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18. 1 Introduction




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brown pigment stercobilin. The characteristic odor of feces is due to the presence of indole and skatole, two of the break­down products of amines arising from the activity of bacteria.

Defecation itself is a complex process involving both reflex and voluntary actions. It is possible to inhibit the reflex con­sciously if the circumstances are not convenient, and under such conditions the urge to defecate will often subside until reiniti­ated by the arrival of further fecal material in the rectum. Eventually, however, the urge to defecate will become over­whelming and the reflex will proceed. Under the influence of the parasympathetic nervous system, the walls of the sigmoid colon and the rectum contract to move feces towards the anus. The anal sphincters relax to allow feces to move through the anal canal. Expulsion of the fecal material is aided by voluntary con­tractions of the diaphragm and the muscles of the abdominal wall as well as closure of the glottis. As a result, intra-abdominal pressure rises and helps to force feces through the relaxed sphinc­ters. The muscles of the pelvic floor relax to allow the rectum to straighten, thus helping to prevent rectal and anal prolapse. The components of the defecation reflex are depicted diagram-matically in Fig. 18.29.

Summary

  1. The large intestine consists of the cecum (which plays no significant
    role in humans), colon, rectum, and anal canal. Its main functions
    are to store food residues, secrete mucus, and absorb remaining water
    and electrolytes from the food residue. Feces are eliminated via the
    anus.

  2. The colon absorbs 400—1000 ml of fluid each day. Sodium is actively
    transported from the lumen to the blood. Chloride moves in
    exchange for bicarbonate, and water moves passively.

3- Intestinal Йога perform fermentation reactions that produce short-chain fatty acids and flatus. The short-chain fatty acids are absorbed by the colonocytes, stimulating salt and water uptake. Intestinal bacteria also synthesize certain vitamins, such as vitamin K.

4. The colon exhibits mixing movements (haustrations) and sluggish
•propulsive movements. 'Housekeeper' contractions occur several

times daily, serving to move intestinal contents over longer distances. In this way feces move into the rectum and this elicits the urge to defecate.

5. Each day 100—150g of feces are eliminated. Defecation involves both
voluntary and involuntary contractions of the anal sphincters and
muscles of the abdominal wall and diaphragm.





Fig. 18.29 Neural pathways involved in the defecation reflex. Sensory axons are shown in red and motor axons in blue.

428 18 The gut and nutrition

18.14 The nutritional needs of the body

The selection of foods eaten by an individual is called the diet. A balanced diet is essential for health. A nutrient is any substance that is absorbed and utilized to promote the activities of cells and, in turn, the functions of the whole body. Nutrients include carbohydrates, proteins, fats, vitamins, mineral salts, and water. Many foods contain a number of different nutrients. Bread and potatoes, for example, although predominantly carbohydrate, also contain small quantities of protein, some vitamins, and water. Certain nutrients are classed as essential because they cannot be made by the body and must be included in the diet.

The concept of the calorific content of foodstuffs is explored in Chapter 24. The chemical characteristics of carbohydrates, fats, and proteins and the importance of water as a solvent and constituent of body tissues are considered in Chapter 2. These aspects of nutrition will not be considered further here.

Carbohydrates

These are found in a wide variety of foods such as sugar, cereals, bread, pasta, convenience foods, fruit, and vegetables. Poly-saccharides, complex molecules consisting of large numbers of monosaccharides, include starches, cellulose, glycogen, and dex-trins. After digestion in the GI tract, they are absorbed in the form of monosaccharides. Some polysaccharides, cellulose for example, are not digested by the human GI tract and pass through virtually unchanged. Carbohydrate provides a source of rapidly available energy and heat, and is utilized in preference to protein. It also provides a store of energy in the form of glycogen in the liver and is laid down as fat if eaten in excess of the body's immediate requirements.

Proteins

Proteins are broken down into small peptides and amino acids before being absorbed in by the small intestine. Amino acids are used to form enzymes, hormones, and structural proteins. Of the 20 a-amino acids that make up the proteins of the body, 12 may be synthesized by the body itself and need not, therefore, be included in the diet These are alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, his-tidine, proline, serine, and tyrosine. They are called nonessential amino acids. The other eight amino acids cannot be synthesized by the body and must be included in the diet. These essential amino acids are isoleucine, leucine, lysine, methionine, phenyl-alanine, threonine, tryptophan, and valine.

