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CHAPTER

27

Digestion, Absorption, &

Nutritional Principles

OBJECTIVES

After studying this chapter, you should be able to:

■

Understand how nutrients are delivered to the body and the chemical processes

needed to convert them to a form suitable for absorption.

■

List the major dietary carbohydrates and define the luminal and brush border pro-

cesses that produce absorbable monosaccharides as well as the transport mecha-

nisms that provide for the uptake of these hydrophilic molecules.

■

Understand the process of protein assimilation, and the ways in which it is compa-

rable to, or converges from, that used for carbohydrates.

■

Define the stepwise processes of lipid digestion and absorption, the role of bile acids

in solubilizing the products of lipolysis, and the consequences of fat malabsorption.

■

Identify the source and functions of short-chain fatty acids in the colon.

■

Delineate the mechanisms of uptake for vitamins and minerals.

■

Understand basic principles of energy metabolism and nutrition.

INTRODUCTION

The gastrointestinal system is the portal through which nutri-

tive substances, vitamins, minerals, and fluids enter the body.

Proteins, fats, and complex carbohydrates are broken down

into absorbable units
(digested),
principally in the small

intestine. The products of digestion and the vitamins, miner-

als, and water cross the mucosa and enter the lymph or the

blood
(absorption).
The digestive and absorptive processes

are the subject of this chapter.

Digestion of the major foodstuffs is an orderly process

involving the action of a large number of
digestive enzymes

(Table 27–1). Enzymes from the salivary glands attack carbo-

hydrates (and fats in some species); enzymes from the stom-

ach attack proteins and fats; and enzymes from the exocrine

portion of the pancreas attack carbohydrates, proteins, lipids,

DNA, and RNA. Other enzymes that complete the digestive

process are found in the luminal membranes and the cyto-

plasm of the cells that line the small intestine. The action of

the enzymes is aided by the hydrochloric acid secreted by the

stomach and the bile secreted by the liver.

Most substances pass from the intestinal lumen into the

enterocytes and then out of the enterocytes to the interstitial

fluid. The processes responsible for movement across the

luminal cell membrane are often quite different from those

responsible for movement across the basal and lateral cell

membranes to the interstitial fluid.