Cooking Oil & Obesity: Popular Oils Linked to Weight Gain

Soybean Oil Linked to Obesity: New Research Uncovers Metabolic Pathway

A growing body of evidence suggests that soybean oil, the most widely consumed cooking oil in the United States, may contribute to weight gain and metabolic issues. New research from the University of California, Riverside, sheds light on the underlying mechanisms, revealing how the oil impacts fat metabolism in mice and potentially, humans.

The Obesity Connection: Beyond Simple Fat Content

For years, scientists have observed a correlation between high soybean oil consumption and increased obesity rates. A recent study conducted at UC Riverside demonstrates that mice fed a high-fat diet rich in soybean oil gained significant weight, while a genetically engineered group remained relatively stable. This difference points to a specific metabolic pathway triggered by the oil.

A Key Liver Protein and Linoleic Acid

The key to this pathway lies in a liver protein, HNF4α, which influences hundreds of genes related to fat metabolism. The genetically engineered mice produced a slightly altered version of this protein, impacting how their bodies process linoleic acid, a major component of soybean oil. According to a UCR biomedical scientist and lead author of the study, “This may be the first step toward understanding why some people gain weight more easily than others on a diet high in soybean oil.”

Human Implications and Genetic Variability

Humans also possess both versions of the HNF4α protein, though the alternative form typically appears only under specific conditions like chronic illness or metabolic stress. This variability, combined with factors like age, sex, medications, and genetics, could explain why individuals exhibit different susceptibilities to the metabolic effects of soybean oil.

Building on Previous Research: Soybean Oil vs. Coconut Oil

This latest research builds upon earlier findings from UC Riverside, which established that soybean oil is more likely to cause weight gain than coconut oil. As a UCR professor of cell biology explained, “We’ve known since our 2015 study that soybean oil is more obesogenic than coconut oil. But now we 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.”

The Role of Oxylipins and Mitochondrial Function

The research team discovered that linoleic acid is converted into molecules called oxylipins. Excessive consumption of linoleic acid leads to increased oxylipin production, which is linked to inflammation and fat accumulation. The genetically engineered mice exhibited significantly fewer oxylipins and healthier livers, despite consuming the same high-fat diet. They also demonstrated enhanced mitochondrial function, potentially contributing to their resistance to weight gain.

Specific Oxylipins Identified as Key Contributors

Researchers pinpointed specific types of oxylipins, derived from both linoleic acid and alpha-linolenic acid (another fatty acid in soybean oil), as necessary for weight gain in regular mice. However, even transgenic mice on a low-fat diet showed elevated oxylipins without becoming obese, suggesting that other metabolic factors also play a role in obesity.

Enzyme Families and Liver-Specific Effects

Further analysis revealed that the altered mice had lower levels of two key enzyme families responsible for converting linoleic acid into oxylipins. These enzymes are highly conserved across all mammals, including humans, and their levels vary based on genetics, diet, and other factors. Importantly, the study found that oxylipin levels in the liver, not the blood, correlated with body weight, suggesting that standard blood tests may not accurately reflect early metabolic changes.

Rising Consumption and Potential Health Concerns

Soybean oil consumption in the US has increased dramatically over the past century, rising from approximately 2% of total calories to nearly 10% today. While soybeans are a good source of plant-based protein and their oil contains no cholesterol, the overconsumption of linoleic acid, particularly in ultra-processed foods, may contribute to chronic metabolic conditions. The UC Riverside study also found that soybean oil consumption was associated with higher cholesterol levels in mice, despite the oil’s lack of cholesterol.

Future Research and Broader Implications

The researchers are now investigating how oxylipin formation causes weight gain and whether similar effects occur with other oils high in linoleic acid, such as corn, sunflower, and safflower oil. A senior researcher noted, “Soybean oil isn’t inherently evil. But the quantities in which we consume it is triggering pathways our bodies didn’t evolve to handle.”

While human trials are not currently planned, the team hopes their findings will inform future research and nutrition policy. As one researcher cautioned, “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.”

Source: UC Riverside.