proportions of Faecalibacterium, Phascolarctobacterium, and Dialister in feces following consumption of both PDX and SCF and a small but statistically significant increase in Lactobacillus following consumption of SCF.

The extent to which a prebiotic (or potential prebiotic) stimulates microbial growth depends not just on the type of substance ingested, but also on its dietary concentration. In a 16-week study of galactooligosaccharides (GalOS) in 18 healthy human volunteers between 19 and 50 years of age, Davis et al. (2010) reported two major findings. First, based on culture enumeration, the concentration of Bifidobacterium in feces increased significantly among volunteers who were fed 5 or 10 grams of GalOS per day, compared to baseline. There was no significant increase in Bifidobacterium in the feces of individuals fed either 0 or 2.5 grams of GalOS per day. Second, individuals fed 10 grams of GalOS per day also had significantly more total anaerobes in their feces compared to baseline. None of the other treatment groups showed a change in total anaerobe concentration. Using quantitative real-time PCR (qRT-PCR) to measure the bifidogenic effects of GalOS, again the researchers found a significant increase in bifidogenic activity among individuals fed 5 or 10 grams of GalOS per day, but not among individuals fed 0 or 2.5 grams per day. With respect to which bacteria are affected by GalOS consumption, Davis and colleagues (2010) pyrosequenced the V1-V3 region of 16S ribosomal DNA (rDNA) and found that GalOS consumption did not impact the diversity of fecal microbes but did impact the relative proportions of bacterial taxa at the phylum, family, genus, and species levels. For example, consumption of 10 grams of GalOS per day increased the proportion of the family Bifidobacteriaceae from 1.56 percent at baseline to 6.14 percent. Within that family, consumption of 10 grams of GalOS per day increased the proportion of Bifidobacterium from 1.28 percent at baseline to 5.20 percent.

Fahey referred to previous speakers’ comments on the considerable variability that exists among individuals with respect to how their microbiota respond to dietary intervention. Not surprisingly, Davis et al. (2010) observed highly variable responses among individuals at the phylum, family, genus, and species levels. Fahey recognized the limitations that individual variability sets up for a study based on a sample size of 18, but asserted that 18 is a manageable number for such an intensive study.

Sometimes prebiotics are used as supplements at very low levels, even though much of the murine research on inulin, for example, involves daily administration of the human equivalent of 30 to 35 grams of fermentable substrate, which is within the range of the dietary reference intake (DRI)



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