Lactose intolerance is the inability to metabolize lactose, because of a lack of the required enzyme lactase in the digestive system. It is estimated that 75% of adults worldwide show some decrease in lactase activity during adulthood.
The frequency of decreased lactase activity ranges from as little as 5% in northern Europe, up to 71% for Sicily, to more than 90% in some African and Asian countries.
Disaccharides cannot be absorbed through the wall of the small intestine
into the bloodstream, so in the absence of lactase, lactose present in
ingested dairy products remains uncleaved and passes intact into the
colon.
The operons of enteric bacteria quickly switch over to lactose
metabolism, and the resulting in-vivo fermentation produces copious
amounts of gas (a mixture of hydrogen, carbon dioxide, and methane).
This, in turn, may cause a range of abdominal symptoms, including
stomach cramps, bloating, and flatulence.
In addition, as with other
unabsorbed sugars (such as sorbitol, mannitol, and xylitol), the
presence of lactose and its fermentation products raises the osmotic
pressure of the colon contents.
Further Reading
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