*By Dr. Devan

For decades, scientists believed that weight gain was a simple matter of calories in versus calories out. But in recent years, a stunning discovery has shifted this understanding: certain bacteria in our gut can make us gain or lose weight—irrespective of how much we eat. These are now popularly known as the “fattening bacteria” or obesogenic microbes.

This revelation has opened an entirely new dimension in the management of weight, where the gut microbiome—the trillions of microorganisms residing in our intestines—plays the central role in determining whether one tends to gain or lose weight easily.

1. The Gut Microbiome and Its Influence on WeightOur gut is home to more than 100 trillion bacteria, belonging mainly to two large families: Firmicutes and Bacteroidetes. The ratio between these two bacterial groups is now known to be a crucial determinant of body weight.

Firmicutes are the so-called fattening bacteria. They extract more energy from food, converting even indigestible fibers into absorbable calories.Bacteroidetes, on the other hand, are slimming bacteria. They are less efficient at extracting calories, leading to lower energy absorption and leaner body types.Studies on identical twins—where one was obese and the other lean—revealed striking differences in their gut flora composition. The obese twin had a much higher proportion of Firmicutes compared to Bacteroidetes. When these “obese microbiota” were transferred into germ-free mice, those mice rapidly gained weight despite eating the same amount of food as their lean counterparts.

2. Mechanism of the Fattening BacteriaFattening bacteria contribute to weight gain through several well-documented mechanisms:

Increased Caloric Extraction:These microbes possess special enzymes that break down complex carbohydrates into short-chain fatty acids (SCFAs), such as acetate, propionate, and butyrate. The body then absorbs these SCFAs as extra calories.

Enhanced Fat Storage:They stimulate the production of certain hormones like insulin and glucagon-like peptide-1 (GLP-1) that encourage fat storage in the liver and adipose tissues.

Altered Satiety Signaling:Some fattening bacteria influence the release of appetite-regulating hormones such as ghrelin and leptin, increasing hunger and promoting overeating.

Reduced Metabolic Rate:These microbes can slow down metabolism by influencing mitochondrial efficiency, leading to reduced calorie burning.

3. Identifying the Fattening BacteriaSome of the key “fattening bacteria” identified in scientific studies include:

Clostridium cluster XIVa and IV – Known for fermenting carbohydrates efficiently.Lactobacillus reuteri and Lactobacillus fermentum – Though beneficial in some contexts, they are linked with weight gain in animals and humans.Firmicutes (especially from genera Ruminococcus and Eubacterium) – Strongly associated with obesity.Methanobrevibacter smithii – A methane-producing archaea that enhances fermentation efficiency, increasing energy harvest from food.4. Using the Fattening Bacteria to Put on WeightWhile most modern focus is on losing weight, there are individuals—particularly those with chronic underweight, malnutrition, or hypermetabolism—who wish to gain healthy body mass. Harnessing the power of fattening bacteria offers a natural and scientific approach to this challenge.

Here’s how they can be deliberately encouraged:

a. Probiotic SupplementationCertain strains like Lactobacillus reuteri and Lactobacillus fermentum are available as probiotics. Studies show these strains can increase body fat percentage and weight gain when taken regularly.

b. High-Fiber Fermentable DietFattening bacteria thrive on fermentable fibers—like those found in bananas, oats, rice, potatoes, and legumes. These fibers serve as prebiotics, feeding Firmicutes and allowing them to multiply.

c. Avoiding Harsh AntibioticsRepeated antibiotic use kills beneficial gut flora, including fattening bacteria. Individuals who wish to gain weight should avoid unnecessary antibiotics and instead repopulate their gut with probiotics post-treatment.

d. Fermented FoodsFoods such as curds, kefir, idli, dosa batter, sauerkraut, and kimchi are rich in live microbes that promote a flourishing gut ecosystem conducive to better nutrient absorption.

e. Gut Microbiome TransferIn clinical research, fecal microbiota transplantation (FMT) from overweight donors to lean subjects resulted in significant weight gain. Though still experimental, it highlights how directly influencing gut flora can change body weight dynamics.

5. The “Lean Microbiome” ProblemUnderweight individuals often have a “lean microbiome,” dominated by Bacteroidetes and lacking the energy-harvesting Firmicutes. They tend to metabolize food inefficiently, have frequent bowel movements, and show reduced absorption of fat-soluble vitamins (A, D, E, K).By strategically increasing the proportion of Firmicutes and reducing excess Bacteroidetes, such individuals can experience better energy retention and gradual weight gain.

6. The Role of Curds and Fermented MilkCurds, or yogurt, hold a special place in traditional diets for a reason. Fresh homemade curds contain live Lactobacillus species—some of which, like L. reuteri, aid in weight gain. When taken with meals, they enhance digestion, nutrient assimilation, and increase caloric efficiency.

A simple daily practice for underweight individuals:→ Eat a bowl of freshly set curds with lunch and dinner.→ Add a teaspoon of jaggery or honey to promote bacterial growth.→ Combine with rice or fermented batter foods to maximize gut flora diversity.

7. Balancing the Microbiome NaturallyWhile fattening bacteria can help one gain weight, an excess can lead to unhealthy obesity and metabolic disorders. The goal is balance, not dominance.

Tips for balanced microbial health:

Rotate between probiotic strains (curds, kefir, fermented vegetables).Eat seasonal fruits and whole grains.Ensure adequate zinc and magnesium intake—minerals that support beneficial bacterial metabolism.Avoid refined sugar, which feeds pathogenic bacteria.8. ConclusionThe discovery of fattening bacteria has revolutionized our understanding of nutrition and metabolism. It is no longer just what we eat, but what our bacteria do with what we eat that determines our body weight.

By cultivating the right microbiota through probiotics, prebiotics, and fermented foods, we can safely and naturally help underweight individuals put on healthy mass—without resorting to synthetic supplements or excessive caloric intake.

The gut, indeed, is the new frontier of medicine—and the key to weight management lies not in the plate, but in the microscopic world within us.

“The body is a reflection of the bacteria it harbors; nurture them right, and they will build you strong.”

*Dr Devan is a Mangaluru-based ENT specialist and author.

Hindusthan Samachar / Manohar Yadavatti