Answer to Carbohydrate Question C-03

Original Question


(b) lactose


Lactose is the milk sugar.


It is formed by Beta-D galactose and D-glucose. The galactose is linked  to the glucose through a beta 1,4-O-glycosidic linkage.



 This linkage is hydrolyzed by a specific disaccharidase called Lactase, located in the brush border of the epithelial cells of the small intestine.  The glucose and galactose  are absorbed and transported through the portal system to the liver.


In spite of the common believe in developed countries, that consume great amounts of dairy products,  most of the humanity are glucose intolerant. Human beings show a decrease in the synthesis of lactase after the 4 years of age. In persons with primary lactose intolerance (the “physiological” one we are talking about), this decrease is usually evident after 6 years of age, in some cases after puberty, and most of the lactose contained in the ingested food remain undigested and continue to the large intestine. where it is fermented by intestinal bacteria, producing gases, organic acids and a decrease in the pH.


It can be manifested with cramps, abdominal pain, and other gastrointestinal disorders, that begin 1 to 2 hours after the ingestion of dairy products. This condition is more frequent among Afro American than among white Americans.


Another kinds of Lactose intolerance are a congenital form, very rare, and lactose intolerance secondary to intestine diseases, including infections.


In fact, primary lactose intolerance can be hardly considered a disease, since there is a genetic programmed decrease in the synthesis of lactase (Humans were not ‘supposed” to drink milk after a few months of life!).  Since around 70 % of the whole population is lactose intolerant, it has been proposed that lactose intolerance should be considered the norm, and that the minority should be considered as a lactase persistent group, that appeared as mutation of the “normal” gen.


Anyway, for those persons living in a society where milk is an important componente of diet, to be intolerant to lactose convey certain minor problems.

The “treatment” consist mainly in avoiding lactose containing products (but recall to supplement calcium and other minerals usually associated to dairy products!)


More information about this condition can be found in:


National Digestive Diseases Information Clearinghouse.


Policy of the American Academy of Pediatrics about Lactose Intolerance (2006)


Marks, J.W.: Lactose intolerance (Lactase deficiency)


Swagerty, D.L.; Walling, A.D.; Klein, R.M.: Lactose Intolerance


Guandalini, S.: Lactose Intolerance



One thought on “Answer to Carbohydrate Question C-03

  1. Pingback: More questions about Carbohydrates and Medicine « The Biochemistry Questions Site

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