Makes Me Sick Exploration of the Basis of Lactose Intolerance Lactose is milk sugar. 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.
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. 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.