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- Understand the barriers to assimilation of water-soluble macromolecules into the body
- Describe dietary sources of carbohydrates, and the pathways involved in the digestion and absorption of carbohydrate polymers, dietary disaccharides, and monosaccharides
- Define the relative roles of luminal and brush border digestion for each type of carbohydrate
- Identify the membrane transporters involved in the uptake of monosaccharides
- Describe how carbohydrate assimilation is regulated during development and by specific dietary components
- Compare and contrast protein assimilation with that of carbohydrates
- Identify essential amino acids, and understand why they must be provided in the diet
- Describe pathways involved in the digestion and absorption of proteins, peptides, and amino acids
- Define the relative roles of luminal, brush border, and cytosolic digestion for each substrate
- Explain how the epithelium can transport a diverse set of peptides into the body
- Understand how protein assimilation is regulated
- Define how key water-soluble vitamins are taken up into the body
- Describe conditions where the absorption of water-soluble components of the diet is abnormal
- Appreciate the basis of lactose intolerance, and why it is common in adults
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It is perhaps ironic that it is only now, at the end of this volume, that we come to discuss in detail the processes that underpin what is arguably the most important physiologic function of the gastrointestinal system—namely, the assimilation of nutrients into the body. However, the author hopes that, by having provided thorough discussions of the secretory and motor functions of the gut, students will now be in a position to rapidly appreciate how these functions are ultimately integrated to respond to the ingestion of a meal.
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Role and Significance
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Carbohydrates and proteins are water-soluble macromolecules of nutritional significance. Together with lipids, which will be discussed in the next chapter, they represent the major sources of calories in the diet, and each supplies specific building blocks for molecules needed for the physiologic function of the body as a whole. Dietary carbohydrates are the major exogenous source of glucose, which is utilized by cells as an energy source. Nutritionally significant carbohydrates include both large polymers and disaccharides consisting of two sugar molecules bound together (
Table 15–1). Proteins supply amino acids, which are resynthesized into new proteins needed by the body. While the body can synthesize glucose
de novo from a variety of substrates, as described earlier, some amino acids cannot be synthesized by the body. These are the so-called
essential amino acids, which will be described in more detail later in this chapter.
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Barriers to the Assimilation of Water-Soluble Macromolecules
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Due to their hydrophilicity, proteins and carbohydrates are “at home” in the aqueous environment of the intestinal lumen. However, neither they, nor the ...