They add that the high-fat diets also adversely affect the immune system, brain function, and a person’s risk of getting COVID-19.

For their research, scientists fed mice three different diets over 24 weeks. At least 40% of the calories from the diets came from fat.

The researchers then examined genetic changes in all four parts of the mice’s intestines.

One group of mice consumed a diet based on saturated fat from coconut oil. The second group took in a diet based on monounsaturated, modified soybean oil. The third group consumed an unmodified soybean oil diet high in polyunsaturated fat.

Even though mouse studies don’t always translate to humans, researchers noted that mice share 97% of people’s working DNA.

When compared to a low-fat control diet, all three groups of mice “experienced concerning changes in gene expression,” which is the process of turning genetic information into functional substances like protein.

The team said the findings are concerning because people in the United States consume more soybean oil than any other oil. It’s also increasingly used in countries such as China, India, and Brazil.

Researchers said some of the changes they observed weren’t surprising, such as those they saw in genes related to gut bacteria and fat metabolism.

What was surprising was the changes seen in genes regulating susceptibility to infectious diseases.

“We saw pattern recognition genes, ones that recognize infectious bacteria, take a hit. We saw cytokine signaling genes take a hit, which help the body control inflammation,” the study authors wrote. “So, it’s a double whammy. These diets impair immune system genes in the host and they also create an environment in which harmful gut bacteria can thrive.”

The research also showed all three high-fat diets increased the expression of ACE2 and other host proteins used by COVID-19 spike proteins to enter the body.

Researchers added that high-fat food increased signs of stem cells in the colon.

“You’d think that would be a good thing but actually they can be precursors to cancer,” said Sladek.

In the study, coconut oil showed the greatest number of changes in terms of gene expression, followed by unmodified soybean oil.

The team reported differences between the two soybean oils, suggesting polyunsaturated fatty acids in unmodified soybean oil — primarily linoleic acid — help alter gene expression.

The team found negative changes to the microbiome were more pronounced in mice given soybean oil. They said this wasn’t surprising as they’d previously documented other negative health effects of high soybean oil consumption.

They noted they had previously found the oil could affect genes in the brain related to autism, anxiety, depression, and Alzheimer’s disease.

The researchers said the findings only apply to soybean oil and not to other soy products, tofu, or soybeans themselves.

“It’s showing three different fat diets negatively impact the immune system and microbiome throughout the entire intestinal tract – from the duodenum to the colon,” said Bilchik, who was not involved in the study. “High fat diets suppress genes that are important in the immune system and alter the function of the trillions of bacteria throughout our bodies (the microbiome).”

“What that means is that in the cell, certain genes can be overactivated and certain genes can have their activity reduced,” said Firoozi, who was not involved in the research. “Based on the study, this effect increased metabolism and decreases the immune response.”

“It also appears to negatively affect the gut bacteria or microbiome, leading to a higher presence of pro-inflammatory bacteria,” he added. “This translates into an increased risk of developing inflammatory bowel disease, diabetes, and cancer.”

“But the evidence demonstrates the foods you eat affect how your body expresses – turns ‘off’ or ‘on’ – the myriad genetic possibilities your DNA has encoded within it,” said Kollmuttathuillam, who also was not involved in research. “Gene expression ultimately plays a major role in cell functions that increase or decrease the long-term risk of diseases such as obesity, heart disease and cancer.”

How should people respond to the study?

“Eating less processed foods and less fast foods can help us to be healthier,” said Ortiz-Pujols, who was not involved in the study. “Even changing our cooking oils to olive oil is a small but realistic change that can promote positive changes in our microbiome.”

“Small changes in our diet can lead to big changes in our health,” she added. “When we talk about diets, we often think about restriction and short-term commitments. I would recommend not thinking about changing what you eat as a diet. Instead, think about making small changes to what and how we eat that will become lifelong habits. If we focus on making small changes, we are more likely to be successful.”