Archived research data on saturated fats which examines the relationship between heart disease risk, lower cholesterol levels and vegetable oils has been found to contain results that are contrary to current thinking. This has led the researchers to question whether other studies have been shelved due to unexpected results and that the weight of evidence may suggest that the benefits of vegetable oils have been overestimated.
Current thinking suggests that saturated fats, such as butter, should be replaced with unsaturated fats which are found in vegetable oils which are rich in linoleic acid. This has been shown to lower cholesterol and prevent build up of plaque on artery walls which narrow the arteries. Preventing the narrowing of the arteries is thought to prevent heart health issues which would therefore extend life.
The data, which was collected 45 years ago, suggests that this current thinking may be wrong. The results of reopening and re-examining the data have been published in the BMJ and may well cause a stir in the scientific community.
The study was designed to uphold initial randomised controlled trials which had shown a reduction in blood cholesterol when linoleic acid replaced saturated fats. Observational evidence had pointed to elevated cholesterol levels being responsible for coronary problems but there was no direct evidence that the events when linked would help to prevent heart disease. The study that has been re-examined was the Minnesota Coronary Experiment or MCE, and it was designed to prove this theory. However the results of the study were not as expected and therefore it was reported as no difference between the groups and very little publicity accompanied the publication of the study. The study, however, was the largest of its kind which examined the effect on cholesterol when saturated fat was replaced by linoleic acid and rigorously followed.
The study chose to use participants who were in nursing homes and mental institutes, following them for a maximum of 4.5 years. This enabled them to control their diets rigidly. The control group ate a diet high in saturated fat while the other group ate corn oil which is rich in linoleic acid. The study was also able to record coronary, aortic and cerebrovascular atherosclerosis through post mortem if participants died. The post mortem were carried out blind, but only 51% of the results have been recovered so far.
Some of the results of the study were expected: the consumption of linoleic acid rather than saturated fats did indeed lead to a reduction of cholesterol, however this drop in cholesterol did not appear to correlate with an increased chance of survival. This group had the highest risk of death. The number of people in the study were 9,570, a good size sample.
This team of researchers had previously examined old data from the Sydney Diet Heart Study (1966-73). The results agreed with MCE in that the group who had replaced saturated fat with linoleic acid had a higher risk of death from coronary heart disease. The study suggested that it is now known that linoleic acid can cause coronary risks to smokers or older people which may account for some of the results, however they would like to track down the missing post mortem results to support their theory.
The researchers are now concerned that current dietary advice is incorrect. They are also concerned that other research studies may also have remained unpublished because the results were not as people expected but which may support the two studies that have already been examined.
The discovery of the role of sugar in obesity has fuelled new interest in dietary needs and old thinking is being reviewed in the light of new research. For now, the best advice for heart health is to eat plenty of fruit and vegetables, fish and wholegrains and avoid consuming too much salt, sugar and trans fats. Maintaining a healthy weight and exercising regularly will help your heart to stay in good shape.
This study has been published in full online.
Ramadan, C.E., et al., Re-evaluation of the traditional diet-heart hypothesis: analysis of recovered data from Minnesota Coronary Experiment (1968-73), BMJ 2016; 353:i1246