Americans have been told to eschew saturated fat for decades, in part because the American Heart Association has said consuming high amounts can increase cholesterol levels — and not the good kind. So in the late 60s, researchers conducted a trial to see if replacing saturated fats with vegetable oil rich in a healthier polyunsaturated fat, called linoleic acid, could lower levels of LDL “bad” cholesterol and reduce rates of coronary heart disease and death. All signs seemed to point to yes, but according to a new study published in BMJ, this hypothesis stands on shakier ground than people have been led to believe.

The trial in question is the Minnesota Coronary Experiment (MCE), the largest and perhaps most rigorously controlled trial on the cholesterol-lowering effects of linoleic acid, according to the study authors. For almost 50 years, these results have fueled the traditional “diet-heart hypothesis,” or the idea that vegetable oils are healthier than butter. The authors of the current study said this hypothesis had never been tested for causality in subsequent trials, which only cast doubt on the effectiveness of the food swap. What’s more, they recovered several documents and raw data, including grants, progress reports, abstracts, and original researcher Steven Kent Broste’s master thesis — a detailed analysis of the full MCE trial — that had never previously been published, or even considered part of the scientific evidence base.

Alarm bells going off, the authors evaluated this “new” data involving the 9,423 participants from state mental hospitals and a nursing home. These locations were ideal partly because there was less of a chance participants would miss meals. And the purpose of it all was to see if replacing saturated fat with linoleic acid reduced cardiac events and outcomes, including heart attacks, strokes, and death.

In place of the usual cooking fats, the hospitals used liquid corn oil in food items like salad dressings, lean ground beef, milk, and cheeses. Margarine was also used in place of butter, so that overall, participants consumed about 50 percent less saturated fat and 280 percent more linoleic acid. Meanwhile, a control group was put on a high-fat diet.

Results at the time showed that eating more oils in place of fat lowered cholesterol levels, but it didn’t translate to survival, the authors said. In fact, participants who saw a greater reduction in blood cholesterol had a higher risk of death. In the Sydney Heart Trial, a study that was similar to MCE from which the authors also recovered unpublished findings, heart disease risk was higher in those who ate more linoleic acid.

The risk of death associated with lower cholesterol was driven by participants aged 65 and older, the authors said — this group was 35 percent more likely to die than their younger counterparts. But when the authors conducted a separate analysis that adjusted for known markers of frailty, which is associated with low cholesterol and death, the risk for death was still high.

Since the analysis was observational in nature, “it is not possible to disentangle the effect of changes in [LDL] cholesterol due to diet from changes due to other factors,” first author of the study Daisy Zamora, a researcher at the University of North Carolina at Chapel Hill, told Medical Daily. She also pointed to some Sydney Trial evidence that suggested high linoleic acid intake could “adversely affect those who are known to have increased linoleic acid oxidation, including older adults, smokers, heavy drinkers, and those with established coronary heart disease.”

It’s important to note that the MCE diet contained almost twice as much linoleic acid as the average American diet, and today, only a small percent of the U.S. population’s diet includes that much. The authors warned, too, that the results should not be generalized to nuts or other unprocessed foods containing linoleic acid.

Considering some reports have also found that hospitals serve food lacking in nutritional value — waterlogged veggies and mystery meat, as CBC News put it — we also asked Zamora whether this might have affected cardiac results.

“In order for the nutritional value of hospital food to bias the results, there would need to be an imbalance where one group receives better/worse food than the other group,” Zamora said. “If both groups receive poor quality food, then there is no imbalance and the experiment tests what it is meant to test: [the] replacement of saturated fat with corn oil.”

She continued, however, that common margarine and shortening — also major sources of potentially harmful trans fat — were present in the control diet, but not the intervention diet. The U.S. Department of Agriculture had a surplus of these food items and provided them to hospitals for free, making it possible that the increased access favored the intervention group.

Zamora and her colleagues conclude that there is not enough evidence to draw strong conclusions about the health effects of vegetable oils. Zamora added that she has been surprised by the lack of causal evidence for a hypothesis that has been widely circulated. That doesn’t necessarily mean it’s wrong — it just means we don’t have the whole story yet.

“It is interesting to speculate how major nutrition policy decisions in the last 40 years would have been affected if this research had been published then,” Zamora said.

Linoleic acid is a bioactive compound that neither Zamora nor her peers fully understand when it comes to how it affects the body. Dr. Robert A. Kloner, vice president of translation at the Huntington Medical Research Institute in Pasadena, Calif. and a professor of medication at USC’s Keck School Of Medicine, told Medical Daily that he’s never recommended this specific fat to his patients. His recommendations have been very general, like to avoid fatty foods and eat more fibrous vegetables.

He found the study to be counterintuitive since generally the measures taken to reduce cholesterol, still a significant risk factor for heart disease, improve overall health. That it didn’t in these trials worries him in terms of what readers will take away.

“My concern is that people will interpret lower cholesterol as bad [since the risk of death was higher],” Kloner said. “But the lower LDL cholesterol is, the better. You can draw a straight line down: when it decreases, so do cardiac events.”

Source: Ramsden CE, et al. Re-evaluation of the traditional diet-heart hypothesis: analysis of recovered data from Minnesota Coronary Experiment (1968-73). BMJ. 2016.