The famous analogy for probiotics and prebiotics has to do with growing a patch of grass. For the lushest, greenest grass you can’t just throw some seeds onto dirt; you need fertilizer to create the best conditions for those seeds to grow. Similarly, probiotics (like the seeds) and prebiotics (like the fertilizer) work together to create the best conditions in the gut for supporting health.

Prebiotics are popularly known as a special class of dietary fibers that boost populations of beneficial bacteria in the gut. The concept of prebiotics has existed for over 20 years, since two scientists introduced it in a 1995 paper, but surprisingly there has never been a widely-accepted scientific definition of the term.

Early attempts to define prebiotics specified the compound in question had to increase intestinal populations of health-promoting groups of bacteria—basically, species of Bifidobacterium and Lactobacillus. By applying this line of reasoning to the scientific studies, three prebiotic compounds became well-known: inulin (a polysaccharide naturally occurring in many plants), fructo-oligosaccharides (FOS; oligosaccharides found in foods or produced industrially), and galacto-oligosaccharides (GOS; a mixture of substances produced from lactose).

Scientists who have proposed more recent definitions, however, have emphasized the value of focusing on the health-promoting activities of microbes, rather than merely their names. That’s because, as knowledge of the human microbiome expands, it’s clear that many different microbes—not just species of bifidobacteria and lactobacilli—can have positive effects on health.

This year, a panel of 12 scientific experts from around the world came together to publish their consensus on the definition of a prebiotic. They kept the new definition simple: “a substrate that is selectively utilized by host microorganisms conferring a health benefit.” That is to say, the new scientific definition of a prebiotic is any compound that improves health through the way it’s metabolized by microorganisms. As shown by scientific data, of course.

The new definition will let many other compounds into the FOS, GOS, and inulin club—provided, of course, that there’s adequate research showing they provide a health benefit through the metabolic activities of microbes in or on the body.

In fact, the doors to the category of prebiotics are thrown wide open with this expanded definition: not only does it allow bacteria besides bifidobacteria and lactobacilli to be implicated in health benefits, but it also allows prebiotics to be expanded to compounds that act outside the gut (for example, to things that improve oral or skin health) and even to products outside the category of food (for example, some drugs). The consensus is a big step forward for the field—and will ultimately provide more clarity for consumers looking to improve their gut health and their overall health.



Gibson GR, Hutkins R, Sanders ME, et al. Expert consensus document: The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of prebiotics. Nature Reviews Gastroenterology & Hepatology. 2017; 14: 491–502. doi:10.1038/nrgastro.2017.75