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Story at-a-glance

• High-fat diets (around 36% fat content) reduce protective IL-22 immune molecules by 50% within just one week, weakening intestinal barriers
• Excess palmitic acid shuts down gut immunity, setting the stage for pathogenic bacteria to overwhelm beneficial ones
• Emulsifiers in ultraprocessed foods destroy beneficial bacteria while promoting harmful strains that cause inflammation
• Even short-term consumption of ultraprocessed foods dramatically reduces protective bacteria like Akkermansia muciniphila within days of exposure
• To restore gut health, eliminate ultraprocessed foods, limit total fats to under 30% of daily calories, consume collagen-rich foods, and reintroduce healthy carbohydrates gradually

Ultraprocessed foods are pervasive in the modern food system. It's estimated that around 60% to 70% of an average American adult's daily calories consist of these products.1 While convenient and tasty, chronic consumption eventually wrecks your health, and the first place that's affected is your gut.

Eating ultraprocessed foods weakens your intestinal lining, allowing harmful bacteria to thrive and creating systemic inflammation. Moreover, your gut's balance shifts dramatically, crowding out beneficial microbes that normally keep your gut healthy and resilient. Upon closer look, evidence shows that high amounts of unhealthy fats are largely to blame.

High Intake of Fat Rapidly Disables Gut Immunity

In a study published in Immunity, researchers from the Garvan Institute of Medical Research investigated how a high-fat diet (HFD) weakens the gut's protective functions by disrupting essential immune cells. At the beginning, the team used two HFD dosages — 36% and 60% total calories as fat — but realized that both produced similar results. As such, they proceeded to just use 36% HFD for the remainder of the study.2

The methodology focused on how different types of dietary fats affected immune activity within the mice intestines, with an emphasis on a critical molecule called interleukin-22 (IL-22). This is produced by a type of immune cell known as ILC3 (innate lymphoid cells), a frontline defender that maintains the gut barrier by strengthening mucus layers and antimicrobial proteins that block harmful bacteria.

• HFD impacts your overall gut function — The researchers found that mice fed with a HFD containing about 36% fat experienced a dramatic decline in IL-22 production within just 48 hours. By seven days, IL-22 output from these immune cells dropped by approximately 50%, significantly weakening the mice's gut defenses.

This loss led to lowered production of critical gut barrier components such as mucus proteins and antimicrobial peptides. Without sufficient IL-22, the mice experienced increased gut permeability, meaning that harmful substances easily crossed into the bloodstream, triggering inflammation and systemic issues.

• A week is enough to lower your gut's defenses — One of the most eye-opening details in the study was how quickly the changes happened. Within just two days, critical defensive molecules like Muc2 (a major mucus protein) and Reg3 (an antimicrobial protein) significantly decreased in the HFD group. After one week, this initial drop evolved into full-blown damage, leaving the gut highly vulnerable to bacteria and harmful substances.

In comparing the dietary fats directly, processed fats such as palmitic acid had a clearly harmful impact, rapidly suppressing IL-22 and leaving the gut compromised. Healthier fats, on the other hand, actively preserved immune functions, creating an internal environment supportive of healthy gut defenses. In short, not all dietary fats have the same effects, highlighting the importance of dietary fat choices to maintain optimal gut health.

• Effects of palmitic acid on gut health — Looking deeper into the specifics, the team discovered a striking difference between fats used in the tests. Processed fats, particularly palmitic acid (largely derived from industrially processed palm oil and cottonseed oil3), were identified as the primary villains causing this rapid shutdown in IL-22 production.

Palmitic acid significantly impaired the ability of ILC3 cells to produce IL-22 by reducing the activity of important genetic regulators within these immune cells.

In contrast, oleic acid showed an entirely different effect. Instead of suppressing IL-22 production, the opposite effect occurred. Mice fed diets with oleic acid retained normal IL-22 levels, maintaining healthy gut barrier function.

• A closer look at the impact of HFDs — Palmitic acid exerts its harmful effects by pushing immune cells into excessive fatty acid oxidation — a process where fats are burned for energy at an abnormal rate. This excessive oxidation inhibits vital genetic pathways, such as signal transducer and activator of transcription 3 (STAT3) and receptor-related orphan nuclear receptor γt (RORγt), directly blocking the production of IL-22 by ILC3 cells.

