Background/aims: Oligofructose and inulin are naturally occurring indigestible carbohydrates. In vitro they selectively stimulate the growth of species of Bifidobacterium, a genus of bacteria considered beneficial to health. This study was designed to determine their effects on the large bowel microflora and colonic function in vivo.
Methods: Eight subjects participated in a 45-day study during which they ate controlled diets. For the middle 15 days, 15 g.day-1 oligofructose was substituted for 15 g.day-1 sucrose. Four of these subjects went on to a further period with 15 g.day-1 inulin. Bowel habit, transit time, stool composition, breath H2 and CH4, and the predominant genera of colonic bacteria were measured.
Results: Both oligofructose and inulin significantly increased bifidobacteria from 8.8 to 9.5 log10 g stool-1 and 9.2 to 10.1 log10 g stool-1, respectively, whereas bacteroides, clostridia, and fusobacteria decreased when subjects were fed oligofructose, and gram-positive cocci decreased when subjects were fed inulin. Total bacterial counts were unchanged. Fecal wet and dry matter, nitrogen, and energy excretion increased with both substrates, as did breath H2. Little change in fecal short-chain fatty acids and breath CH4 was observed.
Conclusions: A 15-g.day-1 dietary addition of oligofructose or inulin led to Bifidobacterium becoming the numerically predominant genus in feces. Thus, small changes in diet can alter the balance of colonic bacteria towards a potentially healthier microflora.