The Pros and Cons of Vegetable Laxatives

The overall sale of laxatives in the United States continues to rise. In 2005 laxative sales accounted for more than $678 million in annual sales1. Most of the laxative products sold in this country are derived from plant sources.

Laxatives are drugs used to facilitate the passage and elimination of feces from the colon and rectum1. Although the term laxative and catharsis are used interchangeably, they imply different intensity of effect. Laxatives produce soft-formed stools while the cathartics produce more fluid evacuation and increased intestinal motor activity. They are used to relieve constipation. Constipation refers to stools that are too small, too infrequent or too difficult to expel. Stool weight and frequency are largely a function of the diet5. Stools produced by a low residue diet (e.g. a Western diet) weigh less, and are not as bulky as stools produced by a high residue diet (e.g. a vegetarian diet). A low residue diet is likely to constipate while a high residue diet containing dietary fiber will maintain regularity.

The wide availability of over-the-counter laxatives creates the potential for self-diagnosis and treatment of constipation without consulting a health professional. Frequently, constipation may be caused by irritable bowel disease but it may also be associated with neurological diseases or systemic diseases, or it may be drug-induced. These problems require primary treatment independent of symptomatic treatment with laxatives. Therefore, indiscriminate use of laxatives is not recommended5.

Laxatives can be classified according to their mechanism of action. There are six classes: emollient, saline, hyperosmotic, lubricant, bulk forming and stimulant laxatives1,3,5. Emollient laxatives, such as docusate sodium, are surfactants, which increase the wetting capacity of intestinal fluids and soften fecal mass. Saline laxatives, made up of inorganic salts such as magnesium sulfate, draw water into the gut by their osmotic action that increases the water content and volume of the stool. Lactulose, a sugar, is a hyperosmotic laxative that exerts an osmotic effect similar to that of saline laxatives. Mineral oil is a lubricant laxative that facilitates the passage of fecal matter through the gut due to its lubricant properties.

This article will focus on vegetable laxatives, most of which are categorized into bulk-forming and stimulant laxatives. Bulk-forming laxatives are natural and semi-synthetic hydrophilic polysaccharides and cellulose derivatives that dissolve or swell in intestinal fluid, forming emollient gels that facilitate passage of the intestinal contents and stimulate peristalsis. The natural stimulant laxatives contain anthraquinone derivatives or are made up of products containing castor oil.

The theories behind the mechanism of action of stimulant laxatives have changed over time. They were first believed to act by increasing the propulsive peristaltic activity of the intestine by local irritation of the mucosa1. It is surmised that these laxatives increase the colonic formation of histamine, serotonin and prostaglandins. The prostaglandins appear to be the most important mediators in the development of increased mass movement and secretion of water and electrolytes associated with these drugs2.

BULK-FORMING LAXATIVES

Dietary Changes: In the majority of cases, modifying the diet by increasing the fiber intake will increase the stool bulk, water retention and the rate of stool transit, which in turn will increase the frequency of defecation. Dietary or plant fibers may be classified into water-soluble or water-insoluble fibers. The water-soluble fibers include pectin-like compounds, gums and storage polysaccharides. The water-insoluble fibers include cellulose, hemicelluloses and lignin that are the most useful in relieving constipation. Wheat products are the most widely recognized food source of insoluble fiber in the American diet, although vegetables and fruits also contain significant amounts. Foods such as wheat bran that are high in insoluble dietary fiber are most efficacious in maintaining regularity. References to foods high in fiber are available in most libraries 1,4.

Psyllium Husk: Psyllium husk sold commercially comes from the seeds of the Indian psyllium or blond psyllium, Plantago ovata Forsk (Family: Plantaginaceae) but often includes black psyllium or Spanish psyllium, P. arenaria Waldst. & Kit. and other species. A physicochemical process is used to remove the mucilaginous layer (husk) from the seed coat6. Commercial preparations in the U.S. containing only psyllium husk include MetamucilR, KonsylR while others such as PerdiemR contain a combination of senna and psyllium. Inexpensive generic preparations are also available.

