Lactose is milk sugar. It is composed of two molecules of "simple" sugars chemically bonded together - glucose and galactose. The enzyme lactase breaks down lactose into glucose and galactose which are easily digested by humans.
Most human infants produce ample quantities of lactase for milk digestion. However, in the vast majority of adult humans, the gene which specifies production of lactase is turned "off" and these individuals cannot digest lactose - they are lactose intolerant. Symptoms of lactose intolerance include cramps and diarrhea. The lactose molecule, which is large, accumulates in the large intestine and affects the osmotic balance there. Since water moves across semipermeable membranes, such as the intestine, from areas of high concentration to low concentration, the addition of large lactose molecules causes water to enter the intestine. This can result in the very unpleasant experience of watery stool or diarrhea. Since lactose is a sugar, it is an ideal food for the bacteria which normally inhabit our intestine (and are essential to digestion). However, the lactose will be fermented by these same friendly bacteria, and organic acids are gas are produced by them and we all know what discomfort intestinal gas can cause! So most folks who are lactose intolerant choose to avoid lactose-containing milk products, or modify the lactose, to avoid the cramps and diarrhea associated with the intolerance syndrome.
There is a science experiment that goes along with this found at exploratorium.edu.
Most human infants produce ample quantities of lactase for milk digestion. However, in the vast majority of adult humans, the gene which specifies production of lactase is turned "off" and these individuals cannot digest lactose - they are lactose intolerant. Symptoms of lactose intolerance include cramps and diarrhea. The lactose molecule, which is large, accumulates in the large intestine and affects the osmotic balance there. Since water moves across semipermeable membranes, such as the intestine, from areas of high concentration to low concentration, the addition of large lactose molecules causes water to enter the intestine. This can result in the very unpleasant experience of watery stool or diarrhea. Since lactose is a sugar, it is an ideal food for the bacteria which normally inhabit our intestine (and are essential to digestion). However, the lactose will be fermented by these same friendly bacteria, and organic acids are gas are produced by them and we all know what discomfort intestinal gas can cause! So most folks who are lactose intolerant choose to avoid lactose-containing milk products, or modify the lactose, to avoid the cramps and diarrhea associated with the intolerance syndrome.
There is a science experiment that goes along with this found at exploratorium.edu.
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