Core Concept: Prebiotics gain prominence but remain poorly defined.

With the gut microbiome increasingly recognized as a major player in shaping human biology, probiotic treatments—introducing a few billion purportedly beneficial micro-organisms into human gut communities composed of trillions of microbes—are under intense investigation. Study designs and results have been a mixed bag, and the impact of probiotics remains unclear. Enter the probiotics’ conceptual cousin, prebiotics. Although probiotics consist of live microbes, prebiotics are microbe food: substances metabolized by microbes that are not digestible by the host. As researchers gain a more complete picture of the forces that shape microbial gut diversity, some think prebiotics could confer health benefits that probiotics have struggled to demonstrate, in part because prebiotics can nourish multiple microbial species already in place, triggering broader and potentially more robust changes in the host’s microbiome and health. Prebiotics are generally nondigestible carbohydrates that get fermented bymicrobes in the gut; they are found in foods that are high in fiber, although not all fibers have prebiotic properties. Despite being a couple of decades behind probiotics in research, prebiotics are potentially key ingredients in the evolving stew of gut influencers. The goal of manipulating whole microbial populations through selective feeding raises practical challenges; as health products, both probiotics and prebiotics exist somewhere in the poorly regulated space between food and drugs. Defining what is and is not a prebiotic will have commercial consequences. Scientifically, too, an ongoing debate over the meaning of the term “prebiotics” reflects diverging views of how they may actually confer benefits. Even the duo that coined the term 20 years ago may not see eye to eye.