REPLACING animal fat in the human diet with vegetable oil seems not to lower heart disease risk, and might even boost it, according to a study published Wednesday that challenges a cornerstone of dietary advice.

Switching from saturated to unsaturated Omega-6 fats did result in lower blood cholesterol in a trial with nearly 10,000 participants, it said, but not the expected reduction in heart disease deaths.

In fact, those with a greater reduction in cholesterol "had a higher rather than a lower risk of death," according to the research published by the medical journal BMJ.

For 50-odd years, animal fat in meat, butter, cheese and cream has been the bad boy of the diet world -- blamed for boosting artery-clogging cholesterol linked to heart disease and stroke.

In 1961, the American Hearth Association recommended vegetable oils replace saturated fats -- a position it still holds even as some research has started to challenge that hypothesis.

The World Health Organization also advises that saturated fats should comprise less than 10 percent of total energy intake.

For decades now, the world has viewed full-fat milk and bacon with suspicion and replaced pork with chicken, and butter with plant-based margarines and cooking oils.

But in the past few years, researchers have started poking holes in the "fat is bad" hypothesis.

The new study, led by Christopher Ramsden at the National Institutes of Health, re-analysed data from a randomised controlled trial conducted 45 years ago with 9,423 residents of state mental hospitals and nursing home in Minnesota.

This is a type of experiment -- generally considered highly reliable -- in which people are randomly divided into groups to receive, or not, the treatment being studied.

'Less certain than we thought'

Part of the Minnesota group had their intake of saturated fat replaced with corn oil, while the rest ate a diet high in animal fat.

"As expected, the diet enriched with linoleic acid (a fatty acid found in plant oils) lowered cholesterol levels," said a statement by The BMJ.

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 team also looked at other randomised controlled trials, and found no evidence anywhere to support the hypothesis that vegetable oils curb heart disease.

"The benefits of choosing polyunsaturated fat over saturated fat seem a little less certain than we thought," Lennert Veerman, a lecturer at the University of Queensland School of Public Health commented on the study.

Further research is needed, he added, to determine whether all Omega-6 type fats provide similar results.

"While we wait for further clarification, we should continue to eat more fish, fruits, vegetables and whole grains," Veerman wrote.

In January, updated US dietary guidelines reiterated that saturated fats should make up less than 10 percent of a day's food intake -- a recommendation that now "will be under increased scrutiny", according to Veerman.

"If blood cholesterol values are not a reliable indicator of risk of cardiovascular disease, then a careful review of the evidence that underpins dietary recommendations is warranted," he wrote in The BMJ.

Other experts stressed there was an established link between high cholesterol and the risk of heart attack or stroke.

"More research and longer studies are needed to assesses whether or not eating less saturated fat can reduce your risk of cardiovascular death," said Jeremy Pearson of the British Heart Foundation.