A new study published in the Journal of Lipid Research suggests that soybean oil, the most widely consumed cooking oil in the United States, may play a direct role in promoting obesity. The effect appears tied to how the body processes one of its main components.

Researchers fed mice a diet rich in soybean oil and tracked how they metabolized linoleic acid which is an omega-6 fatty acid that makes up a large share of soybean oil.

Linoleic acid is broken down in the body into molecules called oxylipins. Eating a lot of linoleic acid can raise the amount of these oxylipins. The study shows that certain oxylipins are linked to weight gain in mice.

Sonia Deol a UCR biomedical scientist and corresponding author of the study explained the significance of the findings.

“This may be the first step toward understanding why some people gain weight more easily than others on a diet high in soybean oil.”

The researchers asked a simple question. If we reduce the mice’s ability to turn linoleic acid into oxylipins will they still get obese on a soybean-oil diet.

To test this they used a genetically engineered line of mice that express a different version of a liver regulatory gene called P2-HNF4α.

The genetic change alters many metabolic pathways including lowering the activity of several enzyme families that normally convert linoleic acid into oxylipins. These enzymes also exist in all mammals including humans and their activity can vary because of genetics, diet and other factors.

The team then fed both the altered mice and normal mice a diet high in soybean oil.

At the end of the experiment the modified mice had healthier livers and gained much less weight compared to normal mice on the same diet.

The researchers pinpointed specific oxylipins made from linoleic acid and alpha-linolenic acid which is another fat in soybean oil that were tied to obesity in the normal mice. These oxylipins were present in higher amounts in the mice that became obese.

The findings suggest that the body’s internal processing of linoleic acid may play a key role in how soybean oil contributes to fat accumulation.

In other words the issue may not be just the calories in the oil itself but what the body turns those fatty acids into once they enter the metabolic system.

The study was conducted entirely in mice and the authors note that human metabolism is more complex. Still the work raises questions about whether high intake of linoleic-acid-rich oils could contribute to obesity through biochemical pathways that go beyond simple energy balance.

Soybean oil is the dominant cooking oil in American households, restaurants and processed foods according to the study. Because of its affordability and neutral taste it is also used heavily in packaged snacks, fast foods and fried items.

The researchers emphasize that the study does not claim soybean oil inevitably causes obesity in humans. Instead it highlights a biochemical mechanism that may help explain why diets high in this oil can promote weight gain in animal models.

The authors also caution that the genetically modified mice differ from humans because they were engineered to express much lower levels of enzymes responsible for converting linoleic acid into its metabolites. That allows scientists to see the effects more clearly but limits how directly the results apply to people.

This research adds to growing concerns about seed oils that have become increasingly popular in American diets over the past several decades. Health advocates have long questioned whether these highly processed oils contribute to rising rates of obesity and chronic disease.

The widespread use of soybean oil in processed foods means most Americans consume significant amounts without even realizing it. Reading ingredient labels reveals soybean oil in everything from salad dressings to crackers to frozen meals.

The study suggests that replacing soybean oil with alternatives lower in linoleic acid might help reduce obesity risk. Options like olive oil, coconut oil and butter contain different fatty acid profiles that may not trigger the same metabolic responses.