for men.12 Human studies show variable effects depending on dietary concentration.

In addition to the type of prebiotic or potential prebiotic ingested and dose, a multitude of other factors influence the way a prebiotic impacts the GI microbiota, including gastric emptying time, intestinal transit time, nutrient digestibility, fecal bulk and frequency of defecation, short-chain fatty acid (SCFA) production, intestinal morphology, gut immune modulation, and the GI microbiota itself.

Are Prebiotics Effective in Achieving Host Health Benefits?

Much of the research on prebiotics is in healthy individuals. Many studies have shown significant increases in the so-called beneficial microbes (i.e., bifidobacteria and lactobacilli) following consumption of GalOS, inulin, and other prebiotics—in healthy individuals. Researchers have also reported increases in butyrate producers (e.g., Eubacterium, Faecalibacterium, Roseburia) following consumption of resistant starch, polydextrose, soluble corn fiber, and other prebiotics—again, in healthy individuals. Fahey urged more studies on diseased populations, given that many microbes are associated with disease. For example, inflammatory bowel disease conditions are known to be associated with a decreased proportion of Faecalibacterium. Studies have shown that SCF, PDX, inulin, fructooligosaccharides, pea fiber, and other prebiotics or potential prebiotics can impact Faecalibacterium. The question remains, Do those same prebiotics or potential prebiotics alleviate inflammatory bowel disease conditions via their impact on Faecalibacterium?

Fahey also urged more consideration of how prebiotics impact microbial metabolites, especially butyrate and other SFCAs. Studies have shown that inulin, fructooligosaccharides, and GalOS can increase SFCA levels, but what impact do prebiotics have on the toxic end products of fermentation such as ammonia, phenols, and indoles? Hooda and colleagues (2012) measured some of those toxic end products as part of a larger microbe-health index principal component analysis and found that Lachnospiraceae, Lactobacillaceae, and Veillonellaceae were all negatively correlated with ammonia, phenols, and indoles. Additionally, Lachnospiraceae and Lactobacillaceae were positively correlated with total SCFA, and Veillonellaceae was positively correlated with fiber intake. Veillonellaceae was also negatively correlated with total branched-chain fatty acids (BCFAs).

As an example of a prebiotic study that both used a disease model and


12 The DRI for total fiber (combination of dietary fiber and functional fiber) for men is 31 grams per day for the 9- to 13-year age group, 38 grams per day for the 14- to 50-year age group, and 30 grams per day for men aged 51 and older (IOM, 2002).

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