Replacing butter with vegetable oil may not decrease heart disease risk

A research team has unearthed more evidence that casts doubt on the traditional “heart healthy” practice of replacing butter and other saturated fats with oils high in linoleic acid, such as corn and vegetable oil.

The findings suggest that using vegetable oils high in linoleic acid might be worse than using butter when it comes to preventing heart disease, though more research needs to be done on that front. This latest evidence comes from an analysis of previously unpublished data of the Minnesota Coronary Experiment (MCE) conducted nearly 50 years ago, as well as a broader analysis of published data from all similar trials of this dietary intervention.

The analyses show that interventions using linoleic acid-rich oils failed to reduce heart disease and overall mortality even though the intervention reduced cholesterol levels. In the Minnesota study, participants who had greater reduction in serum cholesterol had higher rather than lower risk of death.

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 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, randomised 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.

The largest of these trials, the MCE study, was conducted by researchers at the University of Minnesota between1968 and 1973 and enrolled 9,423 patients. Its results did not appear in a medical journal until 1989. The investigators reported then that a switch to corn oil from butter and other saturated fats did lower cholesterol levels but made no difference in terms of heart attacks, deaths due to heart attacks, or overall deaths.

In the course of investigating the health effects of linoleic acid-rich oils, the team of investigators led by Chris Ramsden, a medical investigator at the National Institutes of Health, came across the MCE study and the 1989 paper.

“Looking closely, we realised that some of the important analyses that the MCE investigators had planned to do were missing from the paper,” said Daisy Zamora, PhD, a researcher in the Department of Psychiatry at the University of North Carolina School of Medicine.

Corn oil group had twice the number of heart attacks as the control

After recovering the data from the trial, the team were able to perform the analyses that had been pre-specified by the MCE investigators but never published, the team confirmed the cholesterol-lowering effect of the dietary intervention. But they also found that in the recovered autopsy records, the corn oil group had almost twice the number of heart attacks as the control group.

Perhaps most strikingly, graphed summaries indicated that in the intervention group, women and patients older than 65 experienced roughly 15 percent more deaths during the trial, compared to their control group counterparts.

“We did not recover the individual patient data underlying those graphs and so we couldn’t determine whether those differences were statistically significant,” said Zamora.

She also cautioned that the other analyses were based on only partial recovery of patient data from the MCE files, so it would be premature to conclude from them that replacing saturated fats with corn oil is actually harmful to heart health.

In a much-cited study published in 2013, however, Ramsden, Zamora and colleagues were able to recover unpublished data from a smaller trial, the Sydney Diet Heart Study, and there they also found more cases of heart disease and death among patients who received a linoleic acid intervention (safflower oil), compared to controls.

Following their recovery of data from the MCE study, the researchers added new data to their existing datasets from the Sydney study and the other three published randomised clinical trials of linoleic acid-based dietary interventions. In a meta-analysis of the combined data, they again found no evidence that these interventions reduced deaths from heart disease or deaths from all causes.