The intestinal mucosa represents a semipermeable barrier that controls the flux of solutes and fluid into the systemic circulation. It is highly differentiated at the cellular level, and in particular, enterocytes acquire increasing functional capacity as they migrate along the crypt-villus axis. Maximal absorptive activity thus is found along the villus, characterized by the presence of an enzyme-rich brush border membrane1
and the expression of mRNA for specific transporters.5
Secretory function is concentrated at the base of the crypt.6
Most defects of intestinal transport affect the absorption of a specific nutrient or a group of nutrients. Accordingly, the phenotypic expression varies significantly from none to life-threatening as the result of a wide range of intestinal and extraintestinal effects. Intestinal effects are usually secondary to the presence of excessive amounts of unabsorbed solutes in the lumen, which leads to acute fluid and ionic imbalances; in general, extraintestinal effects reflect the systemic deficiency of a specific nutrient and therefore may be more insidious and progressive. Transport is closely integrated with digestion7
; therefore, this chapter focuses on specific mucosal transport defects and common hydrolytic enzyme deficiencies (primary or secondary) that may be phenotypically indistinguishable.