Mechanism	Condition	Onset	Symptoms	Prevalence
Enzyme deficiency (inability to digest carbohydrate)	Congenital lactase deficiency	Birth	Intractable diarrhea as soon as formula is given; life-threatening	Case reports only
	Primary lactase deficiency	Infancy, childhood	Gas, fussiness, emesis, diarrhea; may be difficult to distinguish from colic	20 percent of Hispanic, Asian, and black children; less common in white children; overdiagnosed in infancy
	Secondary lactase deficiency	Following gastroenteritis, chemotherapy, etc.	Gas, fussiness, emesis, diarrhea; occurs after small bowel injury; temporary	Common
Immunoglobulin E-mediated allergy (antibodies against cow's-milk protein)	Milk protein allergy	Infancy	Eczema (most common presentation), wheezing, or gastrointestinal symptoms	2 to 3 percent of infants