Soybean oil, the world’s most widely used cooking oil and common in processed foods, may contribute toobesity, according to a study from the University of California, Riverside (UCR). The research, published in theJournal of Lipid Research, explores why diets high in soybean oil cause weight gain in mice and uncovers a key liver protein’s role in this process. Soybean oil is rich in linoleic acid-a fatty acid converted inside the body into oxylipins, which are linked to inflammation and fat accumulation. Previous studies showed soybean oil causes more obesity than coconut oil, but this study reveals it’s not the oil itself or linoleic acid alone that’s problematic-it's what obesity-causing fat turns into in the body. Researchers fed normal mice and genetically engineered mice high-fat diets rich in soybean oil. While most normal mice gained weight and developed fatty livers, the engineered mice remained lean and liver-healthy. These modified mice produced a variant of the liver protein HNF4α, which changes how the body metabolizes linoleic acid, reducing oxylipin formation. Oxylipins were found necessary for weight gain in normal mice but were lower in engineered mice. Interestingly, mice on low-fat diets with elevated oxylipins didn’t gain weight, indicating oxylipins alone aren’t enough to cause obesity—other metabolic factors contribute too. The study also found that blood oxylipin levels don’t reliably indicate early obesity; only liver oxylipins correlated with body weight, suggesting common blood tests may miss early signs. Soybean oil consumption in the U.S. has increased fivefold over the past century, now making up nearly 10% of calories. Though free of cholesterol, soybean oil has been linked to higher cholesterol levels in mice and may promote metabolic diseases due to excessive linoleic acid intake, especially from processed foods. The team plans to investigate whether other oils high in linoleic acid—like corn and sunflower oils—affect weight similarly. They caution that while soybean oil isn’t inherently harmful, modern consumption levels exceed what our bodies evolved to handle, triggering harmful metabolic pathways. These findings could influence future nutrition policies aimed at reducing chronic metabolic diseases. REFERENCE:Deol, P., et al. (2025). P2-HNF4α Alters Linoleic Acid Metabolism and Mitigates Soybean Oil-Induced Obesity: Role for Oxylipins. Journal of Lipid Research. doi: 10.1016/j.jlr.2025.100932. Journal of Lipid Research. https://www.jlr.org/article/S0022-2275(25)00195-6/fulltext