By Harshit
RIVERSIDE, CALIFORNIA, DEC 1 —
Soybean oil is the most commonly used cooking oil in the United States and appears in a wide range of processed foods. Research is now shedding light on how this highly consumed ingredient contributes to obesity in mice.
In a University of California, Riverside experiment, most mice fed a high-fat diet rich in soybean oil put on substantial weight. A separate group of genetically engineered mice did not, even though their diets were identical. These modified mice produced a slightly altered version of a liver protein that affects hundreds of genes involved in fat metabolism. The altered protein also changes how the body processes linoleic acid, one of soybean oil’s main components.
“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 UCR biomedical scientist and corresponding author of the study, published in the Journal of Lipid Research.
Liver Protein Variants and Metabolic Impact
Humans produce both forms of the liver protein HNF4α, but the alternative version typically appears only under special conditions, such as chronic illness, fasting, or alcoholic fatty liver. Differences in this protein — along with factors like age, sex, genetics, and medications — may help explain why individuals respond differently to a soybean-oil-rich diet.
The new findings build on a 2015 UCR study linking soybean oil to weight gain. “We’ve known since our 2015 study that soybean oil is more obesogenic than coconut oil,” said Frances Sladek, a UCR professor of cell biology. “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.”
Oxylipins: The Metabolic Link to Weight Gain
Inside the body, linoleic acid breaks down into compounds known as oxylipins. High levels of linoleic acid can increase oxylipins, which are associated with inflammation and fat accumulation.
The genetically engineered mice produced significantly fewer oxylipins and maintained healthier liver tissue, despite consuming the same soybean-oil-rich diet. They also displayed improved mitochondrial function, potentially explaining their resistance to obesity.
Researchers identified specific oxylipins created from both linoleic and alpha-linolenic acids — molecules that regular mice required in order to gain weight.
Why Oxylipins Alone Aren’t the Whole Story
Even though the modified mice had elevated oxylipins on a low-fat diet, they did not become obese. This suggests oxylipins do not independently cause weight gain and that additional metabolic factors must be involved.
Further analysis showed that the modified mice had far lower levels of two enzyme families responsible for converting linoleic acid into oxylipins. These enzyme families function similarly across mammals, including humans, and their activity varies based on a range of biological factors.
Crucially, the study found that only oxylipins in liver tissue — not those found in the bloodstream — correlated with body weight. This means traditional blood tests may not reveal the earliest diet-linked metabolic changes.
Rising Soybean Oil Consumption and Public Health
Soybean oil intake in the United States has surged dramatically over the last century — from roughly 2% of total calories to nearly 10%. Although soybean oil contains no cholesterol, mice consuming it had higher cholesterol levels, highlighting a potential metabolic effect of excessive linoleic acid consumption.
Because linoleic acid appears in many ultra-processed foods, researchers warn that its metabolic impact may be widespread. The UCR team is now studying how oxylipins trigger weight gain and whether similar effects occur with other high-linoleic oils, including corn, sunflower, and safflower oils.
“Soybean oil isn’t inherently evil,” Sladek noted. “But the quantities in which we consume it are triggering pathways our bodies didn’t evolve to handle.”
While human trials are not currently planned, the researchers hope their findings will guide nutrition policy and future dietary studies.
“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.”