Soybean oil is the main cooking oil in the United States and a major ingredient in ultra-processed foods.
Its intake has risen sharply over the past century.
New work in mice suggests that the way the body handles one of its key components, linoleic acid, may help explain why diets high in soybean oil promote weight gain in some individuals.
In high-fat diet experiments at the University of California, Riverside:
- Most normal mice fed a diet rich in soybean oil gained a lot of weight and developed fatty livers
- Genetically engineered mice, eating the same diet, were much more resistant to weight gain and liver fat
These altered mice produce a different version of a liver protein called HNF4α, which regulates hundreds of genes involved in fat metabolism.
This variation changed how their bodies processed linoleic acid.
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Linoleic acid is a common omega-6 fatty acid in many vegetable oils. In the body it is converted into signalling molecules called oxylipins.
In this study:
- Regular mice on a soybean-oil-rich diet produced high levels of certain oxylipins derived from linoleic acid and alpha-linolenic acid
- Higher levels of these specific oxylipins were associated with more weight gain and fatty liver
- Transgenic mice had fewer of these oxylipins and healthier livers despite the same high-fat diet
The altered mice also had better mitochondrial function, which may have protected them from storing so much fat.
Interestingly, transgenic mice on a low-fat diet also had raised oxylipins without becoming obese, which shows that these molecules are part of the story but not the whole picture.
The researchers also found:
- The protected mice had much lower levels of certain enzyme families that convert linoleic acid into oxylipins
- These enzymes exist in all mammals, including humans, and their levels vary with genetics, diet and other factors
This could help explain why some people put on weight more easily than others on a similar high-soybean-oil diet.
Another important detail: only liver oxylipin levels tracked clearly with body weight.
Blood levels did not tell the same story.
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That means standard blood tests may miss early metabolic changes happening in the liver.
Human trials are not yet under way, so for now these findings should be seen as an early warning sign rather than a reason for panic. They add weight to the idea that not all fats – and not all people – respond the same way to modern high-oil diets.
Study: Deol P et al. Journal of Lipid Research 2025.
