Lactose intolerance is caused by the inability to digest lactose due to the absence of the enzyme lactase. It is estimated that as much as 70% of the adult population is unable to digest lactose properly. Lactose intolerance is most common in adults and is rare in infants and very young children. It is thought that the production of lactase is at its highest in babies since all mammals’ milk has lactose in it and that is the primary food for babies. As we age our lactose production slows since we need less milk, and for some lactose production may slow to a crawl or a complete halt.

There are three types of lactose intolerance that may affect infants, a secondary lactose intolerance that is caused by other conditions, developmental lactase deficiency and much less common, congenital lactose intolerance. Secondary lactose intolerance may occur when an infection or inflammation in the small intestine damages the lining where lactase is produced.  This is seen commonly with celiac disease, giardiasis, tropical sprue, diabetic gastropathy and chemotherapy. When the infection or inflammation is halted the lining of the small intestine usually repairs itself and the lactose intolerance goes away.

Developmental lactase deficiency may occur in premature babies since lactase production does not start till the latter part of pregnancy. Babies that are premature may have only partial lactase production and may need to be given lactose free formula or the lactase enzyme until their lactase production can catch up.

Congenital lactose intolerance is very rare and is a genetic condition where a baby receives genes from both parents that cause the baby to not produce enough lactase at birth. A baby that receives two ineffective genes from both parents may produce little or no lactase at all, resulting in severe diarrhea, abdominal pain, vomiting, dehydration, renal tubular acidosis, aminoaciduria, liver damage, and severe bloating and distension.  The baby will fail to thrive and lose weight. Once the congenital lactose intolerance is diagnosed, and the baby is fed a lactose free diet the damage will reverse itself quickly. Some babies will be able to tolerate small amounts of lactose after the age of 6 months.