Replacing saturated fats with vegetable oils rich in linoleic acid lowers blood cholesterol, but doesn’t curb heart disease risk or help you live longer, suggests a new study. The findings add to doubts around the widely held belief that vegetable oils rich in linoleic acid are good for heart health.
The traditional “diet-heart hypothesis” predicts that replacing saturated fat with vegetable oils rich in linoleic acid will reduce cardiovascular deaths by lowering blood cholesterol levels. Although many studies support this theory, this paradigm has never been causally demonstrated in a randomised controlled trial and thus has remained uncertain for over 50 years. Furthermore, key findings from landmark trials on this topic were not published.
So a team of US researchers, led by Christopher Ramsden at the National Institutes of Health and University of North Carolina, set out to re-examine this hypothesis by analysing recovered data from a large randomised controlled trial that took place 45 years ago.
The Minnesota Coronary Experiment (MCE) followed 9,423 participants from state mental hospitals and a nursing home for up to four and a half years. The trial tested whether replacing saturated fat with vegetable oil rich in linoleic acid (corn oil) reduces the risk of coronary heart disease and death by lowering blood cholesterol levels. The control group ate a diet high in saturated fat.
As expected, the diet enriched with linoleic acid lowered cholesterol levels, but this did not translate to improved survival. In fact, participants who had greater reduction in blood cholesterol had higher, rather than lower, risk of death.
In 2013, the researchers examined unpublished data from a similar trial – the Sydney Diet Heart Study – and found that the risk of death from coronary heart disease was higher in those who replaced saturated fat with vegetable oil rich in linoleic acid.
To test their findings further, the team reviewed and analysed the results of all similar randomised controlled trials – and failed to find any reduction in death from coronary heart disease or other causes.
After emphasising limitations of available data, they reason that collective findings from controlled trials “do not provide support for the central diet-heart tenet that the serum cholesterol lowering effects of replacing saturated fat with linoleic acid translate to reduced risks of coronary heart disease and death.” And they conclude that findings from the MCE “add to growing evidence that incomplete publication has contributed to overestimation of the benefits of replacing saturated fat with vegetable oils rich in linoleic acid.”
The researchers also note that, although small amounts of linoleic acid are essential for health, high intakes are not natural to human diets, and preliminary evidence links by-products of linoleic acid to chronic pain and other diseases.
In a linked editorial, Lennert Veerman, senior lecturer at the University of Queensland, says: “The benefits of choosing polyunsaturated fat over saturated fat seem a little less certain than we thought.”
He points to some study limitations that could have influenced the results, and suggests that “a careful review of the evidence that underpins dietary recommendations is warranted.”
In the meantime, he says “we should continue to eat (and to advise others to eat) more fish, fruits, vegetables, and whole grains. We should avoid salt, sugar, industrial trans fats, and avoid over-eating.”
Objective: To examine the traditional diet-heart hypothesis through recovery and analysis of previously unpublished data from the Minnesota Coronary Experiment (MCE) and to put findings in the context of existing diet-heart randomized controlled trials through a systematic review and meta-analysis.
Design: The MCE (1968-73) is a double blind randomized controlled trial designed to test whether replacement of saturated fat with vegetable oil rich in linoleic acid reduces coronary heart disease and death by lowering serum cholesterol. Recovered MCE unpublished documents and raw data were analyzed according to hypotheses prespecified by original investigators. Further, a systematic review and meta-analyses of randomized controlled trials that lowered serum cholesterol by providing vegetable oil rich in linoleic acid in place of saturated fat without confounding by concomitant interventions was conducted.
Setting: One nursing home and six state mental hospitals in Minnesota, United States.
Participants: Unpublished documents with completed analyses for the randomized cohort of 9423 women and men aged 20-97; longitudinal data on serum cholesterol for the 2355 participants exposed to the study diets for a year or more; 149 completed autopsy files.
Interventions: Serum cholesterol lowering diet that replaced saturated fat with linoleic acid (from corn oil and corn oil polyunsaturated margarine). Control diet was high in saturated fat from animal fats, common margarines, and shortenings.
Main outcome measures: Death from all causes; association between changes in serum cholesterol and death; and coronary atherosclerosis and myocardial infarcts detected at autopsy.
Results: The intervention group had significant reduction in serum cholesterol compared with controls (mean change from baseline −13.8% v −1.0%; P<0.001). Kaplan Meier graphs showed no mortality benefit for the intervention group in the full randomized cohort or for any prespecified subgroup. There was a 22% higher risk of death for each 30 mg/dL (0.78 mmol/L) reduction in serum cholesterol in covariate adjusted Cox regression models (hazard ratio 1.22, 95% confidence interval 1.14 to 1.32; P<0.001). There was no evidence of benefit in the intervention group for coronary atherosclerosis or myocardial infarcts. Systematic review identified five randomized controlled trials for inclusion (n=10 808). In meta-analyses, these cholesterol lowering interventions showed no evidence of benefit on mortality from coronary heart disease (1.13, 0.83 to 1.54) or all cause mortality (1.07, 0.90 to 1.27).
Conclusions: Available evidence from randomized controlled trials shows that replacement of saturated fat in the diet with linoleic acid effectively lowers serum cholesterol but does not support the hypothesis that this translates to a lower risk of death from coronary heart disease or all causes. Findings from the Minnesota Coronary Experiment add to growing evidence that incomplete publication has contributed to overestimation of the benefits of replacing saturated fat with vegetable oils rich in linoleic acid.