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

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-analyzed data from a randomized 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.