Colonic SCFA Uptake: pH-Dependent Transport

Colonic Conditions

Luminal pH 6.5

SCFA concentration 40 mM

MCT1 transporter density 10

Simulation speed 1.0×

Live Readouts

Protonated (HA)

4.8%

Deprotonated (A⁻)

95.2%

Passive flux

0 /s

MCT1 flux

0 /s

Transporter occupancy

Total uptake

0 /s

Legend

Free SCFA in lumen (unclassified) Protonated HA — crosses membrane freely Deprotonated A⁻ — needs MCT1 carrier

How it works

Short-chain fatty acids (SCFAs — mainly acetate, propionate and butyrate) are produced by fermentation of dietary fibre by colonic bacteria. Whether a molecule crosses the colonocyte membrane by simple lipid diffusion or needs a carrier protein depends on its ionization state, set by the Henderson–Hasselbalch equation:

pH = pKa + log10( [A⁻] / [HA] )
fraction protonated (HA)   = 1 / (1 + 10^(pH − pKa))
fraction deprotonated (A⁻) = 10^(pH − pKa) / (1 + 10^(pH − pKa))

Butyrate's pKa is about 4.8 — well below the physiological colonic pH range (~5.5–7.5) — so the large majority of luminal SCFA exists as the charged anion A⁻, not the neutral acid HA:

Protonated HA is lipid-soluble and crosses the epithelial membrane by passive nonionic diffusion, at a rate roughly proportional to its concentration — this pathway never saturates.
Deprotonated A⁻ is charged and cannot cross the lipid bilayer directly. It is carried across by the MCT1 transporter (monocarboxylate transporter 1, a H⁺-coupled symporter), which behaves like a Michaelis–Menten enzyme: a fixed pool of carrier sites means the flux saturates once every carrier is occupied, no matter how much substrate is waiting.

This is why a more acidic proximal colon (rich in freshly fermented SCFA, lower pH) favours faster passive uptake, while a more neutral distal colon relies almost entirely on MCT1 — and why increasing "MCT1 transporter density" in this simulation raises the saturation ceiling of the ionized pathway but does nothing to the passive one. Absorbed SCFA fuels colonocyte metabolism directly (butyrate is the epithelium's preferred energy source) and enters the bloodstream via the basolateral side, exactly as modelled by the particles reaching the lower "blood side" layer here.

Colonic lumen Epithelial membrane + microvilli Colonocyte → bloodstream

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Colonic SCFA Uptake: pH-Dependent Transport Across the Gut Epithelium