Soybean oil is the major vegetable cooking oil used in the US, and its popularity is on the increase worldwide. Rich in unsaturated fats, especially linoleic acid, soybean oil can induce obesity, diabetes, insulin resistance and fatty liver in mice.

The researchers tested Plenish, a genetically-modified (GM) soybean oil developed by DuPont. Plenish is engineered to have low linoleic acid, resulting in an oil similar in composition to olive oil, the basis of the Mediterranean diet and considered to be healthful.

The study, published in Nature Scientific Reports, is the first to compare the long-term metabolic effects of conventional soybean oil to those of Plenish. The study also compares both conventional soybean oil and Plenish to coconut oil, which is rich in saturated fatty acids and causes the least amount of weight gain among 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,” says Frances Sladek, a professor of cell biology, who led the research project.

Next, the researchers compared Plenish to olive oil. Both oils have high oleic acid, a fatty acid believed to reduce blood pressure and help with weight loss.

“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,” says Poonamjot Deol, the co-first author of the research paper. “Plenish, which has a fatty acid composition similar to olive oil, induced hepatomegaly, or enlarged livers, and liver dysfunction, just like olive oil.”

Sladek explains that some of the negative metabolic effects of animal fat that researchers often see in rodents could actually be due to high levels of linoleic acid, given that most US farm animals are fed soybean meal.

“This could be why our experiments are showing that a high-fat diet enriched in conventional soybean oil has nearly identical effects to a diet based on lard,” she says.

In response to the study, a spokesperson for DuPont tells NutritionInsight: “We’re pleased that the study found Plenish high oleic soybean oil is similar to olive oil and induces less obesity and insulin resistance than commodity soybeans.”
 
“In developing Plenish high oleic soybeans, we set out to re-invent soybean oil to increase the level of oleic acid (monounsaturated fat) and reduce linoleic and linolenic acids (polyunsaturated fats) to help improve stability and create a healthier oil profile relative to commodity soybeans,” the spokesperson adds.

The researchers further speculate that the increased consumption of soybean oil in the US since the 1970s could be a contributing factor to the obesity epidemic. According to the Centers for Disease Control and Prevention, 35 percent of adults are obese. In some ethnic groups, however, such as Hispanics and African-Americans, between 42 percent and 48 percent of the population is obese. Obesity, officially designated by the American Medical Association in 2013 as a disease, is linked to diabetes, heart disease and cancer.

“Our findings do not necessarily relate to other soybean products like soy sauce, tofu, or soy milk – products that are largely from the water-soluble compartment of the soybean; oil, on the other hand, is from the fat-soluble compartment,” Sladek says. 

Deol and Sladek recommend avoiding conventional soybean oil as much as possible.

“This might be difficult as conventional soybean oil is used in most restaurant cooking and found in most processed foods,” Deol says. “One advantage of Plenish is that it generates fewer transfats than conventional soybean oil.”

“But with its effects on the liver, Plenish would still not be my first choice of an oil," Sladek says. “Indeed, I used to use exclusively olive oil in my home, but now I substitute some of it for coconut oil. 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.”

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