Microorganisms occur throughout the alimentary tract but in healthy individuals their numbers and diversity are maintained within strict limits by the combined effects of intraluminal conditions, rapid transit, and host immunity. The colon and rectum, however, are adapted to facilitate bacterial colonization, and the typical adult human colonic microflora has been estimated to contain about 400 different bacterial species. The largest single groups present are Gram-negative anaerobes of the genus Bacter-oides, and Gram-positive organisms including bifi-dobacteria, eubacteria, lactobacilli, and clostridia. However, a large proportion of the species present cannot be cultured in vitro and are very poorly characterized.
The proximal colon, which receives undigested food residues, intestinal secretions, and the remnants of exfoliated enterocytes from the distal ileum, contains around 200 g of bacteria and substrates in a semiliquid state. These conditions are ideal for bacterial fermentation. Most of the bacteria of the human colon utilize carbohydrate as a source of energy, although not all can degrade polysaccharides directly. Many that are ultimately dependent upon dietary carbohydrate residues for energy are adapted to utilize the initial degradation products of the polysaccharide utilizers, rather than the polymers themselves. It has been estimated that somewhere between 20 and 80 g of carbohydrate enter the human colon every day, about half of which is undigested starch. Around 30 g of bacteria are produced for every 100 g of carbohydrate fermented.
Apart from dietary fiber, there are three major sources of unabsorbed carbohydrate for the colonic microflora. Perhaps the most important is resistant starch, which consists of retrograded starch polymers and starch granules enclosed within intact plant cell walls. Nondigestible sugars, sugar alcohols, and oligosaccharides such as fructooligosac-charides and galactooligosaccharides occur only sparingly in most plant foods, but they are now of great commercial interest because they can be used as prebiotics to selectively manipulate the numbers of bifidobacteria in the human colon. Endogenous substrates including mucus are also important for the colonic microflora. Mucus is an aqueous dispersion of a complex group of glycoproteins containing oligosaccharide side-chains, which are a major source of fermentable substrates. Even when the colon is surgically isolated and has no access to exogenous substrates it still supports a complex microflora.
The beneficial effects of dietary fiber on the alimentary tract were emphasized by another of the founders of the dietary fiber hypothesis, Denis Burkitt, who based his arguments largely on the concept of fecal bulk, developed as a result of field observations in rural Africa, where cancer and other chronic bowel diseases were rare. His hypothesis was that populations consuming the traditional rural diets, rich in vegetables and cereal foods, produced bulkier, more frequent stools than persons eating the refined diets typical of industrialized societies. Chronic constipation was thought to cause straining of abdominal muscles during passage of stool, leading to prolonged high pressures within the colonic lumen and the lower abdomen. This in turn was thought to increase the risk of various diseases of muscular degeneration including varicose veins, hemorrhoids, hiatus hernia, and colonic diverticulas. Colorectal neoplasia was also thought to result from infrequent defecation, because it caused prolonged exposure of the colonic epithelial cells to mutagenic chemicals, which could initiate cancer. Burkitt's overall hypothesis for the beneficial effects of fecal bulk has never really been refuted, and epidemiolo-gical evidence continues to support a protective role of fiber against colorectal cancer, particularly within Europe. However, the origins of intestinal neoplasia are now known to be far more complex than Burkitt was able to envisage, and there is little evidence to suggest a direct causal link between chronic constipation and colorectal cancer. Indeed, in one recent prevention trial, the risk of recurrence of colorectal polyps was slightly increased by prolonged supplementation with a bulk laxative based on one specialized source of cell wall polysaccharides.
Whatever the relationship to disease, it is certainly true that the consumption of dietary fiber is one major determinant of both fecal bulk and the frequency of defecation (bowel habit). However, the magnitude of the effect depends upon the type of fiber consumed. Soluble cell wall polysaccharides such as pectin are readily fermented by the microflora, whereas lignified tissues such as wheat bran tend to remain at least partially intact in the feces. Both classes of dietary fiber can contribute to fecal bulk but by different mechanisms. The increment in stool mass caused by wheat bran depends to some extent on particle size, but in healthy Western populations it has been shown that for every 1 g of wheat bran consumed per day, the output of stool is increased by between 3 and 5g. Other sources of dietary fiber also favor water retention. For example, isphagula, a mucilaginous material derived from Psyllium, is used pharmaceutically as a bulk laxative. Soluble polysaccharides such as guar and oat /3-glucan are readily fermented by anaerobic bacteria, but solubility is no guarantee of fermentabil-ity, as is illustrated by modified cellulose gums such as methylcellulose, which is highly resistant to degradation in the human gut. Fermentation reduces the mass and water-holding capacity of soluble poly-saccharides considerably, but the bacterial cells derived from them do make some contribution to total fecal output. Thus, although all forms of dietary fiber are mild laxatives, the single analytical measurement of total fiber content again provides no simple predictive measure of physiological effect.
Although fermentation of fiber tends to reduce its effectiveness as a source of fecal bulk, it has other very important benefits. The absorption and metabolism of short-chain fatty acids derived from carbohydrate fermentation provides the route for the recovery of energy from undigested polysac-charides. Butyrate functions as the preferred source of energy for the colonic mucosal cells, whilst propionate and acetate are absorbed and metabolized systemically. There continues to be much debate about the importance of butyrate for the colon. In vitro, butyrate causes differentiation of tumor cells, suppresses cell division, and induces programed cell death (apoptosis). These effects are thought likely to suppress the development of cancer, but it is not yet entirely clear whether they also occur in the intact intestine. Research continues on the importance of butyrate and other short-chain fatty acids for human health.
The other major breakdown products of carbohydrate fermentation are hydrogen, methane, and carbon dioxide, which together comprise flatus gas. Excess gas production can cause distension and pain in some individuals, especially if they attempt to increase their fiber consumption too abruptly. In most cases, however, extreme flatus is probably caused more by fermentation of oligosaccharides such as stachyose and verbascose, which are found principally in legume seeds, rather than the cell wall polysaccharides themselves.
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