The three organs your food travels through
You swallow. Then what? Most of us have a vague picture — food goes down, something happens, the rest comes out. The something is three organs doing three different jobs, and only one of them is where fibre matters.
The stomach: a holding tank with acid
Your stomach is not where most digestion finishes. It is a mixing chamber. Muscular walls churn what you have eaten into a slurry, acid begins unravelling protein, and the whole mixture is held back by a valve at the exit while it is worked on.
How long that takes depends on what you ate. Water leaves in minutes. A meal with fat and protein can sit for hours. This is the first place where fibre does something: viscous fibre thickens the slurry, and a thicker slurry leaves more slowly. That is the whole mechanism behind feeling full for longer.
The small intestine: where you get fed
This is the long one — around six metres, folded into your abdomen, lined with microscopic projections that give it an enormous surface area. Almost everything your body absorbs, it absorbs here. Sugars, amino acids, fats, most vitamins and minerals.
Enzymes from your pancreas and your own intestinal wall break food down into pieces small enough to cross the lining and enter your blood. It is efficient, and it is specific: each enzyme handles a particular kind of bond.
Which raises the question this course is really about. What happens to the food your enzymes have no tool for?
The colon: about a metre and a half, and mostly misunderstood
Whatever the small intestine could not absorb arrives here. The traditional description is that the colon reclaims water and forms stool, and it does both. But calling it a waste pipe is like calling a brewery a warehouse for barley.
The colon is where the food you cannot digest meets the organisms that can. We will spend most of this course here.
Go deeper: why the small intestine is so long
Surface area. The lining is folded into ridges, the ridges are covered in finger-like villi, and each villus cell carries a brush border of microvilli. The result is an absorptive surface far larger than the organ's own dimensions suggest — which is why nutrients can be captured during the relatively short time food spends passing through.
The colon takes the opposite approach. It is shorter and smoother, and material moves through it far more slowly — hours to days rather than hours. Slowness is the point: fermentation takes time.
Three organs, three jobs. The stomach mixes and meters. The small intestine feeds you. The colon feeds something else.
What to hold on to
When you read about fibre "slowing digestion", that is the stomach. When you read about fibre "feeding your microbiome", that is the colon. They are different mechanisms in different organs, and a fibre that is good at one is not automatically good at the other.
Next we look at what the small intestine actually takes out — because the list of what it cannot take is short, and fibre is on it.