Mucosa of the small intestine normally produces an enzyme – lactase – that disintegrate lactose (sugar in milk) and thus allows its absorption in the bloodstream. Strong attacks of gastroenteritis (see gastroenteritis in infants) may sometimes temporarily damage the intestinal mucosa which then produces little lactose or does not produce lactose at all.
At first, the consequences can not be seen because the baby is not given milk but boiled water as long as diarrhea and vomiting occur due to gastroenteritis. The reaction to lactase deficiency is noticed when the infant starts consuming cow’s milk. Lactose in milk is not absorbed into the bloodstream, but fermented in the intestines causing vomiting and foamy, ”explosive” diarrhea.
If a doctor, when he is treating gastroenteritis, suspects that a recurrence of diarrhea is not a symptom of gastroenteritis recurrence but a lactase deficiency, he will warn the mother to feed infants with synthetic lactose which contains little lactose or none at all. If this improves the condition, the diagnosis is confirmed, and the mother will continue to feed the baby with synthetic food. In the end, the intestine will begin to work normally and the baby will occasionally be given cow’s milk until it ceases to be a cause of diarrhea.