Without IL-22, your gut barrier weakens rapidly, allowing harmful microbes and toxins to enter your bloodstream, igniting widespread inflammation.

• Don't go overboard with healthy fats — While the findings imply that more oleic acid is good for you, the devil is in the details. Findings also show that the oleic acid group also had reduced RORyt expression, but the palmitic acid group had a more suppressed effect. This shows that even certain healthy fats can have a negative effect on your health when quantity is drastically increased.

Ultimately, the findings from this research underscore the immediate and dramatic influence dietary fats have on gut immunity. The type of fat in your food determines whether your gut barrier remains strong or quickly becomes compromised.

Other Ingredients in Ultraprocessed Foods Allow Harmful Gut Bacteria to Proliferate

In a related study published in Nutrients, researchers from the University of Bologna explored the destructive effects ultraprocessed foods have on your gut bacteria and intestinal health.4

The researchers reviewed over 100 human and animal studies to understand how foods with artificial additives, preservatives, and excessive sugars alter the composition of beneficial microbes in your gut, ultimately harming your health.

• Certain ingredients in ultraprocessed foods hamper gut function — Foods high in artificial emulsifiers, such as carboxymethylcellulose (CMC) and polysorbate 80 (P80), disrupt your gut's mucus lining and cause inflammation. This triggers an increase in harmful bacteria and a decline in beneficial ones, leaving your digestive system prone to inflammation.

• How ultraprocessed foods reshape your gut bacteria — Going deeper into the findings, regular consumption of these products consistently leads to a dramatic decrease in protective keystone species like Akkermansia muciniphila and Faecalibacterium prausnitzii. These beneficial bacteria are your gut's first line of defense because they produce beneficial short-chain fatty acids (SCFAs), such as butyrate, which nourish your colonocytes that result in a stronger barrier.

Simultaneously, ultraprocessed foods cause a rapid rise in harmful bacteria, particularly the Firmicutes and Proteobacteria strains. These thrive on the refined sugars, processed fats, and additives abundant in snacks, fast food, and frozen dinners. Proteobacteria, in particular, are known for increasing inflammation.5

• Harmful ingredients in ultraprocessed food — In comparing various additives used in ultraprocessed foods, the research highlighted emulsifiers as particularly damaging. Emulsifiers like CMC and P80, often found in ice cream, salad dressings, and processed baked goods, were singled out as exceptionally harmful due to their direct effect on your gut lining. As noted by the researchers:6

"CMC and P80 impact gut microbiota gene expression, leading to the increased production of lipopolysaccharide (LPS) and flagellin, both known as inflammatory molecules that can damage the gut barrier. Prolonged exposure to these emulsifiers also results in a reduction in mucus layer thickness, thereby compromising its protective function against pathogens."

• Ultraprocessed foods are a whirlwind of toxins for your gut — As noted above, LPS are toxic molecules released from certain harmful bacteria. When the mucus layer protecting your gut lining is compromised, these endotoxins easily pass through your gut walls into your bloodstream, leading to widespread inflammation. This increases your risk for chronic disease once gut dysfunction sets in:

"A disrupted gut microbiome balance has been linked to various health issues, including obesity, Type 2 diabetes (T2DM), cardiovascular disease (CVD), chronic health conditions, and even some mental health disorders."

• Beneficial gut bacteria are suppressed — Another key mechanism by which ultraprocessed foods cause gut damage involves directly altering the environment in your digestive tract. The excessive sugar and unhealthy fats common in these foods create conditions that favor harmful bacteria over beneficial microbes.

These harmful microbes thrive by fermenting unhealthy ingredients such as refined sugars and processed fats, producing substances that further irritate and damage your gut lining:7

"Of particular significance is the impact of ultra-processed food consumption, characterized by elevated levels of refined sugars, detrimental fats, synthetic additives, and insufficient fiber content. Such dietary patterns can perturb the equilibrium of microbial communities, fostering the expansion of potentially harmful taxa while concurrently diminishing the prevalence of beneficial commensal organisms."

