Thursday, 21 April 2016
‘Republished with permission from New Zealand Doctor’
Experts wary of research questioning vegetable oil benefits to heart health
The benefits of eating polyunsaturated fats are questioned in research published today in the BMJ, but local commentators are unconvinced the findings are relevant for everyday diets. New research concludes the polyunsaturated fat linoleic acid in diet lowers cholesterol but not the risk
of death from coronary heart disease. (BMJ 2016;353:i1246)
However, University of Otago professor of nutrition and medicine, Jim Mann, says the study does not have data over a long enough timeframe to determine whether polyunsaturated fat in diet affects mortality. The new research looks at unpublished data from 1968-73 gathered as part of the Minnesota Coronary Experiment, a study of residential mental health patients at seven facilities in Minnesota.
The retrospective study, a double blind randomised control trial, analysed the Minnesota cohort of 9423 participants of which 2355 participants were given an intervention diet of increased polyunsaturated fat intake for between 15 to 56 months.
Other trials attempting to link unsaturated fat, lowered serum cholesterol and cardiac events, such as a 2015 Cochrane review, show substantial reductions in cardiovascular events but never an effect on mortality since the trials were designed to look cardiovascular events, Professor Mann says. This new study says absolutely nothing new, he adds. “I can’t see this paper making any difference other than for the enthusiasts for high fat diets.”
The Minnesota findings have extremely limited applicability, says epidemiologist Rod Jackson of the University of Auckland. Professor Jackson draws attention to how the original authors substituted saturated fat with linoleic acid. “This was not a substitution,” he says. “The intervention group was given a 280 per cent increased linoleic acid intake with a 50 per cent reduction in saturated fat intake.”
Such an extreme diet, he adds, is beyond anything a person outside of a trial setting would eat. Professor Jackson notes the Minnesota Coronary Experiment’s unusual advantage in its complete control of participants’ diets throughout the study, but the unnatural diet compromises applicability of the findings. “Overall it’s not a very clever study.”
Cardiologist Stewart Mann says there is an awful lot of evidence linking saturated fat with coronary heart disease. From that link, a decrease in saturated fat seems to decrease the risk of CHD says Dr Mann, associate professor of cardiovascular medicine at the University of Otago. There is evidence the Mediterranean diet infers lower incidence of CHD, but this does not take into account other factors such as smoking and salt consumption, he says.
Dr Mann, like Professor Jackson, agrees that given the degree of control over the participants’ diets, the Minnesota study is contributing something.
It is helpful and useful that someone has done a randomised controlled trial as they are often the most powerful form of evidence gathering, he adds.
Dr Mann has reservations however, as to whether linoleic acid was the ideal unsaturated fat to study.
Auckland Heart Group cardiologist Chris Ellis emphasises moderation in one’s diet, including fats. “I think that medicine has learned in the last few years that the previous focus on the low fat diet is probably inaccurate,” Dr Ellis says, adding that the new study data are a reflection of that possible inaccuracy.
There was a rather narrow view of getting people to cut down on fat intakes, Dr Ellis says, but we should eat in moderation and not put on weight.
“You’re allowed to have mono-unsaturated, poly-unsaturated and even saturated fat – but not too much.”
Re-evaluation of the traditional diet-heart hypothesis: analysis of recovered data from Minnesota Coronary Experiment (1968-73) - BMJ 2016;353:i1246
Dietary fats: a new look at old data challenges established wisdom - BMJ 2016;353:i151