Replacing saturated, animal-derived fats such as butter, lard and cheese with vegetable oils that are rich in linoleic acid – such as those derived from corn, sunflower, safflower, cottonseed or soybean – has long been thought to decrease cholesterol and, as a result, coronary heart disease and overall mortality rates.
But a review ofthe Minnesota Coronary Experiment (MCE) - a study that was originally conducted almost 50 years ago has cast this belief - which has formed a key component of dietary advice for years - into doubt.
Daisy Zamora, a researcher at the University of North Carolina and co-author of the review that was published yesterday in the British Medical Journal said: "Altogether, this research leads us to conclude that incomplete publication of important data has contributed to the overestimation of benefits - and the underestimation of potential risks - of replacing saturated fat with vegetable oils rich in linoleic acid."
According to current recommendations from the World Health Organisation (WHO), saturated fats should make up less than 10% of total energy, although it is currently conducting a systematic review of evidence for its forthcoming draft guidance on saturated and trans fats. Once this is complete a public consultation on the guidelines will be held, which is expected to be before summer 2016, a WHO spokesperson told FoodNavigator.
The Minnesota Coronary Experiment
Conducted between 1968 and 1973, the MCE was the largest and, according to the review's authors, “most rigorously executed” dietary trial that looked at the effects of replacing saturated with unsaturated fats in 9,423 patients in six mental hospitals and one state-run nursing home in Minnesota.
The results were originally published in 1989 with the researchers concluding that switching from butter to corn oil lowered cholesterol levels but made no difference to rates of heart attacks, death from heart attacks or overall mortality rate.
But, for unknown reasons, the original researchers did not publish several critical analyses. Zamora and the team of researchers therefore put the recovered data into context and conducted a systematic review and meta-analysis of the trials.
They found that the corn oil group had almost twice as many heart attacks as the control: 41% of participants in the vegetable oil group had at least one myocardial infarct, compared with only 22% of participants in the control group, nor did participants in the intervention group did not have less coronary atherosclerosis or aortic atherosclerosis. The risk of death was also greater among older participants, increasing 35% with every 30 mg/dL decrease in serum cholesterol, even when adjusted for changed in body weight and blood pressure, as markers of frailty.
“The partial recovery of 149 heart and aorta autopsy files provides an intriguing clue that the intervention might have had unfavourable effects. As 146 heart and aorta files and the data on the full cohort of 295 autopsied brains remain missing, however, one cannot draw conclusions from these provisional findings,” write the authors. Given the unlikelihood of a study of this size being replicated again, they say it is essential that the missing autopsy files are recovered and analysed.
The consequences of this incomplete knowledge may have skewed public health policy.
"Whatever the explanation for key MCE data not being published, there is growing recognition that incomplete publication of negative or inconclusive results can contribute to skewed research priorities and public health initiatives," they write.
The authors had previously found similar results in a previous study, the Sydney Diet Heart Study.
Oxidation may be behind the “seemingly paradoxical results” of both studies, they write. “Consumption of vegetable oils rich in linoleic acid produces a wide range of biochemical consequences, including qualitative changes in lipoprotein particle oxidation that could plausibly increase risk of coronary heart disease. Hence the clinical effects of replacing saturated fat with vegetable oils could reflect the net impact of decreasing low density lipoprotein while increasing its susceptibility to oxidation.”
An atypical nutritional phenomenon
Diets high in linoleic acids from vegetable oils are also a “recent and atypical nutritional phenomenon,” write the researchers.
Individuals who eat only minimally processed foods, as everyone did 100 years ago, would have consumed around 2-3% of calories from linoleic acid. In today’s Western societies with its reliance on heavily processed and packaged foods, however, this rises to around 17 g per day, or 7% of calories.
Reacting to the study, associate medical director at the British Heart Foundation, Professor Jeremy Pearson said it was an “interesting” study. “However more research and longer studies are needed to assess whether or not eating less saturated fat can reduce your risk of cardiovascular death. In the meantime, it is advised to eat a balanced diet rich in fish, fruit, vegetables and whole grains to maintain a healthy heart.”
The saturated fat debate has been yo-yo-ing back and forth in recent years. Last year the authors of a Harvard review concluded that replacing intake of dietary saturated fat with unsaturated fats may still be the best for heart health.
Source: British Medical Journal
"Re-evaluation of the traditional diet-heart hypothesis: analysis of recovered data from Minnesota Coronary Experiment (1968-73)"
First published online 12 April 2016, doi.org/10.1136/bmj.i1246
Authors: Christopher E Ramsden, Daisy Zamora, Sharon Majchrzak-Hong et al.