Fiber and your gut – The microbial connection
Gut health: By now, we know that fiber is the food our gut bacteria eat in order to grow and thrive. But why do we need or even have microbes in our guts? Before we delve deeply into the answer, let’s start by putting things in proper “biological” and “evolutionary” context.
Good bug, bad bug
Throughout most of our history we’ve been waging war with microbes. Small pox, yellow fever, malaria, tuberculosis, and cholera are just a few of the important examples of bugs that have been causative agents of change throughout human evolution. The field of microbiology, which led to important discoveries such as Louis Pasteur’s “germ theory of disease,” grew out of society’s view that microbes are disease-causing germs and need to be eradicated.
But today, the metaphor for war (“the only good bug is a dead bug”) is no longer appropriate; instead, humans and microbes have co-evolved that mutually benefit both the host (i.e., us) and our resident microbes. Most of the microbes we come in contact with are not germs, but beneficial bacteria that provide us with many health benefits. For example, our gut is home to a large number of microbes that help us ferment the undigested polysaccharides (i.e., fiber) that we consume on a daily basis. The mutualistic relationship between our gut flora and us has evolved over millennia and is helping us in ways that we are just beginning to understand.
Who am I? The other me
Have you ever stopped to ask what we are really made of? Biologically speaking, the answer may be too obvious. As individuals, we came about by the fertilization of a human egg, which contains genes from both mother and father. Simple enough, right? But does this really paint a complete “biological” image of who we really are? Or is there more to us than meets the eye?
A growing number of scientists (myself included) think that this picture is far from perfect. Over the past 10 years, we’ve learned that the human body is not a self-sufficient island. It is more like a complex ecosystem – a social network if you will – containing trillions of bacteria and other microorganisms that inhabit our skin, genital areas, mouth and especially intestines.
A healthy adult human is thought to harbor about 100 trillion bacteria in his or her gut alone,1-3 outnumbering human cells 10 to one (check out the infographics). Bacteria living in the human gut (or microbiota as it is sometimes referred to) achieve the highest cell densities recorded for any ecosystem.4 So, one could argue that we’re 90% bug and only 10% human, thus begging the question: “Are we really human after all?” But, that’s a whole other philosophical topic. Regardless of how we look at things, one thing is clear: It seems like the human fertilized egg is one (small) part of this intricate biosystem.
Bacteria living in and on us outnumber us by 10 to 1,
proof that we do live in a bacterial world
The 10:1 ratio is going by number of cells only. Genetically speaking, our gut flora are much more diverse and adaptable than us2 (see infographic). No one will dispute the fact that our development is much more elaborate and complex than a bug’s, but here’s what the numbers tell us; our cells harbor some 23,000 genes, whereas our microbiome (e.g., the sum of all the genes coming from the bacterial population within our gut), considered as our “second” genome, brings in excess of 3 million genes to this mix: 150 times more than the number of genes in the human genome!1-3 Saying it another way, there are 150 times more bug genes than human genes in each and every one of us.3
This genetic diversity is possible because of evolutionary selection. Basically, the fastest way for an organism to grow and adapt to its environment is to acquire the necessary genes from another organism. Microbes rapidly adapt to their environment by “borrowing” genes from other microbes via a process called horizontal gene transfer.5 In fact, bacteria share as much as 30% of their genome with other bacteria in their microenvironment.6-12 In contrast to bugs, humans are not endowed with such capability; instead, we form symbiotic associations with the microbes that can do this, a trait that has been taking shape for the past 500 million years13 and probably acted as the underlying evolutionary force toward the establishment of bacteria as our partners in both health and disease. This, indirectly allows for the rapid adaptive extension of our capabilities.14
Our microbiome is genetically much more diverse
and adaptable than we are
So, let’s recap:
- You’re more bug than human. The human body carries about 100 trillion microorganisms in its gut, which is 10 times greater than the total number of human cells in the body.
- Genetically, the gut microbiome contains 150 times the number of genes in our genome. This diversity endows us with the functional features that we have not had to evolve ourselves.
Now, consider this:
You have 100 trillion “good” bacteria in your gut (as many as 1,000 different species), all in a state of “happy” equilibrium. Take good care of it, and you ensure living a healthy life. Start messing with it and you may just have triggered the mobilization of 100 trillion “bug” troops, genetically programmed to survive. Now, you have a full-scale war on your hands. Guess who’s going to come ahead of that fight?
…back to “why we have bugs in our gut”?
