Prebiotics and Synbiotics: Dietary Strategies for Improving Gut Health

Janina A. Krumbeck; Maria X. Maldonado-Gomez; Amanda E. Ramer-Tait; Robert W. Hutkins

Disclosures

Curr Opin Gastroenterol. 2016;32(2):110-119. 

In This Article

Prebiotics in 2016: Emerging Concepts

Despite the substantial industrial and clinical interest in prebiotics, the commercial market has been dominated by only a handful of prebiotics, mainly inulin, fructooligosaccharides, and galactooligosaccharides (GOS).[9,10] Several isomaltooligosaccharides products are also commercially available.[11] In the United States, European Union, and Pacific Rim, these prebiotics are added as functional ingredients in a wide variety of processed foods and beverages. Infant formula products, in particular, are often supplemented with GOS or fructooligosaccharides because of their ability to mimic a bifidogenic response similar to that which occurs with human milk oligosaccharides in breast-fed infants.[12,13]

Also documented to have prebiotic activity are resistant starches, starches that are resistant or slowly resistant to digestion and reach the colon intact. Depending on the type of resistant starches, studies have shown they enhance growth of bifidobacteria, as well as Eubacterium rectale, Ruminococcus bromii, and lactobacilli.[5,14–17] Some of these changes in the microbiota are correlated with glycemic improvements[18] and high butyrate production.[19] The latter could be favorable for the prevention of colon cancer and inflammation.[20,21] In general, these commercial prebiotics consist of mostly linear oligosaccharides or polysaccharides that contain only one or two monomeric sugars (Table 1).[22]

There is also considerable evidence showing that several dietary fibers have prebiotic activity.[23] In contrast to the rather simple, linear composition of commercial prebiotics, the carbohydrates in plants as well as those present in human milk (Table 1) are diverse and structurally complex, with many containing functional groups.[13,24,25] In the gut, they require participation of a more extensive and diverse array of hydrolytic enzymes to degrade these molecules into fermentable substrates.[24,26] Accordingly, there may be different host-dependent responses, as the specific form or type of dietary fiber consumed by an individual may differentially affect the response of the microbiota.[26,27]

Prebiotic fibers are often natural constituents of a variety of foods, especially whole grains, fruits, root and other vegetables, and legumes. Although some foods contain appreciable concentrations of these prebiotics,[28] in most western diets, consumption of these fiber-rich foods is probably too low to contribute much fermentable fiber to the colon. However, for individuals who consume whole grain products and fiber-rich diets, significant effects on the microbiota have been observed, with shifts in the abundance of specific taxa and increased microbial diversity.[18,23,29] In contrast, other studies have shown that whole grain consumption does not always induce changes in gut microbiota[30] or consistently affect clinical end points.[31]

Based on these observations, several researchers have suggested that prebiotics are best defined based on their physiological effects or functional capacities rather than the specific microbial targets affected.[32,33] So-called second generation prebiotics were envisioned as providing specific functional benefits. According to this argument, dietary fibers may have prebiotic activity by causing broad changes in community structure, but without necessarily influencing abundances of bifidobacteria or lactobacilli.[34]

It is worth noting that the convergence of 'prebiotics' and 'fiber' has led to the development of a new lexicon in the prebiotic community.[35] Indeed, 'low-digestible'[36] and 'nondigestible carbohydrates',[37] 'prebiotic fiber',[34] 'functional fiber', and 'fermentable fiber'[38] are among the terms used to describe the food carbohydrates that have microbiome-influencing properties. Neither is there consensus on definitions of prebiotics nor the specific types of fiber.[38,39] Recently, Sonnenburg and Sonnenburg[40] introduced the term 'microbiota-accessible carbohydrate' (MAC) to describe fibers, as well as host-secreted mucin and microbial-produced saccharides, that are available as substrates for the gut community. The absence of these fibers in the colon (as a result of low-fiber diets) may result in gut microbes looking elsewhere for sugars, namely, the mucin layer that protects the host.

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