Important Points:
- Microbiome
- Bacteria
- Storage
What is the human gut microbiome?
The human microbiome is the community of bacteria living in your gut, and they outnumber your own cells. They digest fiber you cannot. They decide how much of a meal becomes stored energy. And the mix in an obese gut is different from the mix in a lean one.
We tend to think of bacteria as dirty, something to keep outside our bodies. But gut bacteria are very important for digestion. They break complex fiber polysaccharides (sugar molecules linked together) into simple sugars we can absorb. This process is called fermentation, and it turns nutrients into calories. The large intestine alone holds over 1400 species of bacteria, numbering over 100 trillion. In fact, the human body has ten times more microbial cells than human cells. And the body depends on the genes in these bacteria for specific metabolic pathways.
Our microbial partners evolved along with us. We share nutrients in a helpful (symbiotic) relationship. Early humans had uneven access to food, so the ability to store energy helped them. When nutrient-dense food was around, eating and storing it helped both them and their bacteria later, when food ran short. But in modern, developed societies, large portions and high-calorie diets are easy to get. There this “benefit” becomes a detriment. We develop a once-rare condition called over-nutrition, over-storage, or obesity.
Obese people have a different mix of bacteria in their guts than thin people. The ratio of Firmicutes to Bacteroidetes (called the F/B ratio) is higher in obese people than in lean people. It drops as those people lose weight. Both Firmicutes and Bacteroidetes break down complex polysaccharides. But Firmicutes are much more efficient at it. So having more Firmicutes in our gut makes more energy available. That leads to more calories taken in and, in time, obesity. Too many Firmicutes is also associated with slowed gut movement, commonly known as chronic constipation.
Studies looked at identical twins with the same genes. Their gut bacteria were found to differ depending on whether the twin is lean or obese. The obese twins had much higher levels of Firmicutes. Obese patients can have gastric bypass surgery. After the operation, their colon bacteria change. They become more like those of normal-weight people, with lower Firmicutes levels.
Overusing antibiotics may also wipe out the beneficial bacteria in your gut along with the bad ones. Conventional farmed meats are dosed with antibiotics. Nearly 50-70% of all antibiotics made in the United States go into the animal feed of healthy livestock. They are used to boost growth and weight gain. Eating meat loaded with these drugs may be a significant factor driving growth and weight in the human population [obesity] as well. Eating grass-fed and organically raised meats of all kinds can cut this antibiotic intake.
The amazing thing to keep in mind is that you can change your gut bacteria to a healthy ratio fast, within 72 hours. Simply cut out refined carbohydrates right away and eat more fiber. Supplements of “get thin bacteria” will soon be available as well.
Frequently asked questions
How can I improve my gut microbiome?
Cut refined carbohydrates right away and eat more fiber. Those two changes can shift your gut bacteria toward a healthier ratio within 72 hours. Choose grass-fed and organically raised meats too. That lowers the antibiotics you take in from conventionally farmed meat. It helps protect the beneficial bacteria you already have.
What ruins your gut microbiome?
Antibiotics are a major culprit. Overusing them can wipe out the beneficial bacteria in your gut along with the harmful ones. You also take in antibiotics indirectly. 50 to 70 percent of the antibiotics made in the United States go to healthy livestock to speed growth and weight gain. Eating that meat may drive weight gain in people as well.
Can gut bacteria cause weight gain?
They can tip the balance. Obese people carry a higher ratio of Firmicutes to Bacteroidetes bacteria than lean people. Firmicutes pull more energy out of the fiber you eat. So more of each meal becomes available to store. In identical twins, the obese twin carries far more Firmicutes. And the ratio falls as people lose weight.
How many bacteria live in your gut?
More than 100 trillion, spread across over 1,400 species in the large intestine alone. The human body holds about ten times more microbial cells than human cells. You depend on the genes in those bacteria for specific metabolic pathways. One is breaking complex fiber down into simple sugars you can absorb, a process called fermentation.
Related reading
Medically reviewed by Dr. Gurpreet Singh Padda, MD, MBA, MHP
Interventional pain physician, addiction medicine specialist and obesity medicine diplomate.
Licensed in Missouri and Illinois.
Last reviewed June 18, 2026
