08/07/2026
Wondering about your gut health? This is s good summary - and should let you know why working with a gut microbiome expert makes sense.
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Your gut isn't a soup. It's zoned real estate.
When people say "gut microbiome," they tend to picture a single uniform broth sloshing around in the intestines. A review this year in Lancet Microbe describes something very different: a structured landscape, organized in space, where who lives where is set by the local chemistry.
Travel from your stomach down to your colon and the number of bacteria climbs by roughly a billion-fold. The stomach stays nearly empty because it's so acidic almost nothing survives. The upper small intestine is kept sparse too, with bile, oxygen, and fast movement flushing bacteria through before they can multiply. Further down, as oxygen runs out, the numbers start to climb. Then comes the colon, where the population explodes. More than 99% of your entire microbiome lives in that final stretch.
Your body actively enforces it. Stomach acid, bile, a protective mucus lining along the gut wall, oxygen near the surface, and the sheer speed of transit through the small intestine all work together to keep the upstream nearly sterile. Density is only allowed to explode once you reach the colon, where transit slows, oxygen runs out, and fermentation can happen.
The review notes a second kind of organization too. Even a single cross-section is layered, from the oxygen-exposed surface near the gut wall inward to the near-airless center. The gut is structured in every direction, not just top to bottom.
"Feed your microbiome" is too vague, because most of your microbiome sits in one place. What you're really doing is feeding the colon. The fuels that matter are the ones that survive the stomach and small intestine intact: fermentable fibers, resistant starch, and polyphenols. These reach the dense community downstream, which ferments them into short-chain fatty acids like butyrate, the primary fuel for your colon's own lining. Rapidly absorbed simple sugars, by contrast, get taken up higher up and tend to feed the sparse populations you don't want expanding.
SIBO (small intestinal bacterial overgrowth) is essentially the gradient breaking down: bacteria colonizing the small intestine at densities that belong in the colon, producing gas and symptoms in a compartment built to stay quiet. The gut keeps the upstream sparse for a reason, and losing that separation is itself the problem.
The specific numbers here are the well-established values the field works from, not a single new measurement, and they vary between people, sampling methods, and studies. Much of what we know still comes from stool, which mostly reports the colon. The review's real emphasis is on new high-resolution mapping methods that are finally letting researchers see this living architecture directly rather than inferring it. SIBO, too, is a simplification of a genuinely messy clinical picture.
Picture zoned real estate, with almost all the tenants downstream, and feed the room that actually houses them.
PMID 27541692.
