Your Favorite Food Might Be Hurting Your Gut

For years, nutrition experts have debated the root causes of the global obesity epidemic. Is it sugar? Fat? Lack of exercise? What we have learned from hundreds of scientific studies, obesity is not a single-cause problem, but rather is the end result of multiple factors interacting with each other, including genetics, type, quantity, and quality of food, lack of exercise, and stress. While all these factors play a role, emerging research points to another, often-overlooked culprit: ultra-processed foods (UPFs).

The current U.S. presidential administration is taking notice. Robert F. Kennedy Jr. (RFK Jr.), the U.S. Secretary of Health and Human Services, recently testified before Congress, highlighting the prevalence of artificial additives and food dyes in U.S. products compared to their European counterparts. A recent study, highlighted in The New Yorker by Dhruv Khullar, sheds light on how these foods may be silently wreaking havoc on our metabolism, microbiome, and long-term health.

What Are Ultra-Processed Foods?

According to the NOVA classification system, foods fall into four categories based on their level of processing:

• Group 1: Unprocessed or minimally processed foods (fruits, vegetables, eggs, milk, and grains).
• Group 2: Processed culinary ingredients (oils, salt, sugar).
• Group 3: Processed foods (cheese, canned vegetables, bread made with minimal ingredients).
• Group 4: Ultra-processed foods (UPFs)—industrially manufactured foods with artificial additives, preservatives, emulsifiers, and synthetic flavors.

UPFs include everything from sodas and packaged snacks to processed meats, flavored, sugary yogurts, and fast food. They are engineered for long shelf life and to be tastier, making them hard to resist and easy to overeat. While the NOVA classification has fallen out of favor with some nutrition and obesity experts, the term UPFs remains a useful term to highlight the culprits of our unhealthy diet.

A study conducted by Dr. Kevin Hall at the National Institutes of Health (NIH) sought to unveil the effects of UPFs on the body. Twenty healthy participants lived in a controlled environment for four weeks, allowing the research team to get an accurate picture of their food consumption. Under close observation by the investigators, study participants spent two weeks on a diet rich in unprocessed, whole foods, and another two weeks consuming UPFs. The results were striking:

• On the UPF diet, participants ate an average of 500 calories more per day than when they consumed whole foods.
• They gained approximately 2 pounds, whereas they lost approximately 2 pounds on the unprocessed diet, despite the diets being nutritionally similar in terms of fat, sugar, and protein content.
• UPFs led to increased insulin secretion and higher blood glucose levels.
• The participants ate faster and reported feeling less satisfied, suggesting that food composition and processing affect satiety signals.

The study’s results were contrary to previous beliefs that weight gain is only about calorie intake, even though the fact that caloric intake was significantly lower in the healthy food group. However, study results emphasize the important role that food composition and quality have on dietary intake, satiety, eating behavior, and metabolic health.

The dangers of UPFs go far beyond weight gain. Numerous epidemiological studies link high UPF consumption to:

• Increased risk of chronic diseases like heart disease and inflammatory bowel disease (IBD).
• Mental health issues, including anxiety and depression. One study suggested that women who ate the most UPFs were 50% more likely to develop depression.
• Gut microbiome disturbances — Dr. Arpana Church and collaborators at the G. Oppenheimer Research Center for Stress and Resilience at UCLA found a positive association between food addiction, brain structural changes, and a distinct gut microbiome profile.

Why Are We Addicted to Ultra-Processed Foods?

Several factors make UPFs difficult to resist:

• Energy Density – Many UPFs are dehydrated for long shelf life, making them energy-dense (chips and cereal). This allows people to eat more calories without feeling full.
• They are engineered to trigger bliss—perfect combinations of sugar, salt, and fat that make them highly palatable and even addictive.
• Taste Adaptation – Regular consumption can dull the sensitivity of taste and satiety receptors, making natural foods seem dull.

The Role of the Gut Microbiome in Processed Food-Related Diseases

A particularly alarming discovery is how UPFs affect the gut microbiome—the trillions of bacteria that play a key role in digestion, immune function, and even mental health.

Dr. Katherine Maki, a microbiome researcher at NIH, analyzed stool samples from participants in Hall’s study. She discovered participants on the UPF diet had reduced microbial diversity and an increase in bacteria, such as Akkermansia muciniphila, B. theta, and B. thetaiotaomicron, which in the presence of fermentable fiber are gut health promoting, but in the absence of adequate fiber (~95% of Americans are not getting enough fiber), they compromise the mucus layer by substituting mucus carbohydrate for the missing dietary fiber.

Artificial sweeteners and preservatives commonly found in UPFs may also affect the microbiome in ways that alter glucose metabolism, potentially leading to insulin resistance and metabolic disorders and worsening blood sugar control.

How To Avoid UPFs?

Due to their convenience, avoiding UPFs entirely can be challenging. However, Dr. Mayer suggests minimizing consumption rather than obsessing over perfection. Here are a few strategies:

• Prioritize whole and minimally processed foods in your daily diet– Choose fresh fruits, vegetables, whole grains, and proteins that contain simple, recognizable ingredients.
• Retrain your sweet receptors by avoiding any products with added sugar, including sugary drinks. After practicing this for a few weeks, you will notice a distinctive change in your taste preferences, avoiding many high sugar items.
• Cook at home more often – Homemade meals give you control over ingredients.
• Be mindful of how food makes you feel – Many people feel bloating, sluggishness, or mood swings after eating UPFs. Listening to these cues can help reinforce better food choices.
• Minimize or avoid snacks and so-called powerbars. Unless you go on long hikes or are an endurance athlete, you don’t need the extra calories between your main meals.

While research into ultra-processed foods is still evolving, the evidence is clear: these foods alter metabolism, disrupt hunger cues, and contribute to chronic disease. Although occasional indulgence is fine, making an effort to prioritize whole, nutrient-dense foods could be one of the most impactful health decisions you make.

By shifting the focus away from counting calories and instead looking at food quality and processing, we may finally get closer to solving the obesity epidemic—and reclaiming our health. Quality, Not Type, of Diet is what we should prioritize. Below are some specific foods you want to avoid and some you may want to add.