In 1968, researchers at the University of Minnesota started a massive project called the Minnesota Coronary Experiment (MCE). They studied more than 9,000 patients in state mental hospitals and nursing homes, comparing the effects of butter vs. corn oil on heart health.
The study ran from 1968 through 1973, but the results weren’t published at the time. Sixteen years later in 1989 they finally reported on the study, but the investigators said that a switch to corn oil from butter made no difference in terms of heart disease or deaths.
Researchers today looking at the data, have found that the people who ate corn oil had a greater reduction in serum cholesterol but had a higher risk of death from heart disease.
Using the recovered data that had been stored away for decades in files and on magnetic tapes but never published, the team found that in the recovered autopsy records, the corn oil group had almost twice the number of heart attacks as the control group who ate butter.
The findings are published this week in the British Medical Journal.
“Altogether, this research leads us to conclude that incomplete publication of important data has contributed to the overestimation of benefits – and the underestimation of potential risks – of replacing saturated fat with vegetable oils rich in linoleic acid,” said co-first author Daisy Zamora, PhD, a researcher in the Department of Psychiatry at the UNC School of Medicine.
“Looking closely, we realized that some of the important analyses that the MCE investigators had planned to do were missing from the paper,” Zamora said.
The team was originally just investigating the health effects of linoleic acid-rich oils, when the lead researcher, Chris Ramsden, a medical investigator at the National Institutes of Health, came across the MCE study and the 1989 paper.
Researchers were able to uncover the decades-old data with the help of Robert Frantz, the son of the deceased MCE principal investigator.
The team also uncovered a master’s thesis written by Steven K. Broste, a student of one of the original investigators. Graphed summaries contained in Broste’s thesis indicated that in the corn oil group, women and patients older than 65 experienced roughly 15 percent more deaths during the trial, compared to their butter-eating counterparts.
The belief that replacing saturated fats with vegetable oils improves heart health dates back to the 1960s, when studies began to show that this dietary switch lowered blood cholesterol levels. Since then, some studies, including epidemiological and animal studies, have suggested that this intervention also reduces heart attack risk and related mortality. In 2009, the American Heart Association reaffirmed its view that a diet low in saturated fat and moderately high (5-10 percent of daily calories) amounts of linoleic acid and other omega-6 unsaturated fatty acids probably benefits the heart.
However, randomized controlled trials – considered the gold standard for medical research – have never shown that linoleic acid-based dietary interventions reduce the risk of heart attacks or deaths.
Linoleic acid-rich oils include corn, safflower, soybean, sunflower, and cottonseed oils.
Why linoleic acid-containing oils would lower cholesterol but worsen or at least fail to reduce heart attack risk is a subject of ongoing research and lively debate. Some studies suggest that these oils can – under certain circumstances – cause inflammation, a known risk factor for heart disease. There is also some evidence they can promote atherosclerosis when the oils are oxidized.