This placebo-controlled dose-response study shows that 7 days of ingestion of scFOS at a dose of 2.5 to 10 g/d, which was well tolerated, led to a significant increase in faecal bifidobacteria in healthy volunteers. This is the first study to demonstrate the bifidogenic effect of sc-FOS with such a low dose (2.5 g/d). Moreover, it must be stressed that the increase in bifidobacteria counts was correlated with the dose of ingested scFOS. We previously reported such an effect in healthy volunteers in another dose-response relationship study. However, the optimal and well-tolerated dose of scFOS leading to a significant increase in faecal bifidobacteria under usual diet was 10 g/d. These differences are probably due to the relatively small size of each sample, which decreased the power of the study. The increase in total anaerobes observed at the dose of 10 g/d is probably due to the high stimulation of colonic microflora induced by prebiotics [8, 9]. The faecal bifidobacteria and anaerobe levels observed in this study were similar to those found in previous studies [3, 5] in healthy volunteers on scFOS.
ScFOS has been studied extensively and its bifidogenic effects demonstrated in well-controlled human trials [3–5, 8]. However, Roberfroid et al. concluded from a compilation work that in a large population, there seems to be no dose-response effect of these non-digestible carbohydrates on bifidobacteria for daily intake doses between 4 and 40 g/d . Here, we found a linear dose-response relationship from 2.5 to 10 g/d, suggesting a dose-effect relationship. Such a relationship has been previously found in a study with scFOS, in which the range of doses was wider, from 5 to 20 g/d , and in another trial using a mixture of galacto- and fructooligosaccharides as supplementation of term infant formula . It was not found with other substrates such as galacto-oligosaccharides alone, resistant starch or soybean oligosaccharides .
We did not find any significant reduction in any other genus. There is little published data in humans available in the literature for comparison. Contradictory results have been reported in this field; while Gibson et al.  showed a significant reduction in Bacteroides using 15 g/d oligofructose, Rao  found an increase in Bacteroides using oligofructose at dose of 5 g/d. Moreover using oligofructose at 8 g/d, 2 authors did not find any effect on Bacteroide [7, 12]. The reasons for these discrepancies are unclear.
A decrease in colonic pH could reduce the risk of developing colonic cancer, since an inverse correlation between stool pH and colon cancer risk was observed [13, 14]. Slight acidification of faecal contents during scFOS ingestion was observed in animals  and in humans . In our previous studies, faecal pH did not change during ingestion of scFOS [5, 8]. However, as the faecal pH is the net sum of the degree of short-chain fatty acid absorption and bicarbonate secretion during passage through the colon, faecal pH does not reflect the pH in the colon under physiological conditions [16, 17].
Symptoms relating to gas production in the gut are widely reported in human prebiotic feeding studies but nevertheless remain very mild at the recommended intake levels [18, 19]. Compared to placebo, we did not find any significant digestive intolerance symptoms except minor bloating with scFOS. Similarly, no dose-response relationship for digestive symptoms was observed. In a threshold study to evaluate symptomatic response to varying levels of scFOS ingested regularly by 14 healthy volunteers, excessive flatus and borborygmi were reported by about 10% of volunteers at 10 g/d of scFOS, and excessive flatus, borborygmi and bloating were recorded for about 20–30% of volunteers at 20 g/d . In another study in which 10 volunteers ingested 15 g/d FOS for 12 days, gaseous symptoms such as abdominal cramps, excess flatus and bloating were all significantly more severe in subjects ingesting the FOS than in control subjects ingesting sucrose (P < 0.05) . However, with the exception of flatulence, these symptoms, if present, were usually mild, and did not increase (or decrease) during the course of the 12-day period. In our previous study comprising 10 healthy volunteers who ingested 12.5 g/d of scFOS for 12 days, only bloating was found to be significantly more frequent during the scFOS ingestion period than during placebo ingestion (P < 0.05), but this was very mild and present in only 5/10 volunteers . From all these results, it appears that the most common symptoms noted during scFOS administration are excess flatus and/or bloating, but only a minority of subjects experiences them and they are usually very mild.