The primary function of the gastrointestinal (GI) tract in humans and other animals is to process food materials and absorb nutrients from these materials that are required for cellular metabolism and for maintaining normal systemic functions. Many of the major causes of abnormal intestinal absorption, including malabsorption syndrome, are presented including selected naturally occurring agents and agents of abuse that may be taken into the GI tract. GI smooth muscle responses to stimulation of non-adrenergic non-cholinergic inhibitory nerves are thought to be mediated by polypeptides, adenosine triphosphate, or another unidentified neurotransmitter. The GI response to diet is mediated by neural and hormonal stimuli. Besides the absorption of dietary nutrients, the intestinal mucosa functions in the absorption of water, electrolytes, vitamins, and minerals. Electrolytes, water, and metabolic substrates are absorbed into the blood and distributed to cells throughout the body for their use. Luminal and membrane digestion of dietary carbohydrates produces glucose, galactose, and fructose.