and they have not been shown to meet all of the specific requirements of the current working definition of a prebiotic.

Prebiotics and prebiotic candidates are produced from a variety of raw materials, such as chicory, artichoke, beet, cow’s milk, starch, and soybean (Mussatto and Mancilha, 2007). Production typically involves extracting an intermediate product (e.g., inulin from chicory and artichoke, sucrose from beet, lactose from cow’s milk, soluble starch from starch, soybean whey and xylan from soybean) and then using one of several processes (i.e., hydrolysis, transglycosylation, isomerization, extraction) to isolate the actual prebiotic.

How Do Prebiotics Modify the Composition of the Microbiota?

The effect of a prebiotic (or potential prebiotic) on bacterial growth depends on the type of prebiotic (or potential prebiotic) ingested. In a study on the effect of resistant starch on fecal microbiota in 10 healthy human volunteers, Martinez et al. (2010) observed significant changes in the relative proportions of various bacterial taxa depending on the type of resistant starch ingested. Researchers fed the volunteers three types of crackers in a 17-week double-blind crossover study: RS2 (crackers made with Hi-Maize 260, a resistant starch 2), RS4 (crackers made with a chemically modified, phosphorylated, cross-linked type 4 resistant starch, Fibersym RW), and native wheat starch (the control). All subjects consumed 33 grams of resistant starch per day. Consumption of RS4 increased the proportion of phylum Firmicutes and decreased the proportions of Bacteroidetes and Actinobacteria relative to the control and, in the case of Firmicutes and Actinobacteria, relative to the consumption of resistant starch 2 as well. At the family level, researchers observed increased proportions of Bifidobacteriaceae and Porphyromonadaceae in the RS4 treatment and decreased proportions of Ruminococcaceae and Erysipelotrichaceae relative to the control. At the genus level, they observed increased proportions of Parabacteroides and Bifidobacterium and decreased proportions of Faecalibacterium and Dorea in the RS4 treatment compared to the control.

As another example of the variable effects of different types of prebiotics, in a randomized, double-blind, placebo-controlled, crossover study of 20 healthy men between 21 and 28 years of age, Hooda et al. (2012) observed significant differences in the proportion of bacterial genera detected in feces depending on which of three types of fiber were consumed. Researchers fed the volunteers three fiber bars per day, with each bar containing either no supplemental fiber or 7 grams of either polydextrose (PDX) or soluble corn fiber (SCF). They observed large and significant increases in the



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