Soybean oil is the most commonly used cooking oil in the United States. You’ll find it in countless processed foods, from salad dressings to snack bars.
But new research from the University of California, Riverside suggests this popular oil may be playing a role in the obesity crisis—and scientists are finally starting to understand why.
In a recent experiment, most mice that were fed a high-fat diet rich in soybean oil gained a lot of weight. But one special group of mice didn’t.
These mice were genetically engineered to have a slightly different version of a liver protein that affects how the body handles fat. This change also seemed to affect how the body processed a fatty acid found in soybean oil called linoleic acid.
Both forms of this protein exist in humans. One version is more active during illness or stress, like during fasting or liver problems caused by alcohol. The way people respond to soybean oil may vary depending on which version of this protein their body uses more. Genetics, medications, age, and sex may all play a role.
This study builds on earlier findings that soybean oil may be more likely to cause weight gain than other fats like coconut oil. Researchers now believe the problem isn’t just the oil or even the linoleic acid itself—it’s what this fat turns into inside the body.
When linoleic acid is broken down, it creates molecules called oxylipins. These molecules are linked to inflammation and fat storage.
In the study, the genetically modified mice had fewer of these oxylipins, even though they ate the same high-fat diet as the regular mice. They also had healthier livers and better-functioning mitochondria, the parts of cells that produce energy. This might explain why they didn’t gain as much weight.
The team also found that the engineered mice had lower levels of certain enzymes that turn linoleic acid into oxylipins. These enzymes work the same way in humans and vary based on diet and genetics. This could help explain why some people gain weight more easily when eating soybean oil.
Interestingly, when regular mice had high oxylipin levels, they gained weight. But the engineered mice with high oxylipins didn’t. This shows that other factors besides oxylipins are also involved in how the body gains fat.
One surprising finding was that only the oxylipins in the liver—and not in the blood—were linked to weight gain. This means that common blood tests may not detect the early changes that soybean oil is causing in the body.
Soybean oil use has grown fast in the past 100 years, from about 2% of the average American’s diet to nearly 10% today. Even though soybean oil doesn’t have cholesterol and soybeans are a good plant-based protein, too much linoleic acid—especially from processed foods—may be harming our health.
In fact, even though the oil has no cholesterol, mice that consumed soybean oil showed higher cholesterol levels. The research team is now looking into whether other oils high in linoleic acid—like corn, sunflower, and safflower oils—have similar effects.
Professor Frances Sladek, one of the lead researchers, says soybean oil isn’t bad by itself. But the large amount we consume today may be too much for our bodies to handle. Over time, this could lead to weight gain, liver problems, and other health issues.
The researchers hope their findings will lead to more studies and eventually influence nutrition policies. They’re not running any human trials yet, but they want people to start thinking more carefully about the oils in their diet.
As Sladek put it, “It took 100 years from the first link between chewing tobacco and cancer to get warning labels on cigarettes. We hope it won’t take that long for people to realize that too much soybean oil could be harming their health.”
If you care about weight loss, please read studies about orange that could help obesity, and a berry that can prevent cancer, diabetes and obesity.
For more health information, please see recent studies about ginger’s journey in weight management, and green tea: a cup of weight loss.
The study is published in the Journal of Lipid Research.
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