD).
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 (ref 3). This reported that CHD “tended to be related” to serum cholesterol values and that these in turn “tended to be related” to the proportion of calories provided by saturated fats in the diet (ref 4). Keys acknowledged that epidemiological studies could reveal relationships, not causation (ref 3). RCTs provide the best evidence (ref 5).
Although a number of reviews of RCTs have been undertaken (refs 6-8), no review has examined the RCT evidence available at the time dietary fat guidelines were introduced. Furthermore, these guidelines have not been changed since they were announced; making the validity of their evidence base as relevant as at the time of introduction.
In our paper, we undertook a systematic review and meta-analysis to find the RCTs available to the committees and to review the findings.
The dietary trials
There were only five trials undertaken to test dietary fat interventions before the US recommendations were introduced in 1977 (ref 9-13). A further study was available to the UK committee, but was published after the US guidelines were introduced (ref 14).
None of these trials studied women. Only one of these trials included healthy subjects (ref 12). The other five were secondary studies, which means they only included men who had already had myocardial infarction. The death rate across all the studies was 30%, reflecting the fact that one of the single biggest causes of death is already having had a heart attack.
Table 1 in the paper summarises the dietary interventions that were tested. Rose et al gave one intervention group 64g of corn oil daily and another intervention group 58g of olive oil (both groups had targets of 80g of oil daily, but the participants found the oil ‘unpalatable’) and compared these to a control group of men receiving no oil. During the two year study, five men died in the corn oil group; three men died in the olive oil group and one in the control group.
The Research Committee low-fat diet compared 123 men randomly allocated to a low-fat diet (no more than 40g of fat daily) and 129 men randomly allocated to continue their normal diet. There were non-significant differences in deaths between the groups.
For just over 3 years, the MRC soya-bean oil study followed 194 control patients who continued their normal diet. The 199 men, randomly allocated to the experimental group, were required “as far as possible” to remove saturated fats from the diet and were instructed to consume 85g of soya-bean oil daily. The intervention group was allowed up to 85g of lean meat daily, any fish, skimmed milk, and clear soups. They were not allowed to consume butter, other margarines, cooking-fat, other oils, meat fat, whole milk, cheese, egg yolk, and most biscuits and cakes. This was not a single dietary intervention, therefore. There were slightly more deaths from any cause in the control group and slightly fewer deaths from CHD in the control group. Neither result was statistically significant.
The Dayton/LA Veterans study was undertaken in a veteran’s home and thus meals were served in a controlled environment. The intervention group was to have no more than 40% of their intake in the form of fat and two thirds of their fat from vegetable oils. There were non-significant differences in deaths between the groups.
The Leren/Oslo study also allocated the intervention group no more than 40% of their intake in the form of fat. This time, 72% of the fat was to come from soya-bean oil. There was nothing of significance in all-cause mortality. The deaths from CHD were lower in the intervention group (significant to a p value of 0.1 but not 0.05).
The Sydney diet heart study was the first to test either of the actual dietary recommendations introduced. It tested 10% saturated fat (and 15% polyunsaturated fat) in the intervention group vs. 14% saturated fat and 9% polyunsaturated fat in the control group. There were significantly more deaths in the intervention group from both all-causes and from CHD.
The key conclusions of our review
* 2,467 males participated in 6 dietary trials: 5 secondary prevention studies and 1 including healthy subjects.
* No randomised controlled trial had tested government dietary fat recommendations before their introduction. (Woodhill tested the 10% saturated fat recommendation after the US guidelines were introduced. The death rate from all-causes was 18% in the intervention group vs. 12% in the control group).
* There were 370 deaths from any cause in both the intervention and control groups. The risk ratio (RR) from meta-analysis was 0.996 (95% CI 0.865 to 1.147).
* There were 207 and 216 deaths from coronary heart disease (CHD) in the intervention and control groups respectively. The risk ratio was 0.989 (95% CI 0.784 to 1.247).
* There were no differences in all-cause mortality and non-significant differences in CHD mortality, resulting from the dietary interventions.
* Mean serum cholesterol levels decreased in both control and intervention groups. The reductions in mean serum cholesterol levels were significantly higher in the intervention groups. This did not result in significant differences in CHD or all-cause mortality.
* Recommendations were made for 276 million people following secondary studies of 2,467 males, no study of women and no study of only healthy people.
* RCT evidence did not support the introduction of dietary fat guidelines.
The studies’ own conclusions
These are the verbatim conclusions from each of the studies:
1965 Rose Corn & olive oil: “It is concluded that under the circumstances of this trial corn oil cannot be recommended as a treatment of ischaemic heart disease. It is most unlikely to be beneficial, and it is possibly harmful.” (ref 9)
1965 Research Committee Low-fat diet: “A low-fat diet has no place in the treatment of myocardial infarction” (ref 10) [heart attack].
1968 MRC soya-bean oil: “There is no evidence from the London trial that the relapse-rate in myocardial infarction is materially affected by the unsaturated fat content of the diet used.” (ref 11)
1969 Dayton LA Veterans study: “Total longevity was not affected favorably in any measurable or significant degree… For this reason, and because of the unresolved question concerning toxicity, we consider our own trial, with or without the support of other published data, to have fallen short of providing a definitive and final answer concerning dietary prevention of heart disease.” (ref 12)
1970 Leren Oslo Diet Heart study: “Epidemiological studies have demonstrated several factors associated with the risk of developing first manifestations of coronary heart disease. Blood lipids, blood pressure and cigarette smoking are such risk variables… In spite of the small numbers this observation lends some support to the view that the multi-factorial approach is the best way to the solution of the coronary heart disease problem.”(ref 13)
1978 Woodhill Sydney Diet Heart Study: “Survival was significantly better in the P [control] Group.” “It must be concluded that the lipid hypothesis has gained little support from secondary intervention studies.” (ref 14)
Only one study, the Leren Oslo study, suggested that there was “some support” for considering diet as part of many other factors. No other study suggested that any evidence had been found for “the diet-heart hypothesis” and many voiced extreme concern about repeating their dietary interventions. There are few stronger cautions than: “a low-fat diet has no place in the treatment of myocardial infarction.”
As our paper says: “The present review concludes that dietary advice not merely needs review; it should not have been introduced.”
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