Modulating gut microbes with fiber, microbiota-accessible carbohydrates and prebiotics: an update of definitions and health benefits

The closely related terms “fiber”, “microbiota-accessible carbohydrates” (or MACs) and “prebiotics” probably sound familiar. But what exactly are they and how can they contribute to promoting health and abrogating disease by shaping the gut microbiome?

Fiber can be utilized by gut microbes, but this is not a requirement

We have known for a while that a diet of fiber-rich foods reduces the risk of developing chronic diseases and lowers people’s mortality rate. However, scientists have only recently started elucidating why fiber is beneficial for human health, helping to keep the gut microbiome and the immune system, mainly located in the small intestine, in good shape, as scientific journalist Carl Zimmer explains in The New York Times.

Fiber is a very broad term and definitions are flexible depending on the country, but one description of dietary fiber includes “the carbohydrates that are fermentable by an individual’s microbiota and those that remain unfermented and serve a bulking role.” Similar to dietary fiber, the term plant polysaccharides as a synonym of “microbiota food” is also inaccurate as it excludes non-plant molecules, such as human milk oligosaccharides, which can shape the gut microbiome.

Traditionally, fiber has been classified according to its physicochemical characteristics, one of which is solubility. For example, insoluble fibers such as cellulose, wheat bran and lignin (found in whole grains and vegetables, among others) are indicated for those who want to resolve episodes of constipation and, in turn, excess intestinal gas. Meanwhile, soluble fibers such as pectins and psyllium (found in carrots, apples and oats, among others) regulate the rhythm of bowel movements and are indicated for people who alternate episodes of constipation with episodes of diarrhea.

However, solubility alone does not predict the functional properties of fiber for the gut microbiome. And the reality is that foods are often a complex mix of soluble and insoluble fibers, while cooking also affects the availability of dietary fiber for gut microbes.

Microbiota-accessible carbohydrates: a novel term, included in the definition of fiber, for carbohydrates that must be metabolized by the microbiota

As each type of fiber affects gut-dwelling microorganisms differently, scientists have coined the term microbiota-accessible carbohydrates (MACs) for one type of fiber, representing “carbohydrates that can be metabolically used by gut microbes”.

MACs include dietary carbohydrates—from plants or animal tissue (e.g. animal cartilage) or food-borne microbes—resistant to digestion and absorption by the host. But they also may be secreted in the intestine by the host (e.g., mucus) or produced by microbes.

One challenge in defining MACs is the lack of nutrition databases that quantify the amount of dietary MACs present in a food source. For other types of carbohydrates, such as resistant starch, which can be fermented by gut microbes, databases document their content in commonly consumed foods. Databases including those food sources are needed to formulate dietary interventions with targeted effects on microbes and examine related health outcomes. This is a complex issue, however, as most of the 26,000 distinct nutritional components in foods have yet to be characterized or quantified.

In addition, gut microbes’ use of MACs in a food source will depend on the make-up of individual gut microbial communities. The individuality of which carbohydrates qualify as MACs is illustrated by the presence of specific genes for digesting the nori alga used in sushi in the gut microbiome of Japanese individuals. These genes are rarely found in North American and European individuals, given that they transferred from a marine environmental bacterium to the Japanese gut bacteria. Simply put, a substrate can be a MAC for one person but not for another, due to the lack of microbes for fermenting it.

Although at first glance MACs seem the same as prebiotics, not all MACs meet the criteria stated in the definition of a prebiotic as having science-backed health effects.

Selective utilization by resident microbes and proven health benefits: key features that differentiate prebiotics from dietary fiber and MACs

The most recent definition of prebiotics was updated in 2017 by experts in microbiology, nutrition and clinical research, who were convened by the International Scientific Association for Probiotics and Prebiotics. It states that they are “substrates that are selectively utilized by host microorganisms conferring a health benefit”.

While fiber can exhibit some gut microbiome modulation properties and has specified Adequate Intake values, prebiotics must be selectively utilized by resident microbes and do not have Adequate Intake or Daily Value levels. Although a daily value of 28 grams/day based on 2000 Kcal/diet has been set for dietary fiber, the good news is that as little as < 5g/day of the right fiber has been shown to be enough to promote the growth of beneficial Bifidobacterium spp.

Dietary carbohydrates including inulin-type fructans (i.e., inulin, oligofructose and fructo-oligosaccharides) and b-galacto-oligosaccharides are the most widely researched prebiotics.

Emerging prebiotics also subject to research include human milk oligosaccharides, polyunsaturated fatty acids and polyphenols, provided they are selectively utilized by the host microbiota and promote health.

Interestingly, dietary sources of insoluble fibers such as resistant starch and grape by-product from winemaking are being explored as novel prebiotics. This highlights that when it comes to effects on the gut microbiome, the definition of fiber as fermentable or non-fermentable makes more sense, compared to classifying fiber as soluble (i.e., swelling via water absorption) and insoluble (i.e., bulking).

Prebiotics have been mainly investigated in gastrointestinal disorders—mainly at low doses for patients with irritable bowel syndrome, while their role in inflammatory bowel disease is controversial—and as a means of immune modulation. For instance, some prebiotics can help improve the effectiveness of seasonal influenza vaccinations in older adults. Emerging data also suggest prebiotics as a novel strategy for ameliorating age-related neuroinflammatory pathologies and brain function.

On the whole, dietary fiber, MACs and prebiotics are science-based tools for ensuring the gut microbiome is in good shape. First, fiber includes MACs and prebiotics. Secondly, while both MACs and prebiotics are utilized by gut microbes, not all MACs are considered prebiotics, with the term used for substrates selectively used by gut microbes that have evidence of a health benefit. Definitions aside, what is clear for scientists is that eating a wealth of fibers is good advice when trying to improve everyone’s gut microbiome, with personalization encouraged as needed and especially in the context of intestinal disorders.

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