Lactose Digestion Kinetics: A 2D Gut-Tract Map

Genetics & Meal

Lactase activity 30% The LCT gene's brush-border enzyme. ~5–10% in most lactase non-persistent adults, 80–100% in persisters (common in populations with a long dairying history).

Lactose dose 12 g ≈12 g in one glass of milk, ≈50 g in a large ice-cream + yogurt meal.

Gut Physiology

Small-intestine transit time 120 min More time in contact with the enzyme means more lactose gets hydrolysed before it reaches the colon.

Colonic fermentation activity 60% Gut-microbiota load that ferments unabsorbed sugar into H₂/CO₂/CH₄ gas.

Live Readouts

Digested in small intestine0%

Reaches colon undigested0 g

Colonic gas production0 mL/h

Bloating / osmotic severity0 / 100

How it works

Lactase (a brush-border disaccharidase) hydrolyses lactose into absorbable glucose + galactose following saturable Michaelis–Menten kinetics:

dC/dt = -Vmax · C / (Km + C)
Vmax = Vmax₀ · (lactase activity / 100)

The concentration curve C(t) is stepped numerically (Euler integration) across the chosen transit time to get the fraction still undigested when the meal reaches the ileocecal valve. Whatever is left is osmotically active — it pulls water into the colon lumen — and is fermented by anaerobic bacteria:

Lactose + gut bacteria → SCFAs + H₂ + CO₂ (± CH₄)
gas volume ≈ undigested grams × yield(fermentation activity)

Lactase activity — the genetically-determined, and population-variable, brush-border enzyme level: this single number is the biophysical reason milk tolerance differs so sharply between individuals and ethnic groups.
Dose & transit time — more substrate or less contact time both push more lactose past the point of saturation and into the colon undigested.
Fermentation activity — how vigorously the resident colonic microbiota converts the leftover sugar into gas, which (together with osmotic water) produces the classic bloating/cramping/diarrhea of lactose intolerance.
Particles travelling the tube are colour-coded live from this same model: yellow = intact lactose, green = hydrolysed monosaccharides absorbed in the small intestine, amber → bubbles = undigested sugar fermenting in the colon.
This 2D map is a schematic top-down layout of the same coiled small intestine → looping colon path as the 3D version — drag to pan, scroll/pinch to zoom.

drag to pan · scroll to zoom

Small intestine → Colon

Intact lactose Absorbed (glucose+galactose) Fermenting / gas