The nutritional value of a protein depends upon the amino acids it contains. Protein foods that contain all the essential amino acids, in the proportions required to maintain health, are termed first-class (complete) proteins. These include meat, fish, soya beans, milk, and eggs. Second-class {incomplete) proteins do not contain all the essential amino acids in the correct proportions. They are mainly of vegetable origin and include cereals and pulses such as peas, beans and lentils. Other foods such as bread,

Table 18.5 Recommended daily intake of dietary protein (in grams) for different age groups



Age (years)

Males

Females

1-3

15

15

4-6

20

20

7-10

28

28

11-14

42

41

15-18

55

45

19-50

56

45

Over 50

53

47

potatoes, and other vegetables also contain some protein, and by eating a wide variety of incomplete proteins it is possible to avoid amino acid deficiencies. This is an especially important consideration for those following a strictly vegetarian diet.

Table 18.5 lists the daily protein requirements of males and females at different ages. If protein is eaten in excess of the body's requirements, the nitrogenous part is detached (deamina-tion) in the liver and excreted by the kidneys. The remainder is used in energy metabolism or is converted to fat for storage in adipose tissue.

Fats

Fats are divided into two groups, saturated and unsaturated (see also Chapter 2). Saturated fat is found in milk, cheese, butter, eggs, meat, and oily fish such as herring and cod. Unsaturated fat is found in most vegetable oils. Cholesterol is synthesized in the body and is also present in fatty meat, egg yolk, and full-fat dairy prod-ucrs. Linoleic, linolenic, and arachidonic acid are polyunsaturated fats which cannot be synthesized by the body. They are therefore known as essential fatty acids and must be included in the diet.

Fats serve a number of important functions in the body. As well as providing the fatty tissue that supports and protects organs such as the kidneys and eyes, fat is an important con­stituent of nerve sheaths and cell membranes. It plays an impor­tant role in cell signaling through arachidonic acid, it is an important source of heat and energy in metabolism, and stores the fat-soluble vitamins A, D, E, and K.

Vitamins

Although required only in very small quantities by the body, vitamins are essential for normal metabolism and health. They are found in a wide range of foods and are subdivided into two categories, fat-soluble vitamins (A, D, E, and K) and water-soluble vitamins (C and В complex). Table 18.6 lists the differ­ent vitamins, their recommended daily requirements, major sources, functions, and deficiency disorders.

Minerals

Mineral salts are required for all cellular processes. The major minerals include calcium, phosphorus, sodium, potassium, iron,

18.14 The nutritional needs of the body

429

Table 18.6 Actions and daily requirements of vitamins



Vitamin Daily Major dietary Functions Deficiency disorders requirement sources (adults)

A (retinol) 700 fig Dairy products, oily fish, Formation of visual pigments, Night blindness, epithelial atrophy, eggs, liver development of bone cells susceptibility to infection

D (cholecalciferol) 10 fig Fish oils, dairy products, Normal bone development, Rickets (children), osteomalacia (adults) synthesized in skin absorption of calcium from gut

E (a-tocopherol) 8 /xg Nuts, egg yolk, wheatgerm, Prevents catabohsm of some Hemolytic anemias milk, cabbage fatty acids (prevents atherosclerosis)

К (coagulation 100 fig Green vegetables, pig liver, Formation of clotting factors Bruising, bleeding vitamin) also synthesized by and some liver proteins intestinal flora

Bt (thiamine) 1.6 mg Lean meat, fish, eggs, legumes Carbohydrate metabolism Beriberi with many neurological green vegetables symptoms, disturbances of metabolism

B2 (ribofiavin) 1.8 mg Milk, liver, kidneys, heart, Constituent of flavine coenzymes Dermatitis, hypersensitivity to light meat, green vegetables

B^ (niacin or 15 mg Most foods, can be synthetized Constituent of nicotinamide Pellagra, listlessness, nausea, dermatitis, nicotinamide) from tryptophan coenzymes neurological disorders

B6 (pyridoxine) 2 mg Meat, fish Amino-acid metabolism, Irritability, convulsions, anemia, , synthesis of hemoglobin and vomiting, skin lesions antibodies

Pantothenic acid 5—10 mg Most foods Component of coenzyme A Neuropathy, abdominal pain

Biotin (vitamin H) 100 fig Liver, egg yolk, nuts, legumes, Fatty-acid synthesis Muscle pain, scaly skin, elevated blood cholesterol

B12 (cyanocobalamm) 1.2 mg Liver, meat, fish (NOT plants) Erythrocyte production and Pernicious anemia amino-acid metabolism

Folic acid 250 fig Liver, dark-green vegetables. Hematopoiesis, nucleic acid Anemias, gastrointestinal disturbances, Also synthesized by synthesis, development of diarrhea intestinal bacteria neural tube

С (ascorbic acid) 50 mg Fresh fruits (especially citrus Protein metabolism, collagen Scurvy, liability to infection, poor fruits), vegetables synthesis wound healing, anemia

and iodine. Calcium is found in milk, eggs, green vegetables, and some fish. Sources of phosphorus include cheese, oatmeal, liver, and kidney. Calcium and phosphorus are needed for the normal mineralization of bone. Calcium is involved in secretion, muscle contraction, and blood clotting, while phosphorus is an important component of cell membranes and of ATP.

