Although many studies support the theory that swapping saturated fat for vegetable oils rich in linoleic acid cut cardiovascular deaths by lowering blood cholesterol levels it has never been casually demonstrated in a randomised controlled trial, said researchers.
The team led by Christopher Ramsden from the National Institutes of Health and University of Carolina re-examined data from the Minnesota Coronary Experiment, a large randomised controlled trial which followed 9,423 residents of one nursing home and six state mental hospitals for four and a half years from 1968.
While people on the study diet swapped saturated fat for the oils rich in linoleic acid the control group ate a diet high in saturated fat.
Their research published in the British Medical Journal (BMJ) found that replacing saturated fat with linoleic acid – such as corn, sunflower, safflower, cotton seed or soybean oils – lowered serum cholesterol but did not lead to a lower risk of death from coronary heart disease.
Instead, paradoxically trial participants who had greater reductions in their serum cholesterol had a higher, rather than a lower risk of death.
The researchers said that although limited the evidence from the randomised trials showed “no indication of benefit on coronary heart disease or all cause mortality from replacing saturated fat with linoleic acid rich vegetable oils.”
They said one reason why the reduction in serum cholesterol did not lead to improved outcomes was that replacing saturated fat with linoleic acid decreases low density lipoprotein which is “often thought a causal mediator of coronary heart disease”.
High intakes of linoleic acid could also have adverse effects on people who are prone to linoleic acid oxidation, such as heavy smokers or drinkers and older people.
Researchers also pointed out that linoleic acid mainly comes from highly concentrated vegetable oils used in cooking, frying and heavily processed food in modern diets and is much higher than the amount consumed in “natural diets” without any addition of oil.