PROCESSES OF DIGESTION AND ABSORPTION

Concepts of vertebrate intestinal function have changed continuously over the past few years and the processes of digestion and absorption are now recognised as being very complex. Broadly, the present state of knowledge indicates the following ... 

It wiU be evident from the section on processes of digestion and absorption that the pancreas plays a central role in the absorptive process for carbohydrates, fats, and proteins. Disorders of the exocrine pancreas are therefore frequently associated with GI symptoms of malabsorption or diarrhea. In this section, pediatric and adult exocrine pancreatic disorders are briefly discussed and tests for assessing exocrine pancreatic function are described. Information on exocrine pancreatic tumors can be found in the later section on GI regulatory peptides. Recent textbooks on gastroenterology or medicine have more detail on the clinical aspects of exocrine pancreatic disorders. 

TAG group. Changes in serum fatty acids and LPL protein mass concentration between DAG and TAG loading did not differ. These results indicated that the differences in the concentration of TG-rich lipoproteins after loading of DAG or TAG were presumably due to differences in the process of digestion and absorption of these oils in the intestine. 

Whilst only 4% of the UK diet is fibre, there are few parts of the process of digestion and absorption which fibre does not influence to some extent. Experimental evidence exists to show that appetite, satiety, gastric emptying, carbohydrate, sterol and mineral absorption may all be affected by fibre. 

However, it is experimentally difficult to separate it from extracellular processes that use similar methods of digestion and absorption (Section 6.1). 

Four major groups of lipoproteins are recognized Chylomicrons transport lipids resulting from digestion and absorption. Very low density lipoproteins (VLDL) transport triacylglycerol from the liver. Low-density lipoproteins (LDL) deliver cholesterol to the tissues, and high-density lipoproteins (HDL) remove cholesterol from the tissues in the process known as reverse cholesterol transport. 

The mucosa of the GIT represents an interface between the external and internal environments. The expansive surface area is necessary for the efficient hydrolysis of foodstuffs and the absorption of energy and nutrients. The mucosa also influences the systemic availability of non-nutrient compounds in the diet, both beneficial and detrimental. Digestion and absorption of glucosinolates are critical determinants of health benefits (see Chapter 4) Similarly, the bioavailability and health benefits of phytoestrogens, such as genistein (see Chapters 5 and 10) are at least partly dependent on the carrier-mediated processes of absorption associated with the GIT (Oitate et al, 2001). Moreover, the metabolic activities of the mucosa can influence the systemic concentrations and forms of dietary phytochemicals, as exemplified by research with soy isoflavones (Andlauer et al., 2000). 

The gastrointestinal tract (GIT) is a highly specialized region of the body whose primary functions involve the processes of secretion, digestion, and absorption. Since all nutrients needed by the body, with the exception of oxygen, must first be ingested orally, processed by the GIT, and then made available for absorption into the bloodstream, the GIT represents a significant barrier and interface with the environment. The primary defense mechanisms employed by the gut... 

The following sections will discuss each region of the digestive system separately. Where appropriate, the basic digestive processes — motility, secretion, digestion, and absorption — will be considered. 

Digestion and absorption in the small intestine. Most digestion and absorption of carbohydrates, proteins, and lipids occurs in the small intestine. A summary of the digestive enzymes involved in these processes is found in Table 18.3. 

Digestion and absorption are the GI processes that generate usable fuels for the body. Understanding the mechanisms of these processes can enhance rational use of EN support. 

Many factors can alter these stepwise processes and interfere with digestion and absorption, such as functional immaturity of the neonatal gut. 

FIGURE 17-1 Processing of dietary lipids in vertebrates Digestion and absorption of dietary lipids occur in the small intestine, and the fatty acids released from triacylglycerols are packaged and delivered to muscle and adipose tissues. The eight steps are discussed in the text. 

The processes for the digestion and absorption of fat- and water-soluble vitamins are different, due to their solubility properties. Fat-soluble vitamins and their precursors (A, [1-carotene, D, E and K) are digested and absorbed by processes similar to those for dietary fats, mainly in the small intestine. Most water-soluble vitamins require specific enzymes for their conversion from natural forms in feed-stuffs into the forms that are ultimately absorbed. Unlike fat-soluble vitamins that are absorbed mostly by passive diffusion, absorption of water-soluble vitamins involves active carrier systems to allow absorption into the portal blood. 

Gastrointestinal motility is an essential function of the digestive and absorptive processes of the gut. It propels intestinal contents, mixes them with digestive juices, and prepares unabsorbed particles for excretion. Gastric motility has been shown to be inhibited by D-glucose in the intestinal fluid. 

The fourth section deals with various aspects Digestion, Absorption, and Nutritional Biochemistry. The chapter Obesity considers current problems with respect to the ever-increasing incidence of imbalance between energy intake and utilization. Key problems of undemutrition are discussed in the chapters Protein-Energy Malnutrition and Vitamin A Deficiency in Children. The chapters Lactose Intolerance, Pancreatic Insufficiency, and Abetalipoproteinemia focus on the biochemical processes underlying food digestion and absorption. Calcium Deficiency Rickets, Vitamin B12 Deficiency, and Hemochromatosis provide discussions of absorption and utilization of vitamin D, vitamin B12, and iron, respectively. 

The digestion and absorption of dietary lipid can be completed only in the presence of adequate amounts of bile salts that are synthesized in the liver and pass, via the bile duct, into the duodenum and thence into the jejunum. Reabsorption of the bile salt micelles occurs in the ileum, from which a large proportion return via the blood to the liver. The bile ducts carry bile salts from the liver to the gallbladder, where they are stored excreted (excess) cholesterol is dissolved in the bile salt micelles. Overall, 90 percent of the bile salts involved in absorption of lipid in the jejunum are recycled, in a process called the enterohepatic circulation, and 10 percent are lost in the feces. Replacement of this amount necessitates conversion from cholesterol. Thus, de novo synthesis of cholesterol itself plays an important part in maintaining the supply of bile salts. 

A major question to address in the future is how structure influences the dynamics of digestion and the signaling processes involved. Armand et al. (1996, 1999) investigated the digestion and lipid absorption from emulsions with different droplet sizes in humans. Healthy subjects received intragastrioally a coarse (10 pm) and a fine (0.7 pm) lipid emulsion of identical composition in random order. Gastric and duodenal aspirates as well as triglyceride appearance in the blood were analyzed. They found an increase in droplet size in the stomach however, the fine emulsion retained droplets... 

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