Plant Fiber Intake in the Pediatric Diet

Dietary fiber has been defined as the part of material in foods impervious to the degradative enzymes of the human digestive tract. The dietary fiber of plants is comprised of carbohydrate cornpounds including cellulose, hemicellulose, pectin, gums, rnucilages, and a noncarbohydrate substance, lignin. These substances, which form the structure of plants, are present in the cell walls of all parts including the leaf, stern, root, and seed.’ Animal tissue also contains indigestible substances. Crude fiber and dietary fiber are not the same thing. Crude fiber refers to the residue left after strong acid and base hydrolysis of plant material. This process dissolves the pectin, gums, rnucilages, and most of the hemicellulose and mainly is a measure of the cellulose and lignin content. Clearly, this method tends to underestimate the total amount of fiber in the food.’ Most food composition tables give only crude fiber values. Current interest in fiber was stimulated by the suggestion that it might help to prevent certain diseases common in the United States, namely diverticular disease, cancer of the colon, irritable bowel syndrome, obesity, and coronary heart disease.2 African blacks in rural areas where the fiber intake was high rarely had these diseases; however, during the past 20 years as this population moved to the cities and adopted Western habits (including a Western diet), they began to suffer from the same “Western-type” diseases. A high-fiber diet increases fecal bulk, produces softer, more frequent stools, and decreases transit time through the intestine.5 These factors may be responsible for the supposed beneficial effects of fiber. A decreased transit time implies less time for potential carcinogens to be in contact with the intestinal mucosa. The increased bulk would dilute potential carcinogens and produce less straining at stool, a factor implicated by Burkitt and co-workers5 in the development of diverticulosis. Obesity is rare in populations eating most carbohydrates as complex carbohydrates, such as rice, beans, lentils, and cereal grains, which contain large amounts of fiber. This could simply be that the increased mastication required for these foods slows caloric ingestion or that the increased bulk acclerates satiety.6 However, it could also be caused by a lower caloric intake. Arteriosclerosis and coronary heart disease may also be inversely related to low fiber intake. Trowe117 suggests that a high fiber intake lowers blood cholesterol by increasing the fecal excretion of bile acids and sterols which are metabolites of cholesterol in man. Human studies have confirmed this. Subjects consuming from 12 to 36 gm ofpectin daily for several weeks have shown a significant reduction of total serum cholesterol ranging from 8% to 30%.’ The different fractions of fiber have different physiologic effects. Pectin has a cholesterol-lowering effect which may be caused by bile acid sequestration and increased stool-fat content. Lignin has bile acid-binding properties in vitro, but these have not yet been demonstrated in vivo. There are conflicting data about whether it lowers serum cholesterol. Cellulose has been shown to increase output of fecal bile acid. It is not thought to have a hypocholesterolemic effect, although this is questionable. Hemicellulose increases fecal excretion of bile acid, but probably by a mechanism other than adsorption. There is some evidence that it has a hypocholesterolemic effect.’ Most evidence for the beneficial effects of fiber is epidemiologic and refers almost entirely to adults, and the diseases mentioned (with the exception of obesity) require years to develop. In addition to fiber intake, there are many variables in the populations studied, such as intake of saturated fats and sucrose, exercise, and stress, all of which are implicated as causative factors in these diseases. Should a recommendation be made for an in-