Sodium is found in most foods, especially meat, fish, eggs, milk, bread, and as table and cooking salt. The daily require­ment for sodium is l-6g, although most people ingest much more. It is the major extracellular cation and plays a crucial part in volume regulation, muscle contraction, nervous conduction, etc.

Potassium is distributed widely in all foods, especially fruit and vegetables. It is the major intracellular cation and is also involved in many cellular processes.

Iron, as a soluble compound, is found in liver, kidney, beef, egg yolk, green vegetables, and wholemeal bread. About 1 mg of iron is required each day to replace that lost from the body. A

higher intake is needed by women, particularly during preg­nancy. Iron is essential for the formation of hemoglobin and is necessary for the oxidation of carbohydrate.

Iodine is found in salt-water fish and in vegetables grown in iodine-rich soil. The daily requirement is 140 /Ag. In areas of the world in which naturally occurring iodine is deficient, small quantities may be added to table salt. Iodine is required for the synthesis of thyroid hormones. Individuals whose diets lack sufficient iodine develop an enlarged thyroid gland (goiter) in an attempt to trap any available iodine from the plasma.

Regulation of dietary intake

Although, to a large extent, the content and size of our meals is dictated by social factors and the daily pattern of activity, both hunger and appetite are important regulators of dietary intake. ^ Hunger refers to a physiological sensation of emptiness, usually accompanied by contraction of the stomach. Appetite refers to the feelings associated with the anticipation of the forthcoming

430

18 The gut and nutrition


food. It may be affected by an individual's emotional state. Nervousness or fear often suppress the appetite. Hunger and appetite, therefore, although related, are different sensations.

The hypothalamus plays an important role in the regulation of food intake. The lateral hypothalamus is thought to contain a region known as the feeding center. Lesions here in rats produce aphagia (lack of feeding) leading to starvation and death. Increased food intake (byperphagia) can be induced by lesions of the ventromedial hypothalamus and this region is called the satiety center. The exact role of these areas in the control of food intake requires further clarification.

Summery

  1. A balanced diet is essential for health. A mixed diet contains ade­
    quate amounts of the essential nutrients and strictly vegetarian diets
    must be carefully controlled.

  2. Carbohydrates are absorbed in the form of monosaccharides. They
    provide an important energy source and may be stored as glycogen or
    fat.

  3. Proteins are broken down into their constituent amino acids which are
    absorbed and used to make structural protein, enzymes, hormones, etc.
    The eight essential amino acids must be included in the diet.

  4. Fat is an important source of heat and energy. It forms protective
    layers and is an important constituent of cell membranes. Certain
    fatty acids are classed as essential since they cannot be manufactured
    by the body.

  5. A wide variety of vitamins and minerals are required for normal cell
    function. Specific absorptive mechanisms opetate in the GI tract for
    many of these.

6. Hunger and appetite are important regulators of food intake.
Feeding and satiety 'centers' are located in the hypothalamus.

Recommended reading

Anatomy

MacKinnon, P. С. В. and Morris, J. F. (1993). Oxford textbook of

functional anatomy. Vol. 2: Thorax and abdomen, pp. 67—90. Oxford University Press, Oxford.

Biochemistry

Elliott, W. H. and Elliott, D. C. (1997). Biochemistry and molecular biology, Chapter 4. Oxford University Press, Oxford.

Histology

Junquiera, L. C, Carneiro, J., and Kelley, R. O. (1992). Basic histology, (8th edn), Chapters 15 and 16. Prentice-Hall International, New

Jersey.

Physiology

Johnson, L. R. (1991). Gastrointestinal physiology, (4th edn). Mosby Year Book, St Louis.

Sandford, P. A. (1992). ^ Digestive system physiology, (2nd edn). Physiological Principles of Medicine Series, Edward Arnold.

Clinical physiology andpathophysiology

Campbell, E. J. M., Dickinson, C. J., Slater, J. D. H., Edwards, С R. W., and Sikora, E. K. (1984). Clinical physiology, (5th edn), Chapter 12. Blackwell Scientific, Oxford.

