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Lactose Intolerance
Lactose intolerance, also called lactase deficiency, is the inability to
digest lactose, a sugar found in milk and to a lesser extent milk-derived dairy
products such as yoghurt, butter and cheese
Lactose intolerant individuals have insufficient levels of lactase, an enzyme that
catalyses hydrolysis of lactose in their digestive system. In most cases this
causes symptoms which may include abdominal bloating and cramps,
flatulence, diarrhoea, nausea, or vomiting after consuming significant amounts
of lactose.
Most mammals normally cease to produce
lactase, becoming lactose intolerant, after weaning but
some human populations have developed lactase
persistence, in which lactase production continues into
adulthood. It is estimated that 75% of adults worldwide
show some decrease in lactase activity during adulthood.
The frequency of decreased lactase activity ranges from
5% in northern Europe through 71% for Sicily to more than 90% in some African and Asian
countries. This distribution is now thought to have been caused by recent natural
selection favouring lactase-persistent individuals in cultures which depend on dairy products
in their diet and have a tradition of dairy farming, and are found frequently in populations of
Europe, India, Arabia and Africa.
Congenital lactase deficiency (CLD) is a very rare, autosomal recessive genetic disorder
that prevents lactase expression from birth. It is particularly common in Finland. People with
congenital lactase deficiency cannot digest lactose from birth, and therefore cannot digest
breast milk. Infants initially rely on breast milk for nutrition until they are weaned onto other
foods. Before the 20th century, babies born with CLD often did not survive but death rates
decreased as lactose-free dairy products have been developed.
Lactose cannot be directly absorbed through the wall of the small intestine into the
bloodstream so, in the absence of lactase, passes intact into the colon. Bacteria in the colon
can metabolise lactose, and the resulting fermentation produces copious amounts of gas (a
mixture of hydrogen, carbon dioxide and methane) that causes the various abdominal
symptoms.
Lactose intolerance is distinct from milk allergy, an abnormal immune response, (usually) to
milk proteins. They may be distinguished in diagnosis by giving lactose-free milk, producing
no symptoms in the case of lactose intolerance, but the same reaction as to normal milk in
the presence of a milk allergy.
Questions
1. What class of molecule does lactose belong to?
2. What is hydrolysis, and what does the hydrolysis of lactose produce?
3. What process produces lactase and how would this be different between populations that
are lactose intolerant and those that are lactose persistent?
4. Which process is responsible for absorption of the products of lactose breakdown? Where
precisely will this occur?
5. Diarrhoea occurs when faeces retain more water than usual, making them much less solid.
With reference to water potential, explain why this occurs when lactose intolerant people
consume dairy products containing lactose.
6. Why do gut bacteria produce hydrogen, carbon dioxide and methane from the lactose?
7. Explain how lactose intolerance is different to a milk allergy.
8. Compare lactose intolerance to the animal kingdom’s inability to digest cellulose.