There are a variety of different types of lactose intolerance, in addition to which there are other causes of intolerance to lactose.

• lactose intolerance is a result of lactase deficiency and is a form of carbohydrate malabsorption. Lactose is hydrolyzed by lactase in the intestinal mucosa
• lactase is one of many beta-galactosidases seen in the small bowel and is most active in the jejunum
• the by-products of lactose hydrolysis are the monosaccharides:
  • (1) glucose and (2) galactose
• when lactase is absent or deficient, hydrolysis of the sugar lactose is incomplete
  • because it is osmotically active, the undigested sugar will pull fluid into the intestine. Hydrogen and lactic acid, in addition to other organic acids, are produced when colonic bacteria act on the undigested sugar
    • the combined osmotic effect of the undigested sugar and organic acids results in the passage of acidic diarrheal stools. These stools can produce significant skin irritation and breakdown
    • infants with lactose intolerance may also present with abdominal distension and vomiting
      • in its most severe forms lactose intolerance can lead to dehydration, electrolyte abnormalities, and failure to thrive. Lactase deficiency has been described as primary, secondary, or congenital
        • congenital lactase deficiency
          • a rare hereditary disorder in which lactase activity is absent
        • primary lactase deficiency
          • the normal gradual reduction in lactase production seen as an individual matures from infancy into adulthood and is expressed variably across populations
        • secondary lactase deficiency
          • lactase deficiency may also be a secondary occurrence because of gastroenteritis, bowel surgery, cystic fibrosis, or immune disorders. It has also been seen transiently in infants exposed to phototherapy and antibiotic therapy
            • if a bout of gastroenteritis or use of antibiotics occurs around the time of the GI symptoms, a secondary lactose deficiency should be suspected. In the intestinal insult that often occurs with secondary lactose intolerance, lactase is the first enzyme to be negatively impacted and the last to recover as the insult resolves

Reference:

• 1. Host A., Clinical course of cow's milk protein allergy and intolerance. Pediatr Allergy Immunol 1998; 9 (Suppl 11):48-52
• 2. Host A. Cow's milk protein allergy and intolerance in infancy. Pediatr Allergy Immunol 1994;5:5-36.
• 3. Wilson J. Milk Intolerance: Lactose Intolerance and Cow's Milk Protein Allergy. Newborn and Infant Nursing Reviews 2005; 5 (4): 203-207.