Mice Study Connects Soybean Oil Intake to Liver Changes and Obesity

Key Takeaways: 

• New research in mice suggests that weight gain linked to soybean oil is driven by the metabolic products of linoleic acid rather than the oil itself.
  
• Genetically engineered mice resistant to obesity on a high-fat soybean oil diet produced fewer oxylipins and showed healthier liver function.
  
• Scientists believe differences in human genetics, enzyme levels, and metabolic stress may influence people’s susceptibility to soybean-oil-related metabolic effects.
  
  

A closer look at soybean oil and obesity

Soybean oil is the most widely consumed cooking oil in the United States and is a key ingredient in many processed foods. A growing body of research has associated high intake of soybean oil with weight gain in animals. A new study from the University of California, Riverside (UCR), published in the Journal of Lipid Research, provides fresh insight into why this may occur.

Researchers found that mice consuming a high-fat diet rich in soybean oil gained considerable weight. However, a separate group of genetically engineered mice did not, despite eating the same diet. These altered mice carried a slightly different version of a liver protein that affects the expression of hundreds of genes involved in fat metabolism.

The findings point toward a metabolic mechanism that may help explain differences in weight gain among individuals exposed to similar diets.

“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.

The role of HNF4α in fat metabolism

In humans, both forms of the liver protein known as HNF4α occur naturally. However, the alternative version typically appears only under certain conditions, such as chronic illness, prolonged fasting, metabolic stress, or alcoholic fatty liver disease. These variations, combined with factors such as age, sex, medication use, and underlying genetics, may influence how different people respond to high levels of soybean oil in their diet.

The UCR team believes that the altered form of HNF4α in genetically engineered mice changes how the body processes linoleic acid, a major fatty acid in soybean oil.

Building on earlier research

The study adds to previous findings from the same research group.

“We’ve known since our 2015 study that soybean oil is more obesogenic than coconut oil,” said Frances Sladek, a UCR professor of cell biology. “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.”

One of the major metabolic products of linoleic acid is a group of molecules called oxylipins. These compounds are associated with inflammation, fat accumulation, and other metabolic changes.

Oxylipins and their link to weight gain

Mice engineered to produce the alternative form of HNF4α showed markedly lower levels of oxylipins in their livers, despite consuming a high-fat soybean oil diet. They also had healthier liver profiles and enhanced mitochondrial function. Improved mitochondrial activity may help explain their resistance to weight gain.

Researchers pinpointed specific oxylipins derived from both linoleic acid and alpha-linolenic acid (another fatty acid found in soybean oil) that appeared necessary for weight gain in regular mice.

However, the picture is complex. Even though transgenic mice on a low-fat diet showed elevated oxylipin levels, they did not become obese. This suggests that while these molecules contribute to weight gain, they are unlikely to be the sole drivers. Other metabolic conditions must also play a role.

Genetic variation in enzyme levels

Further analysis revealed that the modified mice had far 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 can vary significantly from person to person based on factors such as genetics and diet.

The scientists also observed that oxylipin levels in the liver, rather than in the bloodstream, were correlated with body weight. This indicates that standard blood tests may not reliably detect early metabolic disturbances linked to diet.

Soybean oil’s rise in the American diet

Soybean oil consumption in the United States has risen dramatically over the past century. It has increased from around 2 percent of total daily calories to almost 10 percent. Although soybeans provide protein and the oil contains no cholesterol, modern diets deliver far greater quantities of linoleic acid than the body is likely evolved to manage.

In line with this, the UCR study found that soybean oil intake was associated with increased cholesterol levels in mice despite the oil containing no dietary cholesterol. This reflects the complex interplay between dietary fats and internal metabolic pathways.

Questions for future research

The team now aims to understand precisely how oxylipin formation leads to weight gain and whether oils with similarly high linoleic acid content – including corn, sunflower, and safflower oils – trigger comparable effects.

“Soybean oil isn’t inherently evil,” said Deol. “But the quantities in which we consume it is triggering pathways our bodies didn’t evolve to handle.”

Although the researchers have no plans for human trials, they hope the findings will inform future studies and guide public health policy.

“It took 100 years from the first observed link between chewing tobacco and cancer to get warning labels on cigarettes,” Sladek noted. “We hope it won’t take that long for society to recognise the link between excessive soybean oil consumption and negative health effects.”