Long-term tests in mice indicate that a genetically modified (GM) brand of soybean oil causes less obesity and insulin resistance than conventional soybean oil, but doesn’t lead to lower incidences of diabetes or fatty liver. The study, led by Poonamjot Deol, Ph.D., and Frances M. Sladek, Ph.D., at the University of California, Riverside, found that coconut oil, which is high in saturated fats, in fact caused fewer negative metabolic effects than either type of soybean oil, or olive oil, which is commonly perceived to be a “healthy” oil. The results have implicated a causative role for a new class of soybean oil–derived compounds in diet-induced obesity, and the authors say further studies are warranted to investigate the link further.
“The take-home message is that it is best not to depend on just one oil source,” concludes Sladek, who is a professor of cell biology at UC Riverside. “Different dietary oils have far-reaching and complex effects on metabolism that require additional investigation.” The researchers published their results today in Scientific Reports, in a paper entitled “Omega-6 and Omega-3 Oxylipins Are Implicated in Soybean Oil-Induced Obesity in Mice.”
Launched in 2014 by DuPont, Plenish is engineered to be low in linoleic acid and has a fatty acid composition—including high oleic acid levels—that is similar to that of olive oil. The latest studies by Sladek, Deol, and colleagues follow on from their previous, work, which indicated that although soybean oil is assumed to be healthy, a diet rich in soybean oil “does in fact increase adiposity, diabetes, insulin resistance, and fatty liver,” they point out in the new Scientific Reports paper.
The team designed a series of high-fat diets, based on coconut oil, conventional soybean oil, or Plenish, and fed them to experimental mice. Each diet had a total fat content similar to that of the typical, 40kcal% American diet. The long-term results showed that coconut oil caused the least amount of weight gain of all the high-fat diets tested. “We found all three oils raised the cholesterol levels in the liver and blood, dispelling the popular myth that soybean oil reduces cholesterol levels,” added Dr. Sladek, professor of cell biology, who led the research.
And while the Plenish-based diet induced less insulin resistance than conventional soybean oil, its effects on diabetes and fatty liver were similar to those of conventional soybean oil. High-fat diets rich in the GM soybean oil also led to similar degrees of liver enlargement and liver dysfunction as olive oil. “Plenish, which has a fatty acid composition similar to olive oil, induced hepatomegaly, or enlarged livers, and liver dysfunction, just like olive oil,” Deol stated. “In our mouse experiments, olive oil produced essentially identical effects as Plenish—more obesity than coconut oil, although less than conventional soybean oil—and very fatty livers, which was surprising as olive oil is typically considered to be the healthiest of all the vegetable oils.”
The studies also indicated a positive correlation between oxylipin metabolites of linoleic acid and obesity. Previous work has identified a link between oxylipins and detrimental health effects, they note. “Oxylipins in general, as bioactive signaling lipids, are increasingly being associated with inflammation, vascular permeability, and cardiovascular disease as well as diabetes, obesity-induced hypertriglyceridemia, and insulin signaling.” They acknowledge that more work will be needed to understand the link between oxylipins, obesity and other aspects of the metabolic syndrome.
Nevetheless, the UC Riverside researchers note that the global increase in soybean consumption worldwide parallels the rise in obesity. Figures by the U.S. Centers for Disease Control and Prevention (CDC) indicate that 35% of the adult U.S. population is obese, and Dr. Sladek et al. go as far as to suggest that this obesity epidemic may be linked with soybean oil intake. They also speculate that the detrimental effects of animal fat that are observed in experimental rodents may also be related to the high levels of linoleic acid that is contained in the soybean meal fed to farm animals.
“The vast majority of diet-induced obesity studies use lard as the source of fat and assume that they are looking at the effects of saturated fat, as well as cholesterol,” the authors write. “In the U.S., lard comes from animals that are typically fed soybean meal and consequently the levels of LA [linoleic acid] in lard can be quite high (11% or higher). Hence, it is possible that some of the metabolic effects in the literature attributed to saturated fats in these lard-based studies could actually be due to high LA from soybean oil, as others have found with farmed salmon.”
Linoleic acid is an essential fatty acid, and the body requires about 1–2% linoleic acid from the diet. However, people in the U.S. typically consume about 8–10% linoleic acid in their diets. The researchers now recommend that people avoid conventional soybean oil as far as possible. “This might be difficult as conventional soybean oil is used in most restaurant cooking and found in most processed foods,” Deol added.
“But with its effects on the liver, Plenish would still not be my first choice of an oil,” Sladek noted. “Of all the oils we have tested thus far, coconut oil produces the fewest negative metabolic effects, even though it consists nearly entirely of saturated fats. Coconut oil does increase cholesterol levels, but no more than conventional soybean oil or Plenish.”
UC Riverside notes that while DuPont provided Plenish oil for the study, the company played no role in designing the experiments or in preparing the manuscript, and had no knowledge of the results before publication.