Linoleic acid, the primary component of many vegetable oils, has long been thought to be more heart healthy than butter when used for cooking. Now, from a collaboration between the University of North Carolina School of Medicine and the National Institutes of Health (NIH), scientists are saying that linoleic acid-based vegetable oils are not as healthy as previously thought. Even more surprising, there could be a chance that they are more harmful than butter.
Linoleic acid-rich vegetable oils come from corn, safflower, soybeans, sunflower, and cottonseed. Co-first author Daisy Zamora, PhD, describes the misunderstanding of their health qualities as an “overestimation of benefits and the underestimation of potential risks.” In their study published in the British Medical Journal,
UNC and NIH researchers dug up previously unpublished data from two clinical trials, one from over 50 years ago, to see for once and for all the true impact of linoleic acid-rich oils on heart health and mortality.
The battle between unsaturated fats like linoleic acid and saturated fats like butter began in the 1960s, where studies showed linoleic acid lowering blood cholesterol levels. A clinical trial conducted in Minnesota between 1968 and 1973 recruited 9,423 patients in a study called the Minnesota Coronary Experiment (MCE). While this study’s findings, not published in a medical journal until 1989, showed a correlation between reduction in serum cholesterol and choosing linoleic acid-rich oils over butter, the oils were not at all connected to a reduction in heart disease and overall mortality.
Despite the absence of this connection from the MCE, various epidemiological and animal studies conducted since the 1960s have displayed findings that lowered blood cholesterol levels are linked to a reduced risk for heart attack. Hence, the idea that linoleic acid-based oils reduce heart attack risk and heart attack-related death due to their cholesterol-lowering abilities was born and nurtured.
The UNC/NIH team started their investigation with unpublished data from the Minnesota study, performing analyses that had been intended to be completed but never came to fruition.
First, the researchers were able to confirm the data produced by the 1960s study, that the linoleic acid-based oils do have a cholesterol-lowering effect. However, autopsy records from patients consuming corn oil instead of butter showed an increase in heart attack prevalence by almost twofold. Most shockingly, they found that the data actually showed a correlation between decreased levels of cholesterol and a higher
risk of death. The only problem: they couldn’t determine if the results were significantly significant because they could not find the individual patient data from the study.
Next came data from the Sydney Diet Heart Study, published in 2013, which also had unpublished data to be uncovered by UNC and NIH scientists. Specifically focusing on study participants consuming safflower oil versus butter, this study found even more cases of heart disease and death among the former group.
Finally, combining newly collected data with a meta-analysis of three other randomized clinical trials, the MCE study, and Sydney Diet Heart Study, the researchers were finally able to conclude that there was no evidence that these interventions reduced deaths from heart disease or deaths from all causes.
Despite their extensive investigation, the UNC/NIH team is still unsure about why linoleic acid-rich oils reduce cholesterol levels but do not reduce the risk of heart attack or heart attack-related death. They are not finished, though; the next factors to consider in the link between linoleic acid-rich oils and heart health are inflammation and atherosclerosis promotion through oxidation. To be continued!
Source: University of North Carolina Health Care