Understanding the pathophysiology of diarrhea and malabsorption is based on understanding the normal steps in the digestion and absorption of food. The normal gastrointestinal tract is a finely integrated system geared to carry out the assimilation of ingested foodstuffs. Assimilation (the process by which ingested foods reach body fluids and cells) consists of two stages: (1) digestion (the breakdown of large molecules in the lumen of the intestine into their component small molecules) and (2) absorption (the transport across the intestinal mucosa to systemic body fluids).

Many disease processes directly or indirectly alter gastrointestinal physiology in such a manner that normal absorptive mechanisms are compromised, resulting in maldigestion or malabsorption of one or more dietary constituents. Too simplistic an approach to these diseases may be confusing because of the large number of illnesses involved and because of the plethora of diagnostic tests. This chapter will (1) present a classification of malabsorption and (2) outline the usefulness and potential pitfalls of common tests of intestinal function.