Soybean oil, which is the most commonly used cooking oil in the United States, has been linked to obesity in a new study from researchers at the University of California, Riverside. The research found that mice fed a high-fat diet rich in soybean oil gained significant weight. However, genetically engineered mice with a slightly different form of a liver protein did not experience the same weight gain.
The study focused on a liver protein called HNF4α, which exists in two forms in humans. The alternative form is typically produced only under specific conditions such as chronic illness or metabolic stress. Researchers believe that this variation may help explain why some people are more prone to gaining weight when consuming diets high in soybean oil.
“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 published in the Journal of Lipid Research.
Frances Sladek, professor of cell biology at UC Riverside, explained: “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.”
Linoleic acid, a major component of soybean oil, is converted by the body into molecules called oxylipins. High intake of linoleic acid can increase oxylipin levels, which are associated with inflammation and fat accumulation. The genetically modified mice had fewer oxylipins and healthier livers despite eating diets high in soybean oil. They also showed better mitochondrial function compared to regular mice.
The researchers identified specific types of oxylipins derived from linoleic acid and alpha-linolenic acid—both found in soybean oil—as necessary for weight gain in normal mice. However, transgenic mice on low-fat diets also had elevated oxylipins but did not become obese, suggesting other factors contribute to obesity beyond just these molecules.
Further analysis showed that altered mice had lower levels of enzymes responsible for converting linoleic acid into oxylipins. These enzymes are present across all mammals and their activity varies based on genetics and diet.
Importantly, only oxylipin levels measured in liver tissue—not blood—were linked to body weight changes. This suggests standard blood tests may not detect early metabolic effects related to diet.
Soybean oil consumption has increased significantly over the past century—from about 2% to nearly 10% of total calories consumed by Americans today. While soybeans provide plant-based protein and their oil contains no cholesterol, excessive intake of linoleic acid from sources like ultra-processed foods may contribute to chronic metabolic diseases.
Despite its lack of cholesterol content, the UC Riverside study also found an association between soybean oil consumption and higher cholesterol levels in mice.
Researchers plan further studies on how oxylipin formation leads to weight gain and whether similar effects occur with other oils high in linoleic acid such as corn, sunflower, and safflower oils.
“Soybean oil isn’t inherently evil,” Deol said. “But the quantities in which we consume it is triggering pathways our bodies didn’t evolve to handle.”
Although there are currently no plans for human trials based on these findings, researchers hope their work will guide future research efforts and influence nutrition policy decisions.
“It took 100 years from the first observed link between chewing tobacco and cancer to get warning labels on cigarettes,” Sladek said. “We hope it won’t take that long for society to recognize the link between excessive soybean oil consumption and negative health effects.”
