Your colon is not one place
direction of travel →
A chart of how much fermentable fibre is left at each point along the colon, from the caecum on the left to the sigmoid on the right. With only fast-fermenting fibre the amount falls to zero before the transverse colon, so the descending colon and sigmoid receive almost none. With fast and slow fibre together some is still present at the sigmoid.
Almost everything written about fermentation treats the colon as one container. It is not. It is a metre and a half of pipe, and where along that pipe something happens changes what it does for you.
Fermentation front-loads
Fibre arrives at the caecum, the pouch where the colon begins. That is where the bacterial population meets the day's undigested material, and it is where the feeding is most intense.
Then the material travels. As it moves through the ascending and transverse colon it is progressively consumed, so by the time it reaches the descending colon and sigmoid, much of what was fermentable has already been fermented.
You can measure this. When short-chain fatty acids were sampled along the colon, total concentration was highest in the caecum and ascending colon — around 131 mmol per kg of contents — and considerably lower in the descending and sigmoid, around 80 (Cummings et al., 1987). Acidity tracks it: pH sits near 5.7 in the caecum and rises towards 6.6 or higher by the rectum (Fallingborg, 1999).
One correction worth making
A claim circulates that the proximal colon makes acetate and propionate while the distal colon makes butyrate. It is a tidy story and it is the wrong way round.
In that same sampling, the proportions of the three acids stayed roughly constant along the length. Since the total is higher proximally, butyrate in absolute terms is also higher proximally. Butyrate is not a distal speciality. It is most abundant exactly where fermentation is most active — near the beginning.
This matters more than a pedantic correction, because it inverts the problem. If the distal colon were naturally butyrate-rich, there would be nothing to solve. It is not. It is the stretch that runs on the least.
Why the far end is the interesting end
Three things make the descending colon and sigmoid worth caring about.
Contents sit there longest. The distal colon is a holding area. Whatever is present has extended contact with the lining.
It is substrate-poor by default. Everything above has already had first access. What arrives is the remainder.
Its conditions have drifted furthest from where fermentation began. Water has been reabsorbed, so contents are firmer and less well mixed. Acidity has fallen away. The resident community differs from the caecum's, because the environment it lives in does.
Put those together and the geography stops being a detail. The stretch with the longest contact time, and with conditions furthest from where fermentation started, is also the stretch that fermentation reaches last and least.
Go deeper: why the gradient exists at all
Fermentation is substrate-limited, so its rate depends on how much fermentable carbohydrate is available at that point. Availability falls along the length for two reasons at once: upstream organisms consume it, and water is progressively absorbed, so contents become firmer and less well mixed as they travel.
Because the acids produced are absorbed by the lining as they are made, concentration at any point reflects a balance between production and uptake rather than production alone. A low distal concentration means low local production, not efficient distal clearance.
This is also why transit time matters. Slower transit gives more contact and more time to ferment what remains — but it also means a substrate-poor distal colon stays substrate-poor for longer.
Fermentation is a gradient, not an event. It is most intense where fibre arrives and thins out towards the end — the stretch where contents sit longest and conditions have drifted furthest.
What to hold on to
Not all fibre needs to be fermented quickly. A fibre that resists early fermentation is not underperforming; it is arriving somewhere the fast ones never reach.
Which leaves an obvious question. If the distal colon runs short of fibre, the bacteria there do not simply stop. They eat something else — and the next chapter is about what.