Soybean oil is one of the most widely used oils in the American diet, though many people may know it simply as “vegetable oil” on labels and ingredient lists. Because it is inexpensive, neutral-tasting, and common in processed foods, restaurant meals, dressings, and snacks, it can be easy to consume regularly without realizing it.
Credit: Shutterstock
New research suggests that heavy soybean oil intake may disrupt the gut in ways scientists are only beginning to understand.
Soybean oil or “vegetable oil” is everywhere in the American diet. It is used in salad dressings, sauces, fried foods, packaged snacks, frozen meals, and many restaurant meals. Most people may consume it regularly without realizing how much they are getting.
New research from the University of California, Riverside suggests that high soybean oil intake may affect more than body weight. In mouse studies, it has been linked to changes in gut bacteria, a weaker intestinal barrier, greater susceptibility to ulcerative colitis, and metabolic problems.
The findings do not prove that soybean oil causes these diseases in people. But they do raise concerns about how often this inexpensive, widely used oil appears in processed and restaurant foods.
Credit: Sladek lab, UC Riverside
Soybean Oil and Colitis
A study published in Gut Microbes, examined mice fed a diet high in soybean oil for up to 24 weeks. Researchers found that the diet disrupted the gut microbiome. Beneficial bacteria declined, while harmful bacteria increased, including adherent invasive Escherichia coli, a type of E. coli linked to inflammatory bowel disease (IBD) in humans.
The researchers focused on linoleic acid, the main fatty acid in soybean oil. Linoleic acid is essential, meaning the body needs some of it. But the team found that too much may create problems in the gut.
“While our bodies need 1-2% of linoleic acid daily, based on the paleodiet, Americans today are getting 8-10% of their energy from linoleic acid daily, most of it from soybean oil,” said Poonamjot Deol, an assistant professional researcher at UC Riverside. “Excessive linoleic acid negatively affects the gut microbiome.”
Photo shows, from L to R, Frances Sladek, James Borneman, and Poonamjot Deol.
Not All Plant Oils Act the Same
Soybean oil is an unsaturated plant oil, a category often viewed as healthier than saturated fats from animal products. But the researchers say the issue is more complicated.
Soybean oil is currently the most highly consumed cooking oil in the U.S.
Chart depicts consumption of edible oils in the U.S. for 2017/18.
Follow-Up Research
A related study in Scientific Reports looked at how different high-fat diets affected gene activity throughout the mouse intestine.
The researchers compared diets based on coconut oil, conventional soybean oil, and a modified soybean oil lower in linoleic acid and higher in oleic acid, making it more similar to olive oil.
The conventional soybean oil diet caused more disruption in genes tied to metabolism, immune function, gut barrier health, inflammation, and microbiome interactions, supporting the idea that excess linoleic acid may be an important factor.
A different study published in the Journal of Lipid Research looked at soybean oil and obesity. It focused on oxylipins, compounds the body makes when it processes fats such as linoleic acid. The study suggested that soybean oil’s effects may depend partly on what the body turns linoleic acid into after digestion. Mice that were protected from soybean oil-linked obesity had lower levels of certain oxylipins, gained less weight, and were less likely to develop glucose intolerance or fatty liver.
Soybean Oil and Colitis
A study published in Gut Microbes, examined mice fed a diet high in soybean oil for up to 24 weeks. Researchers found that the diet disrupted the gut microbiome. Beneficial bacteria declined, while harmful bacteria increased, including adherent invasive Escherichia coli, a type of E. coli linked to inflammatory bowel disease (IBD) in humans.
The researchers focused on linoleic acid, the main fatty acid in soybean oil. Linoleic acid is essential, meaning the body needs some of it. But the team found that too much may create problems in the gut.
“While our bodies need 1-2% of linoleic acid daily, based on the paleodiet, Americans today are getting 8-10% of their energy from linoleic acid daily, most of it from soybean oil,” said Poonamjot Deol, an assistant professional researcher at UC Riverside. “Excessive linoleic acid negatively affects the gut microbiome.”
Credit: Stan Lim, UC Riverside
In the study, harmful E. coli used linoleic acid as a food source, while some helpful bacteria could not tolerate high levels and died off. The researchers also found that linoleic acid made the intestinal barrier more porous, which can allow toxins and microbes to leak into the bloodstream and fuel inflammation.
“It’s the combination of good bacteria dying off and harmful bacteria growing out that makes the gut more susceptible to inflammation and its downstream effects,” Deol said. “Further, linoleic acid causes the intestinal epithelial barrier to become porous.”
In the study, harmful E. coli used linoleic acid as a food source, while some helpful bacteria could not tolerate high levels and died off. The researchers also found that linoleic acid made the intestinal barrier more porous, which can allow toxins and microbes to leak into the bloodstream and fuel inflammation.
“It’s the combination of good bacteria dying off and harmful bacteria growing out that makes the gut more susceptible to inflammation and its downstream effects,” Deol said. “Further, linoleic acid causes the intestinal epithelial barrier to become porous.”
