In vitro fermentation of swine ileal digesta containing oat bran dietary fiber by rat cecal inocula adapted to the test fiber increases propionate production but fermentation of wheat bran ileal digesta does not produce more butyrate.

This experiment evaluated three hypotheses: i) production of propionate is increased during fermentation of substrate containing oat bran (OB)(6); ii) production of butyrate is increased during fermentation of substrate containing wheat bran (WB) and iii) results of in vitro fermentations using physiological substrates and inocula agree with in vivo data. Ileal digesta collected from swine fed OB and WB were the substrates. Digesta was fermented for 0-96 h in an anaerobic in vitro system using inocula prepared from ceca of rats fed the same fiber sources. Carbohydrate and short-chain fatty acid (SCFA) contents in the fermentations were measured by gas chromatography. Fermentation of WB digesta did not produce more n-butyrate (P > 0.05) and was significantly slower (P < 0.05) than fermentation of OB digesta. OB digesta fermentation produced a significantly greater (P < 0.05) molar proportion of SCFA as propionate. Bacterial mass increased more and was maintained longer during fermentation of OB digesta than the WB digesta. Our results indicate that dilution of undigested WB fiber and not n-butyrate production is one mechanism by which WB may protect colonic mucosa; propionate production is increased during fermentation of beta-glucan in OB; and an in vitro system using physiological sources of inoculum and substrate containing WB and OB yields results that agree with in vivo findings in humans and rats.

Increasing amounts of dietary fiber provided by foods normalizes physiologic response of the large bowel without altering calcium balance or fecal steroid excretion.

Nine healthy, young men consumed constant diets to determine selected large-bowel, serum cholesterol and triacylglycerol, and calcium balance responses to 3 amounts of fiber provided by a mixture of fruit, vegetables, and grains. The diets, each consumed for 1 mo, contained 16, 30, and 42 g total fiber/d, of which 2.9, 4.8, and 7.7 g, respectively, was soluble. Mean daily wet and dry stool weights increased with each fiber addition. The first fiber addition increased defecation frequency and decreased fecal pH, bile acid concentration, and neutral steroid concentration; the second addition had no further effect. Mean weight of each defecation and stool moisture did not increase and serum cholesterol and triacylglycerol concentrations, calcium balance, and gastrointestinal transit time did not decrease as fiber intake increased. We conclude that 1) fiber provided by a mixed-food diet increases stool weight as effectively as does wheat or oat bran; 2) even high amounts of dietary fiber do not change transit time or defecation frequency if they are already approximately 1 and 2-3 d, respectively; 3) food patterns consistent with the food pyramid and incorporating legumes and whole grains are necessary to achieve recommended fiber intakes of 20-35 g/d, even if energy intake is > 12.55 MJ (3000 kcal); 4) soluble fiber provided by a mixture of whole foods has no effect on serum cholesterol concentrations or output of fecal bile acids; and 5) mixed-food fiber has little effect on calcium balance when calcium intakes are high (> or = 1.5 g/d).

Mechanisms by which wheat bran and oat bran increase stool weight in humans.

Generally, stool weight is significantly increased by adding sources of insoluble fiber to the diet. Comparable amounts of fiber provided by wheat and oat brans have the same effect on daily stool output, even though > 90% of wheat bran fiber but only 50-60% of oat bran fiber is insoluble. To determine the bases for these increases in stool weight, stool samples collected from 5 men in 2 constant diet studies that determined the effects of wheat and oat brans on large-bowel physiology were fractionated by using a physicochemical procedure into plant, bacterial, and soluble fractions, which were weighed and analyzed for sugar content and composition. Nitrogen, crude fat, and ash outputs were also determined. Wheat bran increased the fecal concentration of sugars and mass of plant material more than did oat bran, whereas oat bran increased fecal bacterial mass more. Each fiber source increased nitrogen, ash, and fat excretion, but excretion of fat was greater with oat bran. The apparent digestibility of plant-derived neutral sugars decreased significantly when wheat but not oat bran was consumed. The apparent digestibility of neutral sugars provided by wheat bran was 56%; the apparent digestibility of those provided by oat bran was 96%. We conclude that bacteria and lipids are major contributors to the increase in stool weight with oat bran consumption, whereas undigested plant fiber is responsible for much of the increase in stool weight with wheat bran consumption. Results are consistent with the hypothesis that oat bran increases stool weight by providing rapidly fermented soluble fiber in the proximal colon for bacterial growth, which is sustained until excretion by fermentation of the insoluble fiber.

Mechanism of serum cholesterol reduction by oat bran.

Nine normolipidemic young men consumed a constant diet for 2 mo into which oat bran was incorporated during the second month so that we might test the hypotheses that oats lower serum cholesterol concentrations by decreasing bile acid and fat absorption and increasing bile acid synthesis. Bile acid kinetics were determined by measuring the 13C enrichment of serum cholic and chenodeoxycholic acids. Oat bran consumption decreased serum cholesterol levels (p < 0.01) and cholic acid pool size (p < 0.05). Deoxycholic acid pool size (p < 0.01) and the synthesis and fractional turnover rates of both primary bile acids (p < 0.05) increased. Total bile acid pool size did not change. Fecal excretion of total bile acids, the two secondary bile acids and fat increased significantly. The results demonstrate that oat bran lowers serum cholesterol levels in part by altering bile acid metabolism. In addition, the substantial increase in the proportion of the total bile acid pool that was deoxycholic acid is consistent with the hypothesis that oat bran also decreases cholesterol synthesis.

Determination of fermentable carbohydrate from the upper gastrointestinal tract by using colectomized rats.

The primary aim of this study was to characterize the carbohydrate that would be supplied to the colon for fermentation under physiological conditions. Colectomized rats were fed fiber-free diets or diets containing 5% (wt/wt) gum arabic. Four (fucose, galactose, glucosamine, and galactosamine) of 11 analyzed sugars accounted for 77% of the total sugar in ileal excreta from colectomized rats fed fiber-free diets. The three sugars in gum arabic, rhamnose, arabinose, and galactose, accounted for 84% of the total sugars in gum arabic ileal excreta. Comparisons of the sugar compositions of the ileal excreta, the water-soluble fractions of the excreta, and three gel filtration fractions of the water-soluble material with those of the water-soluble fraction of rat mucosa, the acetone-soluble fraction of pancreas, and pancreatin suggested that the major source of endogenous carbohydrate is mucin. Gum arabic increased the daily excretion of the four mucin-derived sugars (fucose, galactose, glucosamine, and galactosamine) by the colectomized rats from 473 mumol per day to 634 mumol per day. We conclude that mucin is the major endogenous carbohydrate excreted from the upper gut and that gum arabic increases the amount of this endogenous carbohydrate.