A common vegetable oil found in countless processed foods on UK supermarket shelves may be a significant, yet overlooked, driver of obesity, according to new scientific research. The study, published on Thursday 4 December 2025, suggests the issue lies not in the oil itself, but in how our bodies process it.
The Hidden Ingredient in Processed Foods
Soybean oil is pervasive in the modern diet, though it is rarely used as a cooking base in British homes. It is a staple ingredient in a vast array of processed items, including salad dressings, margarine, crisps, and many ready meals. While previous studies have linked its consumption to weight gain, the precise mechanism has remained unclear until now.
Scientists from the University of California, Riverside, designed an experiment to unravel this mystery. They fed two groups of mice a diet rich in soybean oil. The first group consisted of normal mice, while the second was genetically engineered to produce a slightly different form of a liver protein involved in fat metabolism.
Genetics Hold the Key to Weight Gain
The results were striking. The normal mice fed the soybean oil diet gained significant weight. However, the genetically modified mice, consuming the exact same high-fat diet, did not. This finding points to a crucial interaction between our genetics and modern food ingredients.
"This may be the first step toward understanding why some people gain weight more easily than others on a diet high in soybean oil," explained Sonia Deol, the study's corresponding author and a biomedical scientist at the university.
Professor Frances Sladek, a cell biology expert involved in the research, emphasised the scale of consumption. "Soybean oil isn't inherently evil," she stated. "But the quantities in which we consume it is triggering pathways our bodies didn't evolve to handle."
The Body's Conversion Process and Inflammation
The research builds on a 2015 study from the same team which found soybean oil to be more "obesogenic" than coconut oil. The new work identifies the specific biological pathway. The oil is rich in linoleic acid, which the body converts into molecules called oxylipins.
Excessive consumption leads to a surplus of oxylipins, which are directly associated with increased inflammation and the accumulation of body fat. The genetically altered mice in the study showed significantly lower levels of the enzymes that create oxylipins, resulting in healthier livers and better mitochondrial function, which may explain their resistance to weight gain.
"We now have the clearest evidence yet that it's not the oil itself, or even linoleic acid. It's what the fat turns into inside the body," Professor Sladek concluded.
While human trials are not yet planned, the researchers hope their findings will influence future nutrition policy. Professor Sladek drew a sobering historical parallel: "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 recognise the link between excessive soybean oil consumption and negative health effects."
