Unexpected weight gain linked to a common cooking oil

Soybean oil is so common that it often goes unnoticed, appearing in home kitchens, restaurant meals, and countless ultra-processed foods.

But new research finds that this everyday ingredient does far more than contribute calories – it can nudge the body’s metabolism in unexpected ways.

A long series of studies from UC Riverside (UCR) show how this happens and why some people react more strongly than others.

How soybean oil affects weight

Regular mice gained a lot of weight on a high-fat soybean oil diet. A special group of engineered mice did not. These mice carried a version of a liver protein that changed how their bodies handled 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 study co-author Sonia Deol.

The engineered mice stayed lean even though they ate the same amount as the others. Their livers stored less fat. Their blood sugar control stayed steadier. Nothing in their behavior explained the difference. Their biology did.

Soybean oil changes molecules

Linoleic acid converts into oxylipins inside the body. These small molecules rise sharply when diets contain a lot of soybean oil.

Regular mice showed high levels of specific oxylipins that tracked closely with weight gain. The engineered mice, however, produced far fewer of them.

Earlier work showed that linoleic acid binds to the liver protein HNF4α. The new research showed something deeper. Different forms of this protein push linoleic acid down different chemical paths.

The engineered mice carried the form that slowed the conversion of linoleic acid into harmful oxylipins.

Several enzymes in that pathway ran at lower levels. Humans also vary in these enzymes, which may explain why reactions to the same diet differ so widely.

Energy shifts inside the body

Metabolomics revealed another layer. The engineered mice had higher levels of molecules tied to mitochondrial activity. These included ketone bodies and several parts of the TCA cycle.

These shifts often point to stronger energy use. Regular mice showed the opposite pattern on soybean oil. Key energy molecules dropped. That drop suggests slower energy turnover, which can push the body toward storing more fat.

Liver lipids also told a clear story. Regular mice built up triacylglycerols, diacylglycerols, and cholesteryl esters on the soybean oil diet. The engineered mice carried much less of these storage fats. Their livers stayed lighter, cleaner, and less stressed.

Four oxylipins stood out as the strongest predictors of obesity. Regular mice showed steep increases in these molecules. The engineered mice did not.

When researchers tried blocking one enzyme in the oxylipin pathway, body weight did not fall. That result narrowed which oxylipins truly drive weight gain.

Differences between the diets

Soybean oil intake in the United States rose sharply over the past century. It now delivers nearly ten percent of daily calories on average.

Much of this intake comes from ultra-processed foods. Despite containing no cholesterol, soybean oil raised cholesterol levels in the mice.

“Soybean oil isn’t inherently evil,” Sladek said. “But the quantities in which we consume it are triggering pathways our bodies didn’t evolve to handle.”

The team also compared soybean oil with coconut oil. Regular mice gained more weight on soybean oil. Engineered mice gained the same amount on both oils. This means their resistance applied to soybean oil’s chemical effects, not to fat intake as a whole.

Inflammation markers did not rise in the liver. Still, the engineered mice carried lower levels of certain cytokines linked with obesity in humans. They also held higher DHA levels, a fatty acid known for its calming effect on tissues.

Where soybean oil turns harmful

The findings point to a simple idea. Linoleic acid itself is not the main problem. The trouble begins after the body converts it into certain oxylipins. Those molecules rise sharply on heavy soybean oil intake.

They push metabolism toward fat storage. When the body makes fewer of them, weight gain slows. Stronger mitochondrial activity adds another layer of protection.

“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.”

Human studies are not underway yet. Still, the message is clear. What the body makes from a food can matter more than the food itself.

Soybean oil seems simple on the surface, but its chemistry inside the body tells a more complicated story.

The study is published in the Journal of Lipid Research.

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