The gastrointestinal tract exists to support nutrition, digesting and assimilating nutrients including water and salts. Disorders of the gut readily impair nutritional status, although there is considerable functional reserve. Many macronutrients are structural components of animals and plants, and therefore ingested in complex molecular forms that cannot be readily absorbed. They must be digested to simpler components in the gastrointestinal tract before absorption and assimilation can occur. For the major complex macronutrients, (fat, carbohydrate, protein), the gut secretes specific enzymes that catalyse the hydrolysis of these nutrients to their basic oligomeric subunits, which are then taken up by specific transport proteins expressed in the epithelial membrane for optimal transport from the lumen into enterocytes. The digestive process is progressive, beginning in the oral cavity and continuing during passage to the small intestine, the key site of most nutrient absorption. The colonic bacterial flora salvages nutrients from otherwise indigestible fibre. Micronutrients (vitamins, minerals), electrolytes and water must be absorbed; specific transport mechanisms exist for each. The magnitude of specialized processes that act in conjunction to enable effective digestion and absorption (along with the regulatory inputs that converge to coordinate these events) demonstrate how finely adapted the gastrointestinal tract is to its function. © 2009.