Soybean oil, which is the most commonly used cooking oil in the United States, may contribute to obesity according to a recent study from UC Riverside. The research, published in the Journal of Lipid Research, examined how mice responded to diets high in soybean oil and found that most gained significant weight. However, genetically engineered mice with a modified form of the liver protein HNF4α did not experience the same weight gain.
“This may be the first step toward understanding why some people gain weight more easily than others on a diet high in soybean oil,” said Sonia Deol, a biomedical scientist at UC Riverside and corresponding author of the study.
The study points out that both forms of the HNF4α protein exist in humans but are typically produced under certain conditions such as chronic illness or metabolic stress. This difference, along with other factors like age and genetics, could help explain varying responses to soybean oil consumption.
Frances Sladek, professor of cell biology at UC Riverside, noted: “We’ve known since our 2015 study that soybean oil is more obesogenic than coconut oil. But now we have the clearest evidence yet that it’s not the oil itself, or even linoleic acid. It’s what the fat turns into inside the body.”
The research found that linoleic acid—a main component of soybean oil—is converted into oxylipins. These molecules are linked to inflammation and fat accumulation when consumed in excess. Transgenic mice had fewer oxylipins and healthier livers despite eating a high-fat diet rich in soybean oil. They also showed better mitochondrial function, possibly explaining their resistance to gaining weight.
Further analysis identified specific types of oxylipins derived from linoleic acid and alpha-linolenic acid as necessary for weight gain in regular mice. However, elevated oxylipin levels alone did not cause obesity if other metabolic factors were absent.
Researchers observed lower levels of two enzyme families involved in converting linoleic acid into oxylipins among genetically altered mice. These enzymes function similarly across mammals and vary due to genetics and diet.
Another finding was that only liver oxylipin levels—not those in blood—were associated with body weight changes. This suggests common blood tests may miss early dietary impacts on metabolism.
Soybean oil consumption has grown significantly over the past century in America, now making up nearly 10% of total calories compared to about 2% one hundred years ago. Although soybeans provide plant-based protein and contain no cholesterol, overconsumption—especially from processed foods—may be contributing to chronic metabolic problems.
The study also reported higher cholesterol levels in mice consuming soybean oil despite its lack of cholesterol content.
Researchers plan further investigation into how oxylipin formation leads to weight gain and whether similar effects are seen with other oils high in linoleic acid like corn or sunflower oils.
“Soybean oil isn’t inherently evil,” Deol stated. “But the quantities in which we consume it is triggering pathways our bodies didn’t evolve to handle.”
While there are currently no plans for human trials, researchers hope these findings will inform future studies and nutrition policy discussions.
“It took 100 years from the first observed link between chewing tobacco and cancer to get warning labels on cigarettes,” Sladek added. “We hope it won’t take that long for society to recognize the link between excessive soybean oil consumption and negative health effects.”
