Underlying pathophysiological condition is responsible for lactase deficiency and subsequent lactose malabsorption. Acute infection (e.g. rotavirus) causing small intestinal injury with loss of lactase containing epithelial cells from tips of villi is a common cause. Immature cells that replace them are lactase deficient. Several reports indicate that lactose malabsorption in children with acute gastroenteritis is not clinically important. (18) Several recent studies and meta-analysis found that children with rotaviral and other infectious diarrheal illness who have no or only mild dehydration can safely continue human milk or standard lactose containing formula without any significant effect on outcome including hydration status, nutritional status, duration of illness or success of therapy. (19-21) However in at risk infant (e.g. younger than 3 months or malnourished) who develop infectious diarrhea, lactose intolerance may be a significant factor that will influence evolution of illness. Giardiasis, cryptosporidiosis and other parasites that infect proximal small intestine often lead to lactose malabsorption from direct injury to epithelial cells by parasite. It can also be seen in celiac disease, Crohn's disease and immune related and other enteropathies. Diagnostic evaluation should be directed towards these entities when infectious etiology is not found for secondary lactase deficiency.

Young infants with severe malnutrition develop small intestinal atrophy that also leads to secondary lactase deficiency. (22) This is common occurrence in developing countries. (23) Lactose malabsorption has also been associated with poor growth in these countries. (24) Most infants and children with malabsorption attributable to malnutrition are able to continue to tolerate dietary carbohydrate including lactose. (25) However World Health Organization (WHO) recommends avoidance of lactose containing milk in children with persistent post infectious diarrhea (diarrhea lasting more than 14 days) when they fail a dietary trial of milk or yogurt. (26)

Thus secondary lactase deficiency and lactose malabsorption attributable to any underlying condition generally does not require elimination of lactose from diet but rather treatment of underlying condition. Once the primary problem is resolved, lactose containing products can often be consumed normally and these excellent sources of calcium and other nutrients need not be unnecessarily excluded from diet.

Lactase and other disaccharidases are deficient until at least 34 weeks gestation. (27) Lactase appears later than other brush border disaccharidases in developing fetal intestine but is present in maximal amounts in full term infant. One study in preterm infants reported benefit from use of lactase supplemented feeds or lactose reduced formula. (28) Use of lactose containing formula and human milk does not seem to have any short or long term deleterious effects in preterm infants. (29) Up to 20% of dietary lactose may reach the colon in neonates and young infants. Bacterial metabolism of colonic lactose lowers faecal pH (5.0-5.5) which has a beneficial effect favoring growth of bifidobacterium and lactobacillus species in lieu of potential pathogens (Proteus sp., Escherichia coli, and klebsiella sp.) in young infants. Antimicrobials may also affect this colonization. Lactose overload in breastfed babies arises if babies are not permitted to nurse long enough during feeds as in clock regulated feeds or if feeding is painful. Babies may also be inefficient feeders and thus deprived of fattier hind milk. Low fat feeds cause fast gastric clearance thus overloading small intestinal capacity to metabolize lactose. Management of lactose overload can be done successfully by ?finishing the first side first' according to Woolridge and Fisher's research. Inadequate fat intake in mother's diet appears to make a direct contribution to intensity of lactose overload symptoms resulting in lower fat and higher lactose levels in breast milk. (34)

It is a rare disorder reported only in few infants. (31,32) Affected newborn infants present with intractable diarrhea as soon as human milk or lactose containing formula is introduced. Small intestinal biopsies reveal normal histology but low or completely absent lactase concentration. (33,34) Unless recognized and treated quickly, this condition is life threatening due to dehydration and electrolyte losses. Treatment is removal and substitution of lactose with commercial lactose free formula.