Research from 2016 casts doubt on the “heart-healthy” practice of replacing butter and other saturated fat sources with vegetable oils rich in linoleic acid, such as corn oil. The findings, published in the BMJ, found using corn oil in place of other fats failed to reduce heart disease deaths and overall mortality. This 2016 evidence comes from a meta-analysis of previously unpublished data from a large controlled trial, the Minnesota Coronary Experiment (MCE), which was conducted by researchers at the University of Minnesota between 1968 and 1973. The MCE included 9,423 patients, aged 20 to 97, residing in six state mental hospitals and one state-run nursing home. The participants were divided into two groups: the first group was put on a diet that replaced saturated fat sources with linoleic acid-rich corn oil and corn oil polyunsaturated margarine; the second group ate a control diet that included usual amounts of saturated fat from animal fats and trans-fats from common margarines and shortenings. After analyzing the reports of deaths from any cause, deaths from heart disease, and the presence and severity of atherosclerosis from autopsies, the authors of the 2016 meta-analysis found that:
Being on the high-linoleic acid diet resulted in a 50% reduction in saturated fat intake and a 2.8-fold increase in linoleic acid consumption compared with the control diet. Being on the control diet resulted in no change in saturated fat intake and a slight increase in linoleic acid intake.
Participants on the high-linoleic acid diet experienced a statistically significant reduction in blood cholesterol levels compared with the participants on the control diet. In addition, those who followed the diet most strictly had the greatest reductions in cholesterol levels.
Despite their lower cholesterol levels, participants eating the high-linoleic acid diet did not have fewer deaths from any cause, less coronary artery disease, or fewer heart attacks compared with those eating the ordinary high saturated- and trans-fat diet.
While these findings go against widely held beliefs about saturated fats, cholesterol, and heart disease, the authors of the 2016 meta-analysis pointed to similar findings from a re-examination of data from a dietary trial in which safflower oil, another linoleic acid-rich oil, was used. How high-linoleic acid vegetable oils bring cholesterol levels down, but fail to reduce cardiovascular disease and deaths, is a subject of ongoing research. Some studies suggest that linoleic acid, which is prone to oxidation, can trigger inflammation and vascular damage in its oxidized state. While we wait for more research to come in, it’s important to remember that unprocessed plant fat sources like nuts, seeds, olives, and avocadoes are an important part of a heart-healthy diet.