Boxes of butter are seen on the shelves of a Japanese supermarket in Singapore October 28, 2015. Reuters/Thomas White

Three months after the US Department of Agriculture declared butter as safe in its Dietary Guidelines for Americans, 2015 report, more studies support the favourite spread as a good alternative even for cooking.

A new study by the University of North Carolina (UNC) School of Medicine and the National Institutes of Health, published in the British Medical Journal, warns that using vegetable oil could lead to worse health outcomes compared to butter. The belief is that oils are better because it could lower cholesterol levels.

Vegetable oil is high in linoleic acid, which makes it worse than butter to prevent heart ailments. The basis of that is analysis of previously unpublished data of a large controlled trial in Minnesota about 50 years ago and a broader analysis of published data on similar trials that substituted vegetable oil for butter.

The study had 9,500 elderly residents of nursing homes and mental hospital. It focused on the serum cholesterol levels of the participants. It found that participants who used oils rich in linoleic acid, such as corn oil, soybean oil and sunflower oil, died earlier that those who used butter, reports the American Council on Science and Health.

Dr Daisy Zamora, researcher at UNC School of Medicine and co-first author of the study, says that interventions that used oils rich in linoleic acid failed to reduce heart ailments and overall mortality even if cholesterol levels were reduced by the intervention. The participants in the Minnesota study, who have greater reduction in serum cholesterol, had higher, instead of lower risk of death.

“Incomplete publication of important data has contributed to the overestimation of benefits – and the underestimation of potential risks – of replacing saturated fat with vegetable oil rich in linoleic acid,” explains Zamora.

But Maryam Farvid, a visiting scientist from Harvard, says the study could not be used to make conclusions about a healthy diet. She cites other research which state that the risk of heart ailment would be lowered if saturated fats, mostly from red meat and dairy fat, are replaced by unsaturated fans from liquid vegetable oils.

Zamora admits that the other analyses were based on only partial recovery of data from the Minnesota study, making it premature to conclude that replacing saturated fats with corn oil is more harmful to cardiovascular health. The cholesterol-lowering ability of oils with linoleic acid which worsens or fails to lower risks of heart attack is the subject of an ongoing research and debate among experts.