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    Lactose or milk sugar occurs in the milk of mammals - 4-6% in cow's milk and 5-8% in human milk. It is also a by product in the the manufacture of cheese. Lactose is made from galactose and glucose units. The galactose and glucose units are joined by an acetal oxygen bridge in the beta orientation. To recognize galactose look for the upward projection of the -OH on carbon # 4. See details on the galactose page towards the bottom.

    Lactose intolerance

    Lactose intolerance is the inability to digest significant amounts of lactose, the predominant sugar of milk. This inability results from a shortage of the enzyme lactase, which is normally produced by the cells that line the small intestine. Lactase breaks down the lactose, milk sugar, into glucose and galactose that can then be absorbed into the bloodstream. When there is not enough lactase to digest the amount of lactose consumed, produce some uncomfortable symptoms. Some adults have low levels of lactase. This leads to lactose intolerance. The ingested lactose is not absorbed in the small intestine, but instead is fermented by bacteria in the large intestine, producing uncomfortable volumes of carbon dioxide gas. While not all persons deficient in lactase have symptoms, those who do are considered to be lactose intolerant.


    Common symptoms include nausea, cramps, bloating, gas, and diarrhea, which begin about 30 minutes to 2 hours after eating or drinking foods containing lactose. The severity of symptoms varies depending on the amount of lactose each individual can tolerate.

    Fortunately, lactose intolerance is relatively easy to treat by controlling the diet. No cure or treatment exists to improve the body's ability to produce lactase. Young children with lactase deficiency should not eat any foods containing lactose. Most older children and adults need not avoid lactose completely, but individuals differ in the amounts and types of foods they can handle. Dietary control of lactose intolerance depends on each person's learning through trial and error how much lactose he or she can handle.

    Acetal Functional Group

    Carbon # 1 (red on left) is called the anomeric carbon and is the center of an acetal functional group. A carbon that has two ether oxygens attached is an acetal. The Beta position is defined as the ether oxygen being on the same side of the ring as the C # 6. In the chair structure this results in a horizontal or up projection. This is the same definition as the -OH in a hemiacetal.


    Compare Lactose and Maltose Acetals

    The position of the oxygen in the acetal on the anomeric carbon (#1) is an important distinction for disaccharide chemistry. Lactose has a beta acetal. The Beta position is defined as the oxygen in the acetal being on the same side of the ring as the C # 6. In the chair structure this results in a horizontal projection. Maltose has an alpha acetal. The Alpha position is defined as the oxygen in the acetal being on the opposite side of the ring as the C # 6. In the chair structure this results in a downward projection. The alpha and beta acetal label is not applied to any other carbon - only the anomeric carbon of the left monosaccharide, in this case # 1 (red).

    Recognize galactose and glucose

    To further identify lactose and maltose, identify the presence of galactose in lactose in the left most structure by the upward -OH on the carbon # 4. Identify glucose in maltose in the left most structure by the horizontal -OH on the carbon # 4.



    Lactose is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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