More than half of the oil consumed in the United States is heavy soybean oil, but studies link the ingredient to changes in gut bacteria, a weakened intestinal lining and issues such as ulcerative colitis.
For one study in Gut Microbes, researchers from the University of California, Riverside fed mice a diet high in soybean oil for up to 24 weeks. Then they measured disruptions in the animals’ gut microbiome — beneficial bacteria declined, while harmful bacteria increased, including adherent invasive Escherichia coli. This type of E. coli has been linked to inflammatory bowel disease in humans.
“Soybean oil … is currently the most highly consumed cooking oil in the U.S. and the second most produced edible oil in the world,” UC Riverside researchers wrote, “and its increased consumption parallels the rise in IBD incidence in humans.”
The researchers focused on linoleic acid, the predominant fatty acid in soybean oil, and found that excessive amounts of it can cause problems in the gut. Linoleic acid is an essential fatty acid, meaning it cannot be produced in the body but is derived from foods.
While people need to consume 1 percent to 2 percent of their fat from linoleic acid, the average American diet derives 8 percent to 10 percent of its linoleic acid from soybean oil, UC Riverside researcher Poonamjot Deol told SciTechDaily.
Harmful E. coli bacteria feed on linoleic acid, while excessive levels cause some beneficial bacteria to die off, they found. The excess linoleic acid also made the intestinal lining more porous, allowing toxins and microbes to leak into the blood, fueling inflammation.
Complicated issue
As an unsaturated plant oil, soybean oil is often viewed as healthier than saturated fats from animal products, but Deol and his colleagues said the issue is more complicated.
“Since studies showed that saturated fats can be unhealthy, it was assumed that all unsaturated fats are healthy,” toxicologist Frances Sladek told SciTechDaily. “But there are different types of unsaturated fats, some of which are healthful. For example, the unsaturated fat fish oil is well known to have many beneficial health effects. People, therefore, assumed that soybean oil is perfectly safe and healthier to consume than other types of oils, without actually doing a direct comparison as we have done.”
While corn oil contains linoleic acid levels similar to those in soybean oil, the researchers said olive oil contains much less and has a well-established health profile. Avocado and coconut oil are similarly low in linoleic acid.
Additional studies
A related study in Scientific Reports compared coconut oil, soybean oil and a soybean oil product modified to have lower linoleic acid to assess their effects on mice. Unmodified soybean oil caused more disruption in genes tied to metabolism, immune function, gut barrier health, inflammation and microbiome interactions, they found, supporting the idea that excess linoleic acid may be an important factor.
A third experiment, published in the Journal of Lipid Research, focused on soybean oil and obesity by measuring oxylipins, compounds the body makes when it processes linoleic acid. Mice that did not have soybean oil had lower levels of the related oxylipins, gained less weight and were less likely to develop glucose intolerance or fatty liver disease.
Soybean oil’s neutral flavor, low cost and versatility in large-scale food production have made it a more common ingredient than most people realize, but experts advise avoiding processed foods, checking labels and using an air fryer, which uses very little oil for similar results.
