A Critical Look at Prebiotics Within the Dietary Fiber Concept.

This article reviews the current knowledge of the health effects of dietary fiber and prebiotics and establishes the position of prebiotics within the broader context of dietary fiber. Although the positive health effects of specific fibers on defecation, reduction of postprandial glycemic response, and maintenance of normal blood cholesterol levels are generally accepted, other presumed health benefits of dietary fibers are still debated. There is evidence that specific dietary fibers improve the integrity of the epithelial layer of the intestines, increase the resistance against pathogenic colonization, reduce the risk of developing colorectal cancer, increase mineral absorption, and have a positive impact on the immune system, but these effects are neither generally acknowledged nor completely understood. Many of the latter effects are thought to be particularly elicited by prebiotics. Although the prebiotic concept evolved significantly during the past two decades, the line between prebiotics and nonprebiotic dietary fiber remains vague. Nevertheless, scientific evidence demonstrating the health-promoting potential of prebiotics continues to accumulate and suggests that prebiotic fibers have their rightful place in a healthy diet.

Consumption of breads containing in situ-produced arabinoxylan oligosaccharides alters gastrointestinal effects in healthy volunteers.

Arabinoxylan oligosaccharides (AXOS) are studied as food compounds with prebiotic potential. Here, the impact of consumption of breads with in situ-produced AXOS on intestinal fermentation and overall gastrointestinal characteristics was evaluated in a completely randomized, double-blind, controlled, cross-over study. Twenty-seven healthy volunteers consumed 180 g of wheat/rye bread with or without in situ-produced AXOS (WR(+) and WR(-), respectively) daily for 3 wk. Consumption of WR(+) corresponded to an AXOS intake of ~2.14 g/d. Refined wheat flour bread without AXOS (W(-)) (180 g/d) was provided during the 3-wk run-in and wash-out periods. At the end of each treatment period, participants collected urine for 48 h as well as a feces sample. Additionally, all participants completed a questionnaire about stool characteristics and gastrointestinal symptoms during the last week of each period. Urinary phenol and p-cresol excretions were significantly lower after WR(+) intake compared to WR(-). Consumption of WR(+) significantly increased fecal total SCFA concentrations compared to intake of W(-). The effect of WR(+) intake was most pronounced on butyrate, with levels 70% higher than after consumption of W(-) in the run-in or wash-out period. Consumption of WR(+) tended to selectively increase the fecal levels of bifidobacteria (P = 0.06) relative to consumption of W(-). Stool frequency increased significantly after intake of WR(+) compared to WR(-). In conclusion, consumption of breads with in situ-produced AXOS may favorably modulate intestinal fermentation and overall gastrointestinal properties in healthy humans.

Prebiotic effects and intestinal fermentation of cereal arabinoxylans and arabinoxylan oligosaccharides in rats depend strongly on their structural properties and joint presence.

SCOPE: Cereal arabinoxylan (AX) is one of the main dietary fibers in a balanced human diet. To gain insight into the importance of structural features of AX for their prebiotic potential and intestinal fermentation properties, a rat trial was performed. METHODS AND RESULTS: A water unextractable AX-rich preparation (WU-AX, 40% purity), water extractable AX (WE-AX, 81% purity), AX oligosaccharides (AXOS, 79% purity) and combinations thereof were included in a standardized diet at a 5% AX level. WU-AX was only partially fermented in the ceco-colon and increased the level of butyrate and of butyrate producing Roseburia/E. rectale spp. Extensive fermentation of WE-AX and/or AXOS reduced the pH, suppressed relevant markers of the proteolytic breakdown and induced a selective bifidogenic response. Compared with WE-AX, AXOS showed a slightly less pronounced effect in the colon as its fermentation was virtually complete in the cecum. Combining WU-AX and AXOS caused a striking synergistic increase in cecal butyrate levels. WU-AX, WE-AX and AXOS together combined a selective bifidogenic effect in the colon with elevated butyrate levels, a reduced pH and suppressed proteolytic metabolites. CONCLUSION: The prebiotic potential and fermentation characteristics of cereal AX depend strongly on their structural properties and joint presence.