According to Sladek and Deol, olive oil, which has lower amounts of linoleic acid, is a healthier oil to consume. Picture: Pexels
California - High use of soybean oil has been associated with obesity and diabetes, as well as autism, Alzheimer's disease, anxiety, and depression.
Add ulcerative colitis, a kind of inflammatory bowel disease (IBD) characterised by chronic inflammation of the large intestine, to the growing list.
Researchers at the University of California, Riverside, examined the gut of mice that were consistently fed a diet high in soybean oil for up to 24 weeks in the lab.
They found beneficial bacteria decreased and harmful bacteria (specifically, adherent invasive Escherichia coli) increased - conditions that can lead to colitis.
Soybean oil is the most commonly used edible oil in the United States and is increasingly being used in other countries, particularly Brazil, China, and India.
In the US, soybean production took off in the 1970s for use as animal feed; a byproduct of the increasing trend in growth was soybean oil. Soybeans, a good source of protein, are easy and cheap to grow.
"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," said Poonamjot Deol, an assistant professional researcher in the Department of Microbiology and Plant Pathology and a co-corresponding author on the paper published July 3 in Gut Microbes, an open access journal.
Deol explained it is linoleic acid in soybean oil that is the main concern.
Deol and her co-authors found that a diet high in soybean oil encourages the growth of adherent invasive E. coli in the gut.
This bacterium uses linoleic acid as a source of carbon to meet its nutritional demands.
Further, several beneficial bacteria in the gut are not able to withstand linoleic acid and die off, which results in harmful bacteria growing out. Adherent invasive E. coli has been identified in humans to cause IBD.
"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."
The barrier function of the intestinal epithelium is critical for maintaining a healthy gut; when disrupted, it can lead to increased permeability or leakiness.
Toxins can then leak out of the gut and enter the bloodstream, greatly increasing the risk of infections and chronic inflammatory conditions, such as colitis.
Toxicologist Frances M. Sladek, a professor of cell biology and a co-corresponding author on the research paper, recalled that heart disease was linked to saturated fats in the late 1950s.
"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."
According to Sladek and Deol, olive oil, which has lower amounts of linoleic acid, is a healthier oil to consume.
"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.