Why does lactase deficiency lead to the retention of lactose within the small intestine and prevent transport across the intestinal lumen?

Basically, Lactose a disaccharides of (glucose and galactose) cannot directly be absorbed through the  wall of small intestine  to reach the blood stream. Usually they are hydrolyzed by LACTASE enzyme  to the constituents   monosaccharides which are readily absorbed through the walls of the small intestine.  

Therefore, in absence or lack of lactase, there is retention/accumulation of intact / unmetabolized   Lactose in the lumen of small intestine, which eventually moved into the COLON.

Intestinal bacterial in the colon breaks down(metabolize) the colon lactose  by fermentation, with the by products of CO2, H2, and CH4.

These gaseous by products lead to different abdominal symptoms(bloating, flatulence) experienced  by   individuals with Lactose tolerance.

Consequently,the increase solute potential of the colon raises the osmotic pressure (since the water potential is very low). These gradients draw water in by Osmosis with the bowel filled with water. leading to swelling

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