By Fitnescity writer in residence, Julita Baker, PHD

The Science of Food Choice

In our last post, we discussed the first type of food that has been shown to provide benefits to our gut microbiology- Probiotics. What else, maybe more prominently, modulates our gut microbiome? Fiber.

 Today we will continue with the next source of food that influences this part of our health, Prebiotics.

Microbe-promoting Prebiotics 

The underlying component within plants, grains, nuts, and seeds, that flourishes (as opposed to contains) healthy strains of microbes, is fiber.

Within this category we can get even more specific, eyeing in on Prebiotics and Resistant Starches as microbiome-benefiting (remember, Probiotics already have live strains of bacteria in them, Prebiotics and resistant starches, don’t).

Let’s look at some examples of Probiotics and Prebiotics:

What are Prebiotics, and how do they benefit our gut health?

Unlike probiotics, which contain live strains of bacteria and act more as transient visitors (4), prebiotics simply promote growth of beneficial bacteria in our digestive system.  

 The term 'prebiotic' was first defined in 1995 by Gibson and Roberfroid as 'a non-digestible food ingredient that selectively induces growth and/or activity of one or a limited number of bacteria in the colon, thereby improving host health' (1). 

Three criteria fulfill the definition of a prebiotic (1):

1. It is non-digestible, resistant to gastric acid and enzymatic hydrolysis, and is resistant to gastrointestinal absorption.

2. It is fermentable.

3. It selectively stimulates growth and/or metabolic activity of intestinal bacteria that are associated with health and well-being.

As we can see in the above image, onions, garlic, leek, asparagus, chicory root, Jerusalem artichoke, and others belong to the prebiotic category. These plants contain compounds called inulin-type fructans, which our microbes absolutely love. Fructans pass through the upper gastrointestinal tract and cannot be fully broken down by our own digestive enzymes. Because of this, these compounds are able to reach the large intestine (colon), where they are fermented by our gut bacteria and are converted into bacterial biomass and organic acids like lactic acid and short-chain fatty acids (interesting tidbit: about 50% of our poop is actually bacteria, not the food itself!). These short-chain fatty acids (otherwise known as SCFA), acetate, butyrate, and propionate have many beneficial metabolic functions to the host (us).  For example, SCFA can regulate our immune system and inflammation (5, 6), regulate our appetite (7), influence our brain physiology and behavior (8), regulate blood pressure (9), protect against metabolic disease (10), and many other health functions. 

Other forms of Prebiotics 

The above list of foods is not the only source of prebiotics, however. Most plant material is in fact a form of prebiotic, as well as other non-carbohydrate dietary components (see image below).  For example, polyunsaturated fatty acids, which are found in nuts, seeds, and fish, can pass through to the colon and promote growth of beneficial bacteria. Phytochemicals and phenolics, which are found in beans, vegetables, herbs and spices, fruit, olives and olive oil, cocoa, tea, coffee, are also significant sources of fermentable substances for our gut microbes. 

The polyphenols found in many families of these plants are specific, such as flavanones in citrus fruit, isoflavones in soy, phloridzin in apples; others, such as quercetin, are found in all plant products such as fruit, vegetables, cereals, legumes, tea, and wine. 

What does the research say about Prebiotics?

In a recent study published in Nature, it was shown that the Mediterranean diet high in prebiotics (fruit, vegetables, whole grains, legumes, fish, nuts, seeds, higher in monounsaturated fat and lower in red meat and saturated fat) has a significant impact on the gut microbiome, producing the SCFA discussed above (2). 

Researchers in this 2021 study saw an inverse association between the Mediterranean diet and several species of bacteria (Clostridia and others) that are typically associated with negative health effects seen with a western-style diet and red meat intake. 

Among the specific food components that were the biggest drivers of differences seen in the positive microbiome changes, vegetables, fruit, and whole grains were the most positive, and red meat/processed meat were the most negative. 

This means: 

Vegetables, fruit, and whole grains = Good bacteria 

Red meat/processed meat = Bad bacteria 

And shows us that specific microbial population growth with plant intake is in the opposite direction as those in red meat/processed meat consumption. 

In this same study, interestingly, no association was seen with Mediterranean diet adherence and increased diversity of the microbiome.  This tells us that specific bacteria may flourish, but overall diversity may not be influenced (remember, diversity typically means better health). 

With these many health outcomes, it is maybe unsurprising that the prebiotic market is expected to reach $7.5 billion by 2023 (3).

Improving your gut health can keep you healthier and also help you improve your overall wellness, energy levels and improve your body composition. A great way to monitor those changes as you start to adjust your meal plan and exercise routine is to get a body composition test as a benchmark and follow-up with additional scans at regular intervals. Not only will it keep you accountable and motivated, but comparing across tests will show you changes that measure much deeper than anything a scale can see.

Next post- Resistant Starch 

So we now know what the second category of foods for our gut microbes is: prebiotics. Coming from plant material high in fiber, they serve our health through the flourishing of specific strains of microbes, microbe fermentation of these foods, and their production of SCFA byproducts. In our next post, we will talk about the last health-benefiting microbe-promoting food source- Resistant Starches.  

References: 

1. Gibson GR, Roberfroid MB. Dietary modulation of the human colonic microbiota - introducing the concept of prebiotics. J Nutr 1995; 125: 1401-1412.

2. Wang, D.D., Nguyen, L.H., Li, Y. et al. The gut microbiome modulates the protective association between a Mediterranean diet and cardiometabolic disease risk. Nat Med 27, 333–343 (2021). https://doi.org/10.1038/s41591-020-01223-3

3. Global Market Insights. Prebiotics market size by ingredient (inulin, GOS, FOS, MOS), by application (animal feed, food & beverages [dairy, cereals, baked goods, fermented meat, dry foods], dietary supplements [food, nutrition, infant formulations]), industry analysis report, regional outlook, application potential, price trends, competitive market share & forecast, 2017 – 2024 Available from: https://www.gminsights.com/industry-analysis/prebiotics-market.

4. https://www.amazon.com/Good-Gut-Taking-Control-Long-term-ebook/dp/B00OZ0TOV2

5. Sathish Sivaprakasam, Puttur D. Prasad, Nagendra Singh. Benefits of short-chain fatty acids and their receptors in inflammation and carcinogenesis. Pharmacology & Therapeutics, Volume 164, 2016

6. Gill, PA, van Zelm, MC, Muir, JG, Gibson, PR. Review article: short chain fatty acids as potential therapeutic agents in human gastrointestinal and inflammatory disorders. Aliment Pharmacol Ther. 2018; 48: 15– 34. https://doi.org/10.1111/apt.14689

7. Byrne, C., Chambers, E., Morrison, D. et al. The role of short chain fatty acids in appetite regulation and energy homeostasis. Int J Obes 39, 1331–1338 (2015). https://doi.org/10.1038/ijo.2015.84

8. Silva Ygor Parladore, Bernardi Andressa, Frozza Rudimar Luiz. The Role of Short-Chain Fatty Acids From Gut Microbiota in Gut-Brain Communication. Frontiers in Endocrinology, 2020. 

9. Pluznick JL. Microbial Short-Chain Fatty Acids and Blood Pressure Regulation. Curr Hypertens Rep. 2017 Apr;19(4):25. doi: 10.1007/s11906-017-0722-5. PMID: 28315048; PMCID: PMC5584783.

10. Sanna, S., van Zuydam, N.R., Mahajan, A. et al. Causal relationships among the gut microbiome, short-chain fatty acids and metabolic diseases. Nat Genet 51, 600–605 (2019). https://doi.org/10.1038/s41588-019-0350-x