Function of the colon

The principal function of the colon is to absorb water and electrolytes. Absorption of nutrients is minimal. Normally, about 1500 mL of fluid enters the colon each day, and approximately 90% gets absorbed. When the colon is working correctly, the extent of fluid absorption is such that the resulting stool is soft (but formed) and capable of elimination without strain. However, when fluid absorption is excessive, as can happen when transport through the intestine is delayed, the resultant stool is dehydrated and hard. Conversely, if insufficient fluid is absorbed, watery stools result.

Frequency of bowel evacuation varies widely among individuals. For some people, bowel movements occur 2 or 3 times a day. For others, elimination may occur only 2 times a week. Because of this wide individual variation, we can’t define a normal frequency for bowel movements. Put another way, although a daily bowel movement may be normal for many people, it may be abnormal for many others.

Dietary fiber

Proper function of the bowel is highly dependent on dietary fiber—the component of vegetable matter that escapes digestion in the stomach and small intestine. Fiber facilitates colonic function in two ways. First, fiber absorbs water, thereby softening the feces and increasing their mass. Second, fiber can be digested by colonic bacteria, whose subsequent growth increases fecal mass. The best source of fiber is bran. Fiber can also be obtained from fruits and vegetables. Ingestion of 20 to 60 gm of fiber a day should optimize intestinal function.

Constipation

Constipation is one of the most common GI disorders. In the United States, people seek medical help for constipation at least 2.5 million times a year, and spend hundreds of millions on laxatives.

Constipation is defined in terms of symptoms, which include hard stools, infrequent stools, excessive straining, prolonged effort, a sense of incomplete evacuation, and unsuccessful defecation. Scientists who do research on constipation usually define it using the Rome II criteria (Table 79–1). As Table 79–1 shows, constipation is determined more by stool consistency (degree of hardness) than by how often bowel movements occur. Hence, if the interval between bowel movements becomes prolonged, but the stool remains soft and hydrated, a diagnosis of constipation would be improper. Conversely, if bowel movements occur with regularity, but the feces are hard and dry, constipation can be diagnosed—despite the regular and frequent passage of stool.

A common cause of constipation is poor diet—specifically, a diet deficient in fiber and fluid. Other causes include dysfunction of the pelvic floor and anal sphincter, slow intestinal transit, and use of certain drugs (eg, opioids, anticholinergics, some antacids).

In most cases, constipation can be readily corrected. Stools will become softer and more easily passed within days of increasing fiber and fluid in the diet. Mild exercise, especially after meals, also helps improve bowel function. If necessary, a laxative may be employed—but only briefly and only as an adjunct to improved diet and exercise.

Indications for laxative use

Laxatives can be highly beneficial when employed for valid indications. By softening the stool, laxatives can reduce the painful elimination that can be associated with episiotomy and with hemorrhoids and other anorectal lesions. In patients with cardiovascular diseases (eg, aneurysm, myocardial infarction, disease of the cerebral or cardiac vasculature), softening the stool decreases the amount of strain needed to defecate, thereby avoiding dangerous elevation of blood pressure. In elderly patients, laxatives can help compensate for loss of tone in abdominal and perineal muscles. As an adjunct to anthelmintic therapy, laxatives can be used for (1) obtaining a fresh stool sample for diagnosis; (2) emptying the bowel prior to treatment (so as to increase parasitic exposure to anthelmintic medication); and (3) facilitating export of dead parasites following anthelmintic use. Additional applications include (1) emptying of the bowel prior to surgery and diagnostic procedures (eg, radiologic examination, colonoscopy); (2) modifying the effluent from an ileostomy or colostomy; (3) preventing fecal impaction in bedridden patients; (4) removing ingested poisons; and (5) correcting constipation associated with pregnancy and certain drugs, especially opioid analgesics.

Precautions and contraindications to laxative use

Laxatives are contraindicated for individuals with certain disorders of the bowel. Specifically, laxatives must be avoided by individuals experiencing abdominal pain, nausea, cramps, or other symptoms of appendicitis, regional enteritis, diverticulitis, and ulcerative colitis. Laxatives are also contraindicated for patients with acute surgical abdomen. In addition, laxatives should not be used in patients with fecal impaction or obstruction of the bowel, because increased peristalsis could cause bowel perforation. Lastly, laxatives should not be employed habitually to manage constipation. Reasons why are discussed below under Laxative Abuse.

Laxatives should be used with caution during pregnancy (because GI stimulation might induce labor) and during lactation (because the laxative may be excreted in breast milk).

Laxative classification schemes

Traditionally, laxatives have been classified according to general mechanism of action. This scheme has four major categories: (1) bulk-forming laxatives, (2) surfactant laxatives, (3) stimulant laxatives, and (4) osmotic laxatives. Representative drugs are listed in Table 79–2.

From a clinical perspective, it can be useful to classify laxatives according to therapeutic effect (time of onset and impact on stool consistency). When these properties are considered, most laxatives fall into one of three groups, labeled I, II, and III in this chapter. Group I agents act rapidly (within 2 to 6 hours) and give a watery consistency to the stool. Laxatives in group I are especially useful when preparing the bowel for diagnostic procedures or surgery. Group II agents have an intermediate latency (6 to 12 hours) and produce a stool that is semifluid. Group II agents are the ones most frequently abused by the general public. Group III laxatives act slowly (in 1 to 3 days) to produce a soft but formed stool. Uses for this group include treating chronic constipation and preventing straining at stool. Representative members of groups I, II, and III are listed in Table 79–3.

Mechanism of action.

Bulk-forming agents have the same impact on bowel function as dietary fiber. Following ingestion, these agents, which are nondigestible and nonabsorbable, swell in water to form a viscous solution or gel, thereby softening the fecal mass and increasing its bulk. Fecal volume may be further enlarged by growth of colonic bacteria, which can utilize these materials as nutrients. Transit through the intestine is hastened because swelling of the fecal mass stretches the intestinal wall, and thereby stimulates peristalsis.

Indications.

Bulk-forming laxatives are preferred agents for temporary treatment of constipation. Also, they are widely used in patients with diverticulosis and irritable bowel syndrome. In addition, by altering fecal consistency, they can provide symptomatic relief of diarrhea and can reduce discomfort and inconvenience for patients with an ileostomy or colostomy.