What the bacteria in your colon actually do all day, what starves them, and how you can tell they're recovering.
Last Updated: August 29, 2026 · Reviewed by Dr. Sarah Kim, RDN
There are roughly as many bacterial cells in your gut as there are human cells in your body, give or take, and most of them live in the last five feet, the large intestine. They aren't passengers. They break down the fiber you can't digest, they make vitamin K and some of the B vitamins, they train your immune system to tell food from threat, and they produce short-chain fatty acids that feed the cells lining your colon. When they're well fed and diverse, digestion is mostly something you don't think about. When they aren't, you get the bloating and the irregularity and the vague sense that something's off.
I find it helpful to think of them as tenants. Good tenants pay rent (the fatty acids, the vitamins) and keep the place tidy (crowding out troublemakers). Bad tenants show up when the good ones move out, and they move out when there's nothing to eat.
Antibiotics, obviously, and they're often necessary; the problem is what you do afterward. A low-fiber diet, which is the big one, because without fiber the bacteria that ferment it starve while the ones that eat the mucus lining your gut do fine. Chronic stress, which changes gut motility and the gut's immune behavior. Poor sleep. Alcohol in quantity. Artificial sweeteners, possibly, though the evidence is messier than the headlines. And, less obviously, constipation itself: material that sits for days ferments differently from material that moves, and the bacterial mix shifts toward the species that thrive in a slow gut.
This is real, and it's also been oversold. The gut and the brain talk through the vagus nerve and through chemicals the bacteria produce, some of which (serotonin precursors, GABA) the brain uses. That's why stress shows up in your stomach and why a bad gut week often comes with a low mood. It doesn't mean a probiotic will fix your anxiety. It means the two systems are connected and it's worth treating gut trouble as something that affects how you feel, rather than a plumbing inconvenience.
You can add bacteria (a probiotic) or you can feed the ones you have (a prebiotic). For most people the second matters more, because the bacteria you already have are adapted to you, and they'll multiply on their own if there's food. Peak BioBoost is four prebiotic fibers that different bacterial groups ferment, so you're feeding a range rather than one species. That's the whole pitch: give the tenants something to eat and let them sort themselves out.
Eat more different plants. The number that keeps coming up in research is thirty different plant foods a week, which sounds absurd until you count herbs, spices, nuts and seeds. Eat some fermented food if you like it (yogurt, kefir, sauerkraut, kimchi). Sleep. Walk. Cut back on ultra-processed food, which tends to be fiber-free and additive-heavy. Don't take antibiotics you don't need. And if your diet's fiber is stuck at 15 grams because you're a normal person with a normal life, add a fiber supplement you'll take.
Regular bloating. Gas that's worse than it used to be. Constipation or diarrhea, or both alternating. Foods you used to handle that now cause trouble. And the softer signs: tiredness, catching every cold, skin flaring up. None of those alone means your microbiome is the problem, but if you've got several and no other explanation, feeding the gut is a cheap thing to try first.
Regularity comes first, usually within a week or two of adding fiber. Then less bloating, over a few weeks, once the bacteria have adjusted to the new food supply and the initial gas has passed. Then the quieter things: fewer stomach complaints, meals that don't sit heavily, better energy. It takes a couple of months for the bacterial population to genuinely shift, so don't judge it at ten days.
About 70% of your immune tissue is in and around the gut, which is one of those statistics that gets used to sell everything. What it actually means is that the gut is where your immune system learns what's normal, and the bacteria are part of that training. A diverse, fed microbiome is associated with a calmer immune system; a depleted one with more inflammation. That's the link. It doesn't make a fiber supplement an immune booster, but it does make gut neglect a whole-body issue.
After a course of antibiotics, a bad stomach bug, or a long stretch of eating badly, the population recovers on its own if you feed it. Fiber, variety, fermented food, time. Some people add a probiotic during and after antibiotics, and there's reasonable evidence for that with specific strains. Most of the recovery, though, is the survivors multiplying once they've got something to eat. Feed them.
