America's most popular cooking oil may be linked to obesity, new study finds

A new study published in the Journal of Lipid Research suggests that soybean oil, the most widely consumed cooking oil in the United States, may play a direct role in promoting obesity, and the effect appears tied to how the body processes one of its main components.

Researchers fed mice a diet rich in soybean oil and tracked how they metabolized linoleic acid, an omega-6 fatty acid that makes up a large share of soybean oil.

Linoleic acid is broken down in the body into molecules called oxylipins, and eating a lot of linoleic acid can raise the amount of these oxylipins. The study shows that certain oxylipins are linked to weight gain in mice.

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"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, in a press release.

The researchers asked a simple question: If we reduce the mice's ability to turn linoleic acid into oxylipins, will they still get obese on a soybean-oil diet?

To test this, they used a genetically engineered line of mice that express a different version of a liver regulatory gene, P2-HNF4α.

The genetic change alters many metabolic pathways, including lowering the activity of several enzyme families that normally convert linoleic acid into oxylipins. 

These enzymes also exist in all mammals, including humans, and their activity can vary because of genetics, diet and other factors.

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The team then fed both the altered mice and normal mice a diet high in soybean oil. 

At the end of the experiment, the modified mice had healthier livers and gained much less weight compared to normal mice on the same diet.

The researchers pinpointed specific oxylipins made from linoleic acid and alpha-linolenic acid (another fat in soybean oil) that were tied to obesity in the normal mice.

These oxylipins were present in higher amounts in the mice that became obese.

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The findings suggest that the body's internal processing of linoleic acid may play a key role in how soybean oil contributes to fat accumulation.

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In other words, the issue may not be just the calories in the oil itself, but what the body turns those fatty acids into once they enter the metabolic system.

The study was conducted entirely in mice, and the authors note that human metabolism is more complex. 

Still, the work raises questions about whether high intake of linoleic-acid-rich oils could contribute to obesity through biochemical pathways that go beyond simple energy balance.

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Soybean oil is the dominant cooking oil in American households, restaurants, and processed foods, according to the study. Because of its affordability and neutral taste, it is also used heavily in packaged snacks, fast foods and fried items.

The researchers emphasize that the study does not claim soybean oil inevitably causes obesity in humans. 

Instead, it highlights a biochemical mechanism that may help explain why diets high in this oil can promote weight gain in animal models.

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The authors also caution that the genetically modified mice differ from humans because they were engineered to express much lower levels of enzymes responsible for converting linoleic acid into its metabolites. 

That allows scientists to see the effects more clearly but limits how directly the results apply to people.