That innocent-looking cookie, small pack of fries, or occasional cheeseburger might feel like a harmless indulgence—especially if you’re otherwise eating healthy or don’t struggle with your weight. But new research suggests these tiny treats could be doing silent, long-term damage to your gut—even in small doses.

A recent Australian study published in the journal Immunity (May 2025) has found that just two days of eating a high-fat diet , mainly one rich in saturated fats, can disrupt gut defences and increase inflammation throughout the body. These are the same types of fats found in everyday favourites like muffins, deep-fried snacks, processed meats, and fast food.


Silent inflammation starts with everyday fat
The study, titled “Acute exposure to high-fat diet impairs ILC3 functions and gut homeostasis”, was led by Dr Cyril Seillet, a researcher from Monash University and The Walter and Eliza Hall Institute of Medical Research. Under this study, using mouse models, Dr Seillet and his team showed that a high-fat diet rapidly impairs the function of a specific group of immune cells known as ILC3s (group 3 innate lymphoid cells). These cells are vital for maintaining the gut’s protective barrier, largely by producing a molecule called interleukin-22 (IL-22).

IL-22 helps the gut produce antimicrobial peptides, mucus, and tight junction proteins that keep harmful bacteria and toxins from leaking into the bloodstream. When ILC3s can’t produce enough IL-22, the gut lining becomes more permeable—a condition often described as “leaky gut.” This allows inflammatory substances to seep into circulation, silently fueling long-term inflammation and increasing the risk of chronic disease .

“The more saturated fats we eat, the more inflammation that builds up,” said Dr Seillet. “But this inflammation remains silent for years and only shows up later as chronic conditions.”


Not all fats are equal: How saturated vs unsaturated fats impact your gut

The study revealed that not all fats affect the gut the same way. Mice fed with saturated fats like palmitic acid, found in butter, fatty meats, and palm oil, experienced reduced IL-22 production and gut barrier breakdown. In contrast, those consuming unsaturated fats like oleic acid, which is found in olive oil and avocados, maintained healthier gut function.

Saturated fats were found to trigger harmful fat oxidation pathways in immune cells, while unsaturated fats formed protective lipid droplets that preserved immune function. The effects were so immediate that even after just 48 hours, mice on a high-fat diet showed gut inflammation and immune suppression. By day seven, the changes became even more pronounced.


Fatty foods damage gut bacteria, but your gut can heal
The study also looked at the gut microbiome , the community of bacteria living in the intestines. Just one week of a high-fat diet caused dramatic shifts: beneficial bacteria declined, and harmful species flourished. Some of these bad actors, like Enterococcus gallinarum, are known to erode the gut lining and promote inflammation.

To test how these dietary changes might affect disease recovery, researchers induced colitis in mice fed different diets. Those on high saturated-fat diets fared worse—losing more weight and showing greater gut damage—than mice fed unsaturated fats or regular food.
What’s encouraging is that these negative effects can be reversed. When mice returned to a standard, lower-fat diet, their gut function began improving within two days, and most immune functions returned to normal within a week. This suggests that the body can bounce back—if we catch the damage early and adjust our eating habits.


Your gut remembers: How fatty foods leave a lasting impact
Although the study was conducted in mice, similar effects were observed in cultured human immune cells, giving weight to the findings. This research may help explain why people experience digestive discomfort after periods of poor eating—like during holidays or fast-food binges. It also reinforces why obesity is now understood as a state of chronic, silent inflammation, often triggered and worsened by gut dysfunction.

Experts say these findings could also be linked to rising rates of inflammatory bowel disease (IBD), autoimmune conditions, and metabolic disorders like diabetes and fatty liver disease—all of which have roots in gut health and inflammation.

“Even an occasional indulgence in high-fat foods can disrupt your gut if it’s not balanced out with healthier choices,” said Dr Rakesh Kochhar, former head of gastroenterology at PGI Chandigarh, in response to the study. “The key lies in using unsaturated fats and limiting ultra-processed snacks and fried foods.”

You don’t need to completely avoid fats; your body needs them. But the type of fat, how often you consume it, and what you pair it with matter greatly. The occasional cookie might seem harmless, but when such indulgences become regular, they quietly add up in ways that affect your gut, immune system , and overall health. In short, hidden obesity and inflammation can begin where you least expect it, in the snacks you don’t think twice about. Your body can recover quickly with the right diet and lifestyle choices. That’s a reason to rethink your next "harmless" bite.


Also Read: Why asthma gets worse in monsoon and what you can do about It