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Copyright © 1997 by Academic Press. All rights of reproduction in any form reserved.

The gastrointestinal tract holds a special place in the history of endocrinology; it was the site of discovery, by Bayliss and Starling in 1902, of secretin, which was the first hormone to be described and which permitted the formulation of the concept that hormones are chemical messengers.

B. Resume of the Gastrointestinal Hormones

Table 8-1 tabulates the 11 gastrointestinal (GI) hormones, including the 3 major GI hormones, gastrin, secretin, and cholecystokinin (CCK), and summarizes their sites of production and biological actions; Table 8-2 summarizes the neurotransmitter hormones produced by the gastrointestinal tract. As will be reviewed in Table 8-3, many of the gastrointestinal hormones play key roles in the plethora of steps and processes associated with the digestion and absorption of food.

C. Problems of Food Processing and Digestion

All animals require continued and regular intake of food so that they can meet their bodily nutrient requirements. Briefly, these food components include the following: (a) Adequate caloric intake of carbohy drate, fat, and protein to provide the spectrum of substrates required for the generation of metabolic energy, which is indispensable for the biosynthesis, modification, and repair of tissue components, and the generation of mechanical (muscle contraction) and electrical energy (nerve impulse); (b) adequate intake of essential substances not capable of being biosynthesized by the animal in question (i.e., essential amino acids, essential fatty acids, and all vitamins); and (c) adequate intake of macro and trace minerals. Thus, the term food is a general label to include the variety of chemical substances, including proteins, carbohydrates, fats, minerals, and vitamins, required to provide nourishment (i.e., maintenance of life).

However, the simple process of swallowing food does not mean that these food substances are available to sustain life. It is only after they have left the lumen of the intestine and appeared in the blood or lymph that they can be said to be "in the body." Although some food substances, such as minerals and free amino acids, can be absorbed with little or no change in the chemical form in which they are ingested, other food components must be subjected to extensive physical and chemical modification prior to their entry into the lymph or bloodstream. Thus, proteins, polysaccharides (both macromolecules), fats (triglycerides), and phospholipids must all be degraded to their constituent

TABLE 8-1 Summary of Gastrointestinal Hormones"

Hormone

Stimulatory agent(s)

Site of production

Major action(s)

Major GI Hormones

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