Strategies to Help You Optimize Fat Intake

If your diet regularly includes fast food or highly processed snacks, you've likely weakened your gut defenses. But the good news is that you have the power to reverse this damage and repair your gut's protective barriers. Here are my recommendations to get you started:

1. Ditch ultraprocessed foods immediately — These are loaded with unhealthy fats, emulsifiers, and other additives that strip away your gut's protective lining, promoting inflammation and weakening your immune defenses. Take stock of fried foods, commercial salad dressings, and foods cooked in vegetable oils such as soybean, corn, and canola oil, then ditch them entirely from your diet.

2. Cut out vegetable oils — The processed oils used to cook fast food damage your gut's immune function by shutting down essential protective molecules like IL-22. I recommend avoiding oleic acid as well.

While the Immunity study noted that oleic acid is better than palmitic acid, other research shows that oleic acid turns on metabolic switches that tell your body to store more fat, even if you're not overeating.

To protect your health, cook with healthier fats such as grass fed butter, ghee, or animal fats like tallow, which support rather than harm your gut lining. These fats are both lower in oleic acid and LA, and are much more stable when heated.

3. Limit overall fat intake — Getting more healthy fats (while minimizing LA) is important for optimal wellness. However, remember to limit your total intake — as I discuss in a separate article, excess oleic acid creates many of the same problems as LA. That's because it will integrate into your mitochondrial membranes, which displaces cardiolipin — a special fat required for energy production.

I recommend keeping your total fat consumption to less than 30% of your daily calories. When you consume more than that, your body turns on a metabolic switch known as the Randle cycle, making it burn fat as energy. This isn't what you want to happen because glucose, especially from healthy sources, is your body's ideal source of energy.

4. Repair your gut barrier — Collagen is key to healing your gut lining by providing necessary building blocks for your gut's protective mucus layer. Include high-quality collagen daily by making your own bone broth at home from grass fed meats.

Here's another tip — aim for your protein intake to be around 15% of your total daily calories. Approximately one-third (about 5%) of this should ideally be collagen. The remainder of your calories will then come from healthy carbohydrates.

5. Slowly reintroduce healthy carbohydrates — Continuing the point above, if you've previously relied heavily on processed foods, your gut bacteria will need to adjust to healthier carbs.

Begin by introducing easily digestible carbohydrates like well-cooked white rice and ripe fruits. Avoid jumping directly into high-fiber vegetables at first, as these can initially irritate your gut. Once your gut function improves, gradually expand your diet to include root vegetables, legumes, and well-tolerated whole grains. Finally, aim for 200 to 250 grams of carbohydrates per day.

Frequently Asked Questions (FAQs) About the Impact of Ultraprocessed Foods

Q: How quickly do ultraprocessed foods damage my gut?

A: Your gut defenses begin deteriorating within days of consuming ultraprocessed foods. Within a week, the damage to your gut barrier becomes significant, leaving you vulnerable to inflammation and chronic diseases.

Q: Which fats cause the most harm to my gut?

A: Vegetable oils, found in ultraprocessed foods and fast food, contain unhealthy fats that rapidly disable your gut's immune cells, reducing essential protective substances and opening the door to harmful bacteria and inflammation.

Q: Are all fats equally harmful for my gut health?

A: No. Healthy fats from nutritious sources such as grass fed butter, tallow, and ghee help maintain gut health by supporting the immune cells responsible for protecting your intestinal lining.

Q: What are the other ingredients in ultraprocessed foods that weaken my gut?

A: Emulsifiers such as carboxymethylcellulose (CMC) and polysorbate-80 (P80), common in ice cream, salad dressings, and processed baked goods, harm your gut lining, reduce protective mucus, and trigger inflammation.

Q: How can I heal my gut after consuming ultraprocessed foods?

A: Immediately cut out junk foods and snacks cooked in vegetable oils. In addition, minimize unhealthy fat intake, and repair your gut lining with collagen-rich foods. Finally, gradually reintroduce easily digestible carbohydrates like fruits and cooked white rice to restore gut function.

Sources and References

• 1 UC Davis, March 17, 2025
• 2 Immunity, Volume 58, Issue 5, 1185 - 1200.e8
• 3 Front Nutr. 2022 Mar 23;9:861664
• 4, 6, 7 Nutrients. 2025 Feb 28;17(5):859
• 5 Biomed Res Int. 2017 Nov 2;2017:9351507