High consumption of soybean oil has been linked to obesity and diabetes and possibly autism, Alzheimer’s disease, anxiety and depression. Now add to this growing list ulcerative colitis, a form of inflammatory bowel disease, or IBD, characterized by chronic inflammation of the colon.
Researchers at the University of California, Riverside, examined the intestines of mice in the lab that were consistently fed a diet high in soybean oil for up to 24 weeks. They found that beneficial bacteria decreased and harmful bacteria (particularly adherent invasive Escherichia coli) increased – conditions that can lead to colitis.
Soybean oil is the most widely used edible oil in the United States and is increasingly used in other countries, especially Brazil, China and India. In the US, soybean production boomed in the 1970s for use as animal feed; a by-product of the accelerating growth trend 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 healthier,” said Poonamjot Deol, an assistant research scientist in the Department of Microbiology and Plant Pathology and a co-corresponding author of the article published July 3 in Gut microbesan open access journal.
Deol explained that the main concern is the linoleic acid in soybean oil.
“While our bodies require 1% – 2% linoleic acid daily, based on the paleo diet, Americans today get 8% – 10% of their energy daily from linoleic acid, most of it from soybean oil,” she said. “Excessive linoleic acid negatively impacts the gut microbiome.”
Deol and her co-authors found that a diet high in soybean oil stimulates the growth of adherent invasive E. coli in the gut. This bacteria uses linoleic acid as a carbon source to meet its nutritional needs. Furthermore, several beneficial bacteria in the gut are not resistant to linoleic acid and die, resulting in harmful bacteria growing out. Adherent invasive E. coli has been identified as a cause of IBD in humans.
“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. “Furthermore, linoleic acid causes the intestinal epithelial barrier to become porous.”
The barrier function of the intestinal epithelium is crucial for maintaining a healthy gut; when disturbed, it can lead to increased permeability or leakage. Toxins can then leak from the gut and enter the bloodstream, significantly increasing the risk of infections and chronic inflammatory conditions, such as colitis. The researchers note that the increase in IBD parallels the increase in US soybean oil consumption and hypothesize that the two may be related.
Toxicologist Frances M. Sladek, professor of cell biology and co-corresponding author of the research paper, recalled that heart disease was associated with saturated fats in the late 1950s.
“Since studies have shown 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 healthy. For example, the unsaturated fish oil is known to have many beneficial health effects. People therefore assumed that soybean oil is completely safe and healthier to consume than other types of oils, without actually making a direct comparison as we have.”
Sladek noted that linoleic acid is an essential fatty acid. The soybean oil that the researchers used in their experiments contained 19% linoleic acid. The American Heart Association recommends that 5% to 10% of daily calories come from omega-6 polyunsaturated fatty acids, such as linoleic acid, to help maintain heart health. Many seed oils, for example safflower and sunflower oil, are sources of linoleic acid. Animal fat can also be a source.
“Every animal needs to get linoleic acid from the diet,” Sladek said. “No animal can make it. A small amount is needed by the body. But just because something is needed doesn’t mean a lot of it is good for you. Different membranes in the body, for example in the brain, require linoleic acid for the cells to function properly. If we ate only 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.”
According to Sladek and Deol, olive oil, which contains less linoleic acid, is a healthier oil to consume.
“Olive oil, the foundation of the Mediterranean diet, is considered very healthy; it causes less obesity and we have now found that, unlike soybean oil, it does not increase mice’s susceptibility to colitis,” Sladek said.
James Borneman, professor of microbiology and plant pathology at UCR and a co-corresponding author of the paper, is an expert on the gut microbiome. He has worked with several groups at UCR on research projects, including studies of how gut microbes prevent obese people from losing weight. For the current study, he teamed up with Deol and Sladek to examine the gut microbes of the mice fed a diet high in soybean oil.
“Adhering invasive E. coli contributes to IBD in humans, and the fact that we find this E. coli in these mice is concerning,” he said. “Sometimes it can be unclear how research in mice translates to humans, but in this study it’s pretty clear.”
The research team were also surprised to find that the mice fed a diet high in soybean oil showed a reduction in the gut of endocannabinoids, cannabis-like molecules naturally made by the body to regulate a wide variety of physiological processes. regulate. At the same time, the gut showed an increase in oxylipins, which are oxygenated polyunsaturated fatty acids that regulate inflammation.
“We previously found that oxylipins in the liver correlate with obesity,” Deol said. “Some oxylipins have also been shown to be bioactive in colitis studies. The bottom line is that a soybean oil enriched diet, similar to the current American diet, causes gut oxylipin levels to rise and endocannabinoid levels to fall, which is consistent with IBD by people.”
Most processed foods in the US contain soybean oil, which may explain why many Americans consume more than the recommended daily intake of linoleic acid. Furthermore, most restaurants in the US use soybean oil because it is relatively cheap.
“Try to stay away from processed foods,” Sladek advised. “If you buy oil, read the nutrition facts label. Air fryers are a good option because they use very little oil.”
The researchers use olive oil for cooking and salads. Other healthy options for cooking, they said, are coconut oil and avocado oil. They warned that corn oil, on the other hand, contains the same amount of linoleic acid as soybean oil.
“We recommend keeping track of the soybean oil in your diet to make sure you’re not consuming too much linoleic acid,” Deol said. “That’s our message to take home.”
More information:
Poonamjot Deol et al, Diet high in linoleic acid dysregulates the intestinal endocannabinoid system and increases susceptibility to colitis in mice, Gut microbes (2023). DOI: 10.1080/19490976.2023.2229945
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