And such in the effect at the higher dose, that the researchers behind the new study, from the University of Reading, suggest recommendations should err on the high side for maximum gut health benefits.
Modern recommendations for inulin and oligofructose intake are between five and eight grams per day.
"Inulin exerted a bifidogenic effect in health human adults at doses of five and eight grams per day," wrote the researchers. "In light of both doses proving effective, the higher dose may be preferable as no significant side effects were noted and a higher percentage of the study population responded to [the prebiotics]."
Sofia Kolida and Glenn Gibson from the University of Reading and Diederick Meyer from Holland-based Sensus Operations (which funded the study), recruited 30 healthy volunteers (15 women, average age 26.5) to take part in the double-blind, placebo-controlled, crossover human study.
The volunteers consumed a chocolate drink containing placebo (maltodextrin, 8 g/day), 5 g/day inulin and 8 g/day inulin for two week periods, with each period separated by a one-week washout period.
Samples of the volunteers' faeces were collected at baseline and then at the end of each intervention and washout period, and populations of gut microflora recorded for Bifidobacteria, Bacteroides Prevotella, Lactobacillus, Enterococcus and Clostridium perfringens - histolyticum subgroup.
Writing in the European Journal of Clinical Nutrition, Kolida and co-workers state that levels of Bifidobacteria increased significantly upon ingestion of the low and the high inulin dose, by 9.78 and 9.79 log10 cells/g faeces, compared to placebo (9.64 log10 cells/g faeces).
"The fact that both inulin doses promoted bifidobacterium growth is of importance as it is essential to estabilish the efficacy of inulin to exert a bifidogenic effect on the gut microflora at lower doses," wrote Kolida. "When excess fermentable oligosaccharides are delivered to the colon side effects such as bloating and flatulence may occur."
The researchers also noted that at the higher dose (eight grams per day) levels of Clostridium perfringens - histolyticum subgroup decreased significantly, a result noted as beneficial.
"Bifidobacteria do not produce gas during carbohydrate fermentation, whereas clostridia are prolific gas producers. This may explain why the high dose was better tolerated," they said.
Considerable research has already focussed on the role of inulin and oligofructose in bone health and colorectal cancer, and the science is now expanding in ever-increasing circles to cover potential benefits for the immune system, weight management, and intestinal health.
Source: European Journal of Clinical Nutrition Advance online publication; doi: 10.1038/sj.ejcn.1602636 "A double-blind placebo-controlled study to establish the bifidogenic dose of inulin in healthy humans"
Authors: S. Kolida, D. Meyer and G.R. Gibson