There are so many cooking oils on the market these days—avocado, grapeseed, flaxseed, canola, and extra-virgin olive, just to name a few. Having at least one of these oils in your pantry is probably pretty standard, as experts suggest cooking with oils loaded with healthy fats and unsaturated fatty acids over butter or margarine when possible. But researchers have recently discovered that using one specific type of oil could actually be doing more harm than good.

A December 2025 study published in the Journal of Lipid Research found that a diet high in soybean oil may lead to increased weight gain. If you think you don’t typically consume soybean oil, think again—in a news release about the study, the University of California, Riverside (UCR) states the oil is “the most widely consumed cooking oil in the United States.” Soybean oil is also used in a wide range of processed foods—and its consumption has reportedly risen over the past 100 years from about 2% of calories consumed to nearly 10%, according to UCR.

For the study, university researchers fed a group of mice a high-fat diet that was rich in soybean oil, which caused the mice to gain “significant” weight. But a select group of mice, which were genetically engineered, did not gain weight. These mice formed a slightly altered version of a liver protein that’s also found in humans called HNF4α. This protein affects genes involved in fat metabolism as well as how the body processes linoleic acid, a main part of soybean oil.

According to UCR, “both versions” of HNF4α are found in humans, “but the alternative form is typically produced only under certain conditions, such as chronic illness or metabolic stress from fasting or alcoholic fatty liver.”

“This may be the first step toward understanding why some people gain weight more easily than others on a diet high in soybean oil,” says Sonia Deol, a UCR biomedical scientist and corresponding author. However, she does note one important caveat: “Soybean oil isn’t inherently evil. But the quantities in which we consume it is triggering pathways our bodies didn’t evolve to handle.”