Soybean oil is the most widely used cooking oil in the United States. It’s found in many everyday foods, especially processed ones.
Now, scientists at the University of California, Riverside have discovered more about how this popular oil may be linked to weight gain—at least in mice.
Their new research offers clues about why some people may be more likely to gain weight when eating foods that are high in soybean oil.
In the study, most mice that ate a high-fat diet with lots of soybean oil became heavier. But there was one group of special mice that didn’t gain much weight, even though they ate the same high-fat diet. These mice were genetically modified to produce a slightly different version of a liver protein.
This protein, known as HNF4α, plays a big role in how the body handles fat. The altered protein also changes how the body processes linoleic acid, which is a major part of soybean oil.
Humans also make both types of this protein, but the alternate version usually appears only under stress or illness. The researchers believe that differences in this protein—along with age, sex, genetics, and medications—might explain why some people gain more weight than others when eating the same amount of soybean oil.
The study adds to earlier research from the same university that found soybean oil caused more weight gain than coconut oil. However, this new work shows it’s not just the oil or linoleic acid itself that’s the problem—it’s what these fats turn into once inside the body.
Linoleic acid is broken down into chemicals called oxylipins. These compounds can cause inflammation and lead to fat buildup.
The genetically altered mice made fewer of these oxylipins and had healthier livers and better energy production in their cells, even while eating a high-fat, soybean oil-rich diet. Regular mice, on the other hand, produced more oxylipins and gained more weight.
The researchers found that oxylipins made from both linoleic acid and another fatty acid in soybean oil, called alpha-linolenic acid, were necessary for the mice to gain weight. Still, oxylipins alone did not cause obesity.
Mice that had high oxylipin levels but ate a low-fat diet didn’t become obese. This means that oxylipins are just one part of a bigger picture that includes diet and metabolism.
They also noticed that only the oxylipins in liver tissue—not those found in the blood—were linked to weight gain. This suggests that common blood tests might miss early signs of diet-related metabolic changes.
Soybean oil intake in the U.S. has grown a lot over the last century. It used to make up about 2% of daily calories, but now it’s closer to 10%.
Even though soybean oil contains no cholesterol and soybeans provide healthy plant protein, too much linoleic acid may be causing long-term health problems. In fact, the study showed that mice fed soybean oil had higher cholesterol levels, even though the oil itself doesn’t contain cholesterol.
The researchers are now studying other oils that are also high in linoleic acid, such as corn, sunflower, and safflower oils, to see if they have the same effects.
According to Frances Sladek, one of the lead scientists, soybean oil isn’t necessarily “bad,” but our bodies didn’t evolve to handle the large amounts we eat today. While there are no human trials planned yet, the team hopes this research will lead to better food choices and smarter nutrition policies.
“It took decades for people to accept that tobacco caused cancer,” Sladek said. “We hope it doesn’t take that long for people to recognize the health risks linked to too much soybean oil.”
If you care about weight loss, please read studies that hop extract could reduce belly fat in overweight people, and early time-restricted eating could help lose weight .
For more health information, please see recent studies about a simple path to weight loss, and results showing a non-invasive treatment for obesity and diabetes.
The study is published in Journal of Lipid Research.
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