Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Glycolysis: The Central Pathway of Glucose Catabolism
Disaccharides must first undergo hydrolysis to monosaccharides

Disaccharides on their own cannot enter Glycolysis. If, for instance, they are introduced directly into the bloodstream, they will not be utilized. In order for the body to utilize the disaccharides present in food, they must first undergo Enzymatic Hydrolysis within the Cells lining the Small Intestine—that is, they must be broken down into the hexose units of which they are composed:

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The resulting simple sugars are then absorbed into the bloodstream and transported to the Liver, where they are phosphorylated and converted into glycolytic intermediates, as described above.

Lactose intolerance (Sections 11.5 and 24.1, a) in certain population groups (with the exception of the populations of Northern Europe and parts of Africa) is associated with the complete or partial disappearance of lactase activity in the intestinal epithelial cells of adults. In such individuals, lactose is incompletely digested and absorbed in the intestine, leading to diarrhea. This issue is not overly serious, since adults living in regions where lactose intolerance is prevalent do not consume milk. No connection has been found between lactose intolerance and galactosemia; lactose intolerance is very common, whereas galactosemia is an extremely rare condition.