What is fermentation, actually?
Fermentation is one of those words that sounds like it means something specific and usually is not made to. Here is what it actually is, and why it produces gas.
Breaking things down without oxygen
Fermentation is how an organism extracts energy from a molecule when oxygen is not available to finish the job.
With oxygen, a cell can take a fuel molecule apart completely and release a lot of energy. Without it, the process stops partway — the fuel is broken into smaller molecules that still hold usable energy, and those are released as by-products.
Those by-products are the interesting part. In your colon they are short-chain fatty acids, and they are the subject of Part III.
Why it happens where it happens
The colon is oxygen-poor, so the organisms living there ferment rather than respire. The material available to them is what your own enzymes could not use. The residence time is long enough for slow work.
Every condition required for fermentation is met in that one organ, and nowhere else in you at any comparable scale.
The gas
Fermentation produces gas. Hydrogen and carbon dioxide, mostly, and in some people methane. This is not a malfunction and it is not a sign of a problem — it is what the chemistry does.
Most of it is dealt with quietly. Some is absorbed and breathed out, some is consumed by other microbes, and some passes. A person eating a reasonable amount of plant material produces gas continuously and rarely thinks about it.
What people notice is not gas but rate. Gas produced gradually is cleared as it forms. Gas produced faster than it can be cleared accumulates, distends, and becomes the sensation people describe as bloating.
This is the single most useful thing to understand about fibre tolerance. Two fibres can yield a similar total amount of gas over a day and feel completely different, because one delivers it over hours and the other over minutes.
What that does not mean
It does not mean gas is a sign that something is working, and it does not mean discomfort should be pushed through. Some fermentation is normal and expected. Persistent, severe or worsening symptoms are worth taking to a clinician rather than interpreting yourself — a fibre course cannot tell you which of those you have.
It also does not mean slow fermentation is simply better. A fibre that ferments very little produces little gas and little else besides; that can be exactly what you want, or not, depending on what you are after. We will come back to this trade-off properly in Part IV.
Go deeper: why some people make methane and others do not
Certain archaea consume hydrogen and produce methane from it. Whether you host them in appreciable numbers varies between individuals, and it changes both the gas mixture and how much total gas volume is present, since several hydrogen molecules are consumed to make one of methane.
This is one of several reasons two people can eat the same fibre and report genuinely different experiences, without either of them being wrong about what they felt.
Fermentation is energy extraction without oxygen. Gas is its by-product, not its failure. What determines comfort is how quickly that gas arrives.
What to hold on to
Rate, not amount. Hold on to that phrase — it is the hinge that Part IV turns on.
Now: what exactly are those by-products, and why would the rest of your body care about something made in your colon?