Gums and Mucilages: Gums are often classified as pathological products (e.g. gum tragacanth is produced in response to injury to the plant) while mucilages are physiological products (e.g. the mucilage produced naturally in psyllium seeds). A variety of gums have been used in conjunction with other laxatives. They have bulk-forming properties but are often used as emulsifying agents. Gum tragacanth and gum arabic, are obtained from Astragalus gummifer Labillardiere and Acacia senegal L., respectively (Family: Fabaceae). Agar, obtained from the seaweed Gelidium cartilagineum L. (Family: Gelidiaceae), has been used in conjunction with other polysaccharides. Other gums and mucilages used as laxatives or in combination with other laxatives include karaya gum from Sterculia urens Roxburgh (Family: Sterculiaceae), guar gum from Cyamopsis tetragonolobu Taubert (Family: Fabaceae), and Irish Moss from the sea weed Chondrus crispus L. (Family: Gigartinaceae)6.

Cellulose and Cellulose Derivatives: Cellulose and its derivatives such as methylcellulose are prepared from fibrous plant materials. Products containing methylcellulose such as CitrucelR are used alone as bulk-forming laxatives or in combination with other laxatives.

Barley Malt Extract: Malt extract sold as MaltsupexR is prepared from an artificially germinated grain of one or more varieties of barley, Hordeum vulgare L. (Family: Gramineae). It is relatively safe and often administered to infants suffering from constipation.

STIMULANT LAXATIVES.

Anthraquinone Derivatives: The anthraquinone glycosides found in plants referred to below are hydrolyzed by colonic bacteria into the active compounds. A number of these anthraquinone glycosides are found in the fruits and leaves of senna, cascara sagrada (Spanish for sacred bark), buckthorn bark, roots and rhizomes of rhubarb, and dried juice or latex from the leaves of aloe. The cathartic effect of the anthraquinones occurs primarily on the colon, 8 to 12 hours after administration of the drug1.

Senna: Senna extracts and its active principles, the sennosides A & B, are prepared from the dried leaves and the seed pods of Cassia acutifolia Delile, known in commerce as Alexandria senna, or from C. angustifolia Vahl, known commercially as Tinnevelly senna, (Family: Fabaceae)6. Senna is available in various dosage forms. To mention a few, it is sold as tablets, granules, and suppositories by the trade name SenokotR, in liquid form as Fletcher’s CastoriaR and in the form of tablets containing pure glycosides under the name Ex-Lax Gentle NatureR.

Cascara sagrada: The name cascara sagrada in Spanish means sacred bark. Its extractive casanthrol is obtained from the aged dried bark of Rhamnus purshianus DeCandolle (Family: Rhamnaceae), a plant indigenous to the North West of the U.S.6. During the one- year aging process the powerful cathartic glycosides are converted to its milder forms. Cascara is available as a generic product in form of tablets or in liquid form as Cascara Sagrada Aromatic Fluid Extract, or in capsules, as a combination of casanthrol and docusate sodium.

Buckthorn: Buckthorn or Frangula bark is obtained from Rhamnus frangula L. (Family: Rhamnaceae) that resembles the cascara bark in its laxative properties but is milder. The bark as well as the fluid extract of the buckthorn bark is commonly used in Europe6.

Other Anthraquinone Drugs: Miscellaneous drugs containing anthraquinone glycosides include rhubarb and aloe. Rhubarb is obtained from the roots and rhizomes of Rheum officinale Baillon & other Rheum species (Fam: Polygonaceae). Aloin from aloe is the latex obtained from the outer pericyclic cells of the leaf of Aloe vera L. & other Aloe species (Family: Liliaceae)6. Aloin must not be confused with the aloe-gel that is obtained from the inner parenchymatous cells of the aloe leaf and is used topically. Rhubarb and aloe are no longer prescribed as laxatives in the U.S. because of their very irritating and drastic cathartic effects.

Castor Oil: It is the purified fixed oil obtained from the seeds of Ricinus communis L. (Family: Euphorbiaceae). The oil is composed mostly of triricinolein that on hydrolysis in the intestine yields ricinoleic acid and glycerol. Ricinoleic acid is the principle responsible for colonic mobility6. Castor oil produces evacuation within 2-6 hours after oral administration. Although not commonly used, it is available in suspension form in products such as NeoloidR.

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