Not All Plant Oils Act the Same
Soybean oil is an unsaturated plant oil, a category often viewed as healthier than saturated fats from animal products. But the researchers say the issue is more complicated.
Credit: Stan Lim, UC Riverside
“Our work challenges the decades-old thinking that many chronic diseases stem from the consumption of excess saturated fats from animal products, and that, conversely, unsaturated fats from plants are necessarily more healthful,” Deol said.
Frances M. Sladek, a toxicologist and professor of cell biology at UC Riverside, said the assumption that all unsaturated fats are healthy became widespread without enough direct comparison among different oils.
“Since studies showed that saturated fats can be unhealthy, it was assumed that all unsaturated fats are healthy,” she said. “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.”
Linoleic acid is not inherently bad. It is essential, meaning the body needs it and cannot make it on its own. It helps maintain cell membranes, including in the brain. The concern is whether modern diets deliver far more than the body needs.
“Every animal has to get linoleic acid from the diet,” Sladek said. “No animal can make it. A small amount of it is needed by the body. But just because something is needed does not mean a lot of it is good for you. Several membranes in the body, in the brain, for example, require linoleic acid for the cells to function properly. If all we ate was saturated fats, our cell membranes would become too rigid and not function properly. Future studies are needed to determine the tipping point for how much daily linoleic acid consumption is safe.”
“Our work challenges the decades-old thinking that many chronic diseases stem from the consumption of excess saturated fats from animal products, and that, conversely, unsaturated fats from plants are necessarily more healthful,” Deol said.
Frances M. Sladek, a toxicologist and professor of cell biology at UC Riverside, said the assumption that all unsaturated fats are healthy became widespread without enough direct comparison among different oils.
“Since studies showed that saturated fats can be unhealthy, it was assumed that all unsaturated fats are healthy,” she said. “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.”
Linoleic acid is not inherently bad. It is essential, meaning the body needs it and cannot make it on its own. It helps maintain cell membranes, including in the brain. The concern is whether modern diets deliver far more than the body needs.
“Every animal has to get linoleic acid from the diet,” Sladek said. “No animal can make it. A small amount of it is needed by the body. But just because something is needed does not mean a lot of it is good for you. Several membranes in the body, in the brain, for example, require linoleic acid for the cells to function properly. If all we ate was saturated fats, our cell membranes would become too rigid and not function properly. Future studies are needed to determine the tipping point for how much daily linoleic acid consumption is safe.”
Credit: USDA
Olive Oil Did Not Show the Same Effect
According to Deol and Sladek, olive oil may be a better choice because it contains much less linoleic acid than soybean oil. Olive oil is also a key part of the Mediterranean diet, which has been linked to many health benefits.
“Olive oil, the basis of the Mediterranean diet, is considered to be very healthy; it produces less obesity and we have now found that, unlike soybean oil, it does not increase the susceptibility of mice to colitis,” Sladek said.
The researchers also pointed to avocado oil and coconut oil as other cooking options. They cautioned that corn oil contains a similar amount of linoleic acid as soybean oil.
Olive Oil Did Not Show the Same Effect
According to Deol and Sladek, olive oil may be a better choice because it contains much less linoleic acid than soybean oil. Olive oil is also a key part of the Mediterranean diet, which has been linked to many health benefits.
“Olive oil, the basis of the Mediterranean diet, is considered to be very healthy; it produces less obesity and we have now found that, unlike soybean oil, it does not increase the susceptibility of mice to colitis,” Sladek said.
The researchers also pointed to avocado oil and coconut oil as other cooking options. They cautioned that corn oil contains a similar amount of linoleic acid as soybean oil.
Follow-Up Research
A related study in Scientific Reports looked at how different high-fat diets affected gene activity throughout the mouse intestine.
The researchers compared diets based on coconut oil, conventional soybean oil, and a modified soybean oil lower in linoleic acid and higher in oleic acid, making it more similar to olive oil.
The conventional soybean oil diet caused more disruption in genes tied to metabolism, immune function, gut barrier health, inflammation, and microbiome interactions, supporting the idea that excess linoleic acid may be an important factor.
A different study published in the Journal of Lipid Research looked at soybean oil and obesity. It focused on oxylipins, compounds the body makes when it processes fats such as linoleic acid. The study suggested that soybean oil’s effects may depend partly on what the body turns linoleic acid into after digestion. Mice that were protected from soybean oil-linked obesity had lower levels of certain oxylipins, gained less weight, and were less likely to develop glucose intolerance or fatty liver.
The Practical Takeaway
Soybean oil is common because it is cheap, neutral-tasting, and useful in large-scale food production. That also makes it easy to consume in large amounts without noticing.
“Try to stay away from processed foods,” Sladek advised. “When you buy oil, make sure you read the nutrition facts label. Air fryers are a good option because they use very little oil.”
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