Subject: Re: Lactose intolerance gene

From: [email-masked] (Paul S Winalski)
Date: 29 Jul 1995 21:09:03 GMT

In article <[email-masked]>,
me <[email-masked]> wrote:

> Is lactose intolerance exclusive to people from west africa?  If not
> what other racial groups find it hard to digest lactose?  What is the
> reason for this intolerance?  Is there any advantage?-Peter

In infant mammals, the small intestines secrete a digestive
enzyme called lactase, which splits the disaccharide lactose
(milk sugar) into its component monosaccharides: galactose
and glucose.  Mammals normally stop producing lactase after
they are weaned.  Since milk is not normally part of an adult
mammal's diet, the enzyme isn't needed any more.

Human beings also follow this general pattern.  In most humans,
lactase is not secreted after childhood.  Sometimes lactase will
continue to be secreted as an adult, but only as long as lactose
continues to be present in the diet.  If consumption of dairy
products stops, so does secretion of lactase.  If lactase is not
secreted, lactose is not broken down and instead is fed on by
bacteria, which produce fermentation gases and waste products
that irritate the gut.  The result is the symptoms of lactose
intolerance:  gas, cramps, or diarrhea after consuming a meal
containing lactose (i.e., dairy products).

In fact, caucasians, especially northern and western Europeans,
are the only ethnic group that generally retains the ability to
digest lactose as adults.  And even among caucasians, lactose
intolerance is fairly common.  Adult lactose intolerance is
almost universal among orientals, which is why Chinese, Japanese,
and Southeast Asian cuisines are almost completely devoid of
dishes with milk products as an ingredient.

As to the genetic advantage of lactose intolerance:  If there
is no lactose in the diet (the usual case for adult mammals),
then there is no point in secreting the enzyme to digest it.
That's a waste of metabolic energy that could be very valuable
under starvation conditions.

--PSW