Porth, С. М. (1994). Pathophysiology, concepts of altered health states, (4th edn), Chaptets 41-43. J. P. Lippincott , Philadelphia.

Pharmacology

Rang, H. P., Dale, M. M., and Ritter, J. M. (1995). Pharmacology, (3rd edn), Chapter 19. Churchill-Livingstone, Edinburgh.

Nutrition

Bender, D. A. (1997). Introduction to nutrition and metabolism, (2nd edn). Taylor and Francis, London.

Self-test questions

The following statements are true or false. The answers are given below.

1. a. The ileum contains Brunner's glands.

b. Villi are present in all parts of the intestine, с The myenteric plexus lies between the longitudinal and circular smooth muscle layers in the gut wall.

d. The GI tract absorbs about 10 liters of fluid each day.

e. The serosa is the innermost layer of the gastro­
intestinal wall.

2. a. Swallowing is a purely voluntary activity.

b. The food bolus is propelled down the esophagus by

segmentation movements, с The saliva contains an enzyme that digests starch.

d. The pH of saliva rises as its rate of secretion increases.

e. Nerves are more important than hormones in the
regulation of salivary secretion.

3. a. Intrinsic factor is secreted by G-cells in the gastric

glands.

b. Cholecystokinin inhibits gastric secretion, с Gastric secretion does not begin until food enters the

stomach.

d. Gastric acid is secreted by parietal cells of the gastric
glands.

e. Most of the acid and pepsinogen produced by the
stomach are secreted during the intestinal phase of
gastric secretion.

4. a. The most vigorous mixing movements in the stomach

take place in the antral region, b. Gastric emptying is inhibited by the enterogastric

reflex, с Venous blood draining the stomach after a meal has a

higher pH than blood in the right atrium.

d. The plasma bicarbonate concentration will be lower
than normal following prolonged vomiting.

e. Persistent vomiting often leads to metabolic alkalosis.

Answers 431

5. a. Pancreatic acinar cells contain trypsin.

b. Cholecystokinin inhibits secretion from the exocrine

pancreas, с Loss of pancreatic enzymes will result in weight loss

due to poor protein digestion.

d. The introduction of acid into the duodenum stimu­
lates pancreatic secretion.

e. The chloride content of pancreatic juice falls as the
rate of secretion rises.

6. a. 70 per cent of the blood flow to the liver is via the

portal vein.

b. The cystic duct drains the gallbladder, с Bile is diluted in the gallbladder.

d. In obstructive jaundice, the feces are pale and fatty.

e. Micelles are found in the lacteals after a fat-rich meal.

7. a. Bile salts are the breakdown products of hemoglobin,
b. The reabsorption of bile salts in the intestine

stimulates bile secretion, с Most bile salts are absorbed in the terminal ileum.

d. Loss of bile salts will lead to poor absorption of
vitamin E.

e. Bile salts are hydrophobic molecules.

8. a. The Na+,K+-ATPase of the basolateral membrane of

intestinal epithelial cells plays an important role in

the absorption of salts and water, b. Intestinal digestive enzymes ate secreted by cells of

the crypts of Lieberkuhn. с About half of the ingested carbohydrate is absorbed

in the small intestine

d. Amino acids are absorbed in the small intestine by
cotransport with sodium.

e. Cellulose cannot be digested or absorbed by the
human small intestine.

f. The first organ to receive the bloodborne products of
digestion is the liver.

9. a. Total gastrectomy leads to malabsorption of vitamin

B12. b. Total gastrectomy will lead to a reduction in plasma

osmolality after meals, с Parasympathetic activity inhibits intestinal motility.

d. Distension of the ileum inhibits gastric motility.

e. The presence of large amounts of fat in the chyme
will accelerate gastric emptying.

10. a. Vitamin К deficiency may increase the risk of thrombosis.

b. Vitamin B12 deficiency can result in disorders of
night vision.

c. Foods derived from plants contain all the essential
amino acids.

d. The hypothalamus plays an important role in the
regulation of food intake.

e. Within the body, carbohydrates are stored in the form
of glycogen.

11. a. The mucosa of the anal canal is covered by stratified

squamous epithelial cells, b. Aldosterone stimulates the absorption of sodium and

water by the large intestine, с Dietary fiber reduces the rate at which food residues

move through the colon.

d. Vitamin К is synthesized by the intestinal flora.

e. Gastrin facilitates ileal emptying.

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