A new animal study suggests that soybean oil may contribute to obesity in mice, raising questions about whether the most widely used cooking oil in the United States could also be playing a role in the country’s weight problems.
Researchers at the University of California, Riverside, published their work in the Journal of Lipid Research in October. They examined how mice metabolize linoleic acid — an omega-6 fatty acid that is abundant in soybean oil — by feeding them a high-fat diet based on this common cooking oil.
Although the experiments were done in mice, Frances Sladek, a UCR professor of cell biology and one of the study’s authors, told Newsweek that the results may be relevant for people, since “the pathways we found involved in soybean oil-induced obesity are highly conserved between mouse and human.”
Why This Matters
Soybean oil is, by a wide margin, the most commonly used cooking oil in the U.S., followed by rapeseed (canola) oil and palm oil, according to Statista data. Sladek noted that soybean oil is composed of more than 50 percent linoleic acid.
That’s important because of both its popularity and the country’s high obesity rates. In the U.S., roughly one in five children and two in five adults meet the definition of obesity, with a Body Mass Index (BMI) over 30.
Obesity, in turn, is linked to higher risks of diabetes, heart disease and stroke.
Previous research has also raised concerns about the typical American diet. A study published last year reported that most Americans follow eating patterns that promote inflammation, increasing the risk of conditions such as heart disease, obesity, diabetes, depression and certain cancers.
Inside the Study: Oxylipins and Weight Gain
The new UCR research focused on molecules called oxylipins. These compounds are formed when linoleic acid is broken down in the body. The more linoleic acid someone consumes, the more of these oxylipins are produced.
Other fatty acids can also be converted into oxylipins, but the team found that those specifically derived from linoleic acid were the ones associated with obesity in mice. The researchers had reported this link before; this time, they explored it in a more targeted way.
In the new study, male mice were genetically engineered to carry a different version of a liver regulatory gene called P2-HNF4α. This genetic change altered their metabolic pathways by reducing the activity of enzymes that convert linoleic acid into oxylipins.
When both the genetically modified mice and a control group were fed the same soybean oil-rich high-fat diet, the modified mice developed healthier livers and gained less weight than the controls. This outcome further supported the idea that oxylipins are involved in obesity.
“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 co-author of the study.
However, the team also observed that the genetically modified mice had elevated oxylipin levels on a low-fat diet without becoming obese, indicating that other metabolic factors are likely involved.
Sladek explained that the researchers found “it is the levels of the oxylipins present in the liver, not circulating in the blood, that correlate with obesity.” At the same time, he noted that they still do not know “exactly how the oxylipins drive obesity.”
How Much Soybean Oil Are Americans Eating?
Some linoleic acid is essential for human health, and small amounts are considered part of a balanced diet. But the researchers pointed out that linoleic acid intake in the U.S. has risen dramatically over the past 50 years, largely through growing use of soybean oil.
According to the study’s authors, the amount of linoleic acid needed for health is about 1 to 2 percent of a person’s total daily calories. At those levels, it supports important biological functions.
In reality, many Americans are consuming far more — roughly 15 to 25 percent of their daily caloric intake from linoleic acid, the researchers said.
Sladek emphasized that modest amounts of soybean oil are “perfectly safe and provide a good source of the essential fatty acid linoleic acid.” The concern, he said, is how much of it is now embedded in processed food.
He noted that a growing share of the diet comes from processed products that often contain soybean oil or other seed oils such as corn, safflower and sunflower oil — all of which are high in linoleic acid.
“So in general, we are taking in much more of these seed oils, all of which have high levels of linoleic acid, than our body needs,” he said.
What Might Soybean Oil Be Doing in the Body?
The authors stressed that their mouse data cannot directly predict how soybean oil affects people, and that more work is needed to understand what these findings mean for human health.
They do, however, argue that the possible link between soybean oil, oxylipins and obesity is strong enough to justify further study.
Sladek drew a comparison to how long it took to formally recognize the dangers of other substances, saying: “It took 100 years from the first observed link between chewing tobacco and cancer to get warning labels on cigarettes. We hope it won’t take that long for society to recognize the link between excessive soybean oil consumption and negative health effects.”
He also told Newsweek that earlier research from the team suggests soybean oil may affect the intestines, the microbiome and the hypothalamus, an area of the brain involved in appetite, hormones and energy balance.
Sladek added that while there is increasing evidence that dietary linoleic acid can benefit heart health, “we have not looked at the impact of a soybean oil diet on the heart.”
“Different organs will respond differently to dietary linoleic acid and more research in general needs to be done,” he said.
Why Some Experts Remain Skeptical
Not all specialists agree that these mouse findings should change how people view soybean oil.
Dr. Dariush Mozaffarian, director of the Food is Medicine Institute at Tufts University, told Newsweek that much of the earlier research pointing to harms from omega-6 polyunsaturated fatty acids, including linoleic acid, comes from rodent studies like this one.
“We’ve since clearly learned that humans are not mice, and that these effects don’t translate to what’s seen in humans,” Mozaffarian said. “In mice, for example, high fat diets (from any source) cause obesity, whereas in humans, carbs are the problem.”
He pointed out that in controlled human trials, soybean oil and omega-6 polyunsaturated fatty acids have been shown to improve blood cholesterol, support better glucose control and reduce the risk of heart attacks. In observational studies, he added, these oils have been “linked to less weight gain and obesity risk.”
Mozaffarian described soybean oil as a “healthy oil for cooking,” noting evidence that about a tablespoon and a half per day may lower the risk of coronary heart disease, that it does not appear to promote inflammation, and that people who consume around 5 grams daily have been found to have a lower risk of death from all causes.
In short, the new study adds to an ongoing scientific debate. While mouse data from UCR suggest that very high intakes of linoleic acid from soybean oil could encourage obesity through oxylipins, other experts argue that human evidence still paints soybean oil — in moderation and within a balanced diet — as beneficial, especially for heart health. Further research will be needed to clarify where the balance truly lies.
