Gut microbiota and obesity: how are they connected?

The gut microbiota is far more than a passive resident of the digestive system, it functions as an additional metabolic organ.

“As the composition of the microbiota changes, so can a person’s tendency to gain weight or maintain a healthy body weight through several biological mechanisms,” explains Dr. Grandi.

These include:

• Regulation of metabolism. Gut bacteria ferment indigestible carbohydrates, such as dietary fiber, producing short-chain fatty acids (SCFAs)—including acetate, propionate, and butyrate, which act as important metabolic signaling molecules.
• Regulation of hunger and satiety. The gut microbiota communicates directly with both the central nervous system and the endocrine system. By influencing the production of gut hormones such as ghrelin and peptide YY (PYY), intestinal bacteria help regulate appetite and satiety signals sent to the brain.

Gut dysbiosis and obesity: what is the connection?

For optimal function, the gut microbiota requires eubiosis, a balanced and diverse microbial ecosystem. When this balance is disrupted, the condition is known as gut dysbiosis.

“What is dysbiosis? It is a qualitative and quantitative alteration of the intestinal bacterial community, characterized by the loss of beneficial species and the overgrowth of potentially harmful bacteria,” explains Dr. Grandi.

Numerous studies have shown that people living with obesity often exhibit a characteristic pattern of dysbiosis, commonly involving an altered balance between two major bacterial groups: Firmicutes and Bacteroidetes.

“A higher abundance of Firmicutes is frequently associated with obesity. What we know today is that the lifestyle typically associated with obesity, including physical inactivity and a diet rich in processed foods, sugars, and saturated fats, promotes an increase in Firmicutes. However, it has not yet been definitively established whether elevated levels of these bacteria directly contribute to obesity.

Current research suggests that the inflammatory state associated with an excess of Firmicutes may promote low-grade systemic inflammation, creating conditions that favor the development of obesity, in which chronic inflammation plays an important role.”

Dysbiosis may also impair the intestinal barrier, allowing bacterial toxins to enter the bloodstream. This can trigger chronic low-grade inflammation, a condition strongly associated with insulin resistance and the accumulation of visceral fat.

“Yes, but not directly,” says the specialist.

Probiotics (live microorganisms that provide health benefits to the host) and prebiotics (dietary fibers that nourish beneficial bacteria) may help restore a healthy microbial balance.

Some probiotic strains, including certain varieties of Lactobacillus gasseri and Bifidobacterium, have shown promising results in reducing visceral fat and improving metabolic health.

“However, supplements are effective only when they are part of a comprehensive lifestyle intervention. They cannot replace the benefits of a balanced diet and regular physical activity,” Dr. Grandi cautions.

Diet is the most powerful tool we have for cultivating a healthy gut microbiota.

Foods to include

• Fiber and complex carbohydrates: vegetables, fruit, legumes, and whole grains provide the preferred fuel for beneficial bacteria.
• Fermented foods: plain yogurt, kefir, and fermented foods or beverages such as sauerkraut and kombucha supply live beneficial microorganisms.
• Polyphenol-rich foods: berries, cocoa, green tea, and extra virgin olive oil contain antioxidant compounds that promote the growth of beneficial bacteria.

Foods to limit

• Refined sugars and artificial sweeteners, which may promote inflammatory bacteria and negatively affect glucose metabolism.
• Ultra-processed foods and poor-quality saturated fats, which can damage the intestinal barrier and significantly reduce microbial diversity.

Lifestyle, the gut microbiota, and obesity

“It’s not only what we eat that affects our gut bacteria,” Dr. Grandi continues. “The microbiota is highly sensitive to overall lifestyle.”

Key lifestyle factors include:

• Sleep. Sleep deprivation disrupts circadian rhythms, promotes dysbiosis, and has been associated with increased cravings for calorie-dense foods.
• Stress. Chronic stress increases intestinal permeability and alters the composition of the gut microbiota, encouraging the accumulation of body fat—particularly visceral abdominal fat.
• Physical activity. Regular exercise increases the production of short-chain fatty acids and promotes the growth of bacterial species associated with better metabolic health.

If you experience persistent gastrointestinal symptoms such as bloating, irregular bowel habits, or slow digestion, or suspect that your gut microbiota may be out of balance, self-treatment is not recommended.

It is important to consult a nutrition specialist (physician specializing in nutrition or registered dietitian) or a gastroenterologist. These professionals can:

• assess your intestinal health;
• recommend appropriate tests to evaluate the composition of your gut microbiota when indicated;
• develop a personalized nutritional and therapeutic plan, including supplements when appropriate, to restore intestinal balance and support long-term health.

The Clinical Dietetics and Nutrition Service at Policlinico San Pietro specializes in the diagnosis and treatment of nutritional disorders, providing care for patients with overweight and obesity, eating disorders, diabetes mellitus, gastrointestinal conditions (including irritable bowel syndrome, celiac disease, and gastroesophageal reflux disease), food allergies, and other nutrition-related conditions.

The service adopts a multidisciplinary approach, bringing together physicians specializing in clinical nutrition, registered dietitians, and nutrition professionals to provide individualized patient care.