Soybean oil, the most widely used cooking oil in the United States, contributes to obesity through a specific genetic mechanism, according to a new study from UC Riverside.Published in the Journal of Lipid Research, the study explains why consuming this oil leads to significant weight gain in mice, providing potential clues about human metabolism.In the experiment, researchers fed mice a high-fat diet rich in soybean oil. While regular mice gained a lot of weight, a group of genetically engineered mice stayed lean. These “transgenic” mice produced a variant of a liver protein called HNF4α, which changes how the body processes linoleic acid, the main fat 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, the study’s corresponding author.The researchers found that the issue isn’t the oil itself, but what the liver turns it into. Linoleic acid is metabolized into molecules called oxylipins. High levels of certain oxylipins are linked to inflammation and fat buildup.Genetically engineered mice exhibited significantly lower levels of oxylipins and had a different profile of enzymes responsible for their production. As a result, these mice maintained healthier livers and better mitochondrial function, despite consuming the same high-fat diet as the obese mice.Soybean oil consumption in the U.S. has increased fivefold over the past century, now accounting for nearly 10% of total caloric intake.“We’ve known since our 2015 study that soybean oil is more obesogenic than coconut oil,” said Professor Frances Sladek. “But now we have the clearest evidence yet that it’s what the fat transforms into inside the body.”The team noticed that oxylipin levels in the liver, not the blood, are associated with weight gain, suggesting common blood tests may fail to detect early metabolic damage.“Soybean oil isn’t inherently evil. But the quantities in which we consume it are triggering pathways our bodies didn’t evolve to handle,” Deol noted.