Replacing saturated fat with common vegetable oils may not curb heart disease risk or help patients live longer, even though cholesterol levels are lowered, according to a US study.
The findings add to doubts around the widely held belief that vegetable oils are good for cardiovascular health, suggested the study authors in the British Medical Journal.
The traditional “diet-heart hypothesis” predicts that replacing saturated fat with vegetable oils, rich in linoleic acid (corn oil), will reduce cardiovascular deaths by lowering blood cholesterol levels.
Researchers from the US National Institutes of Health and University of North Carolina set out to examine the hypothesis by analysing data recovered from a large trial that took place 45 years ago.
The Minnesota Coronary Experiment followed 9,423 participants from state mental hospitals and a nursing home for up to four and a half years.
It tested whether replacing saturated fat with vegetable oil reduced the risk of coronary heart disease and death by lowering blood cholesterol levels. A 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.
The researchers also examined unpublished data from a similar trial – the Sydney Diet Heart Study – and found the risk of death from coronary heart disease was higher in those who replaced saturated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 trials, failing to find any reduction in death from coronary heart disease or other causes.
They concluded 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”.
They added that, although small amounts of linoleic acid are essential for health, high intakes are not natural to human diets.
In an editorial in the same journal, Lennert Veerman, senior lecturer at the University of Queensland, said: “The benefits of choosing polyunsaturated fat over saturated fat seem a little less certain than we thought.”
He suggested that “a careful review of the evidence that underpins dietary recommendations is warranted”, but in the meantime he said patients should continue to be advised to eat “more fish, fruits, vegetables, and whole grains” and “avoid salt, sugar, industrial trans fats”.