The large and dynamic bacterial community in our guts extends our biological and nutritional capabilities in many important ways and plays a major role in our overall well-being. So much so that our gut microflora is sometimes referred to as our ‘‘forgotten organ’’.15 Functionally, one of the benefits that humans have gained from the acquisition of a microbial partner in our gut is the ability to ferment a greater range of plant polysaccharides (e.g., fiber) that would otherwise pass through our intestines partially or completely undigested.2, 6-10 These undigested products are the foods that our bacterial partners need in order to survive, grow and sustain our health.
Many of these beneficial effects are mediated by the short chain fatty acids (SCFAs) such as acetate, propionate, and butyrate, which are produced during fermentation of the undigested fiber (such as inulin, resistant starch and Arabica gum)16-19 in the colon.
Now, I think you are starting to get the picture as to why “gut bacteria” is so vital to a healthy life. Like us, our gut bacteria need good nourishment to flourish and keep pathogenic bugs in check. Fiber, which is essentially food for bugs, is the nourishment that will keep the good bacteria happy and growing, providing you with the beneficial side products your body is not capable of making such as short chain fatty acids (SCFA), vitamins K and B, etc.
Our gut microflora extends our biological and nutritional
capabilities in many important ways and plays a major
role in our health and disease.
Stay tuned, as we will be exploring the many facets of our gut flora and its effects on health.
- Backhed, F., Ley, R.E., Sonnenburg, J.L., Peterson, D.A. & Gordon, J.I. Host-bacterial mutualism in the human intestine. Science 307, 1915-1920 (2005).
- Gill, S.R., Pop, M., Deboy, R.T. et al. Metagenomic analysis of the human distal gut microbiome. Science 312, 1355-1359 (2006).
- Qin, J., Li, R., Raes, J. et al. A human gut microbial gene catalogue established by metagenomic sequencing. Nature 464, 59-65 (2010).
- Whitman, W.B., Coleman, D.C. & Wiebe, W.J. Prokaryotes: the unseen majority. Proceedings of the National Academy of Sciences of the United States of America 95, 6578-6583 (1998).
- Liu, L., Chen, X., Skogerbo, G. et al. The human microbiome: a hot spot of microbial horizontal gene transfer. Genomics 100, 265-270 (2012).
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- Garrido, D., Barile, D. & Mills, D.A. A molecular basis for bifidobacterial enrichment in the infant gastrointestinal tract. Advances in nutrition 3, 415S-421S (2012).
- Gibson, G.R., Macfarlane, G.T. & Cummings, J.H. Sulphate reducing bacteria and hydrogen metabolism in the human large intestine. Gut 34, 437-439 (1993).
- Roberfroid, M.B., Van Loo, J.A. & Gibson, G.R. The bifidogenic nature of chicory inulin and its hydrolysis products. The Journal of nutrition 128, 11-19 (1998).
- Sela, D.A., Li, Y., Lerno, L. et al. An infant-associated bacterial commensal utilizes breast milk sialyloligosaccharides. The Journal of biological chemistry 286, 11909-11918 (2011).
- Faith, J.J., McNulty, N.P., Rey, F.E. & Gordon, J.I. Predicting a human gut microbiota’s response to diet in gnotobiotic mice. Science 333, 101-104 (2011).
- Turnbaugh, P.J., Ridaura, V.K., Faith, J.J. et al. The effect of diet on the human gut microbiome: a metagenomic analysis in humanized gnotobiotic mice. Science translational medicine 1, 6ra14 (2009).
- Ley, R.E., Lozupone, C.A., Hamady, M., Knight, R. & Gordon, J.I. Worlds within worlds: evolution of the vertebrate gut microbiota. Nature reviews. Microbiology 6, 776-788 (2008).
- Rosenberg, E., Sharon, G., Atad, I. & Zilber-Rosenberg, I. The evolution of animals and plants via symbiosis with microorganisms. Environmental microbiology reports 2, 500-506 (2010).
- O’Hara, A.M. & Shanahan, F. The gut flora as a forgotten organ. EMBO reports 7, 688-693 (2006).
- Calame, W., Weseler, A.R., Viebke, C., Flynn, C. & Siemensma, A.D. Gum arabic establishes prebiotic functionality in healthy human volunteers in a dose-dependent manner. The British journal of nutrition 100, 1269-1275 (2008).
- Conlon, M.A. et al. Resistant starches protect against colonic DNA damage and alter microbiota and gene expression in rats fed a Western diet. The Journal of nutrition 142, 832-840 (2012).
- McOrist, A.L. et al. Fecal butyrate levels vary widely among individuals but are usually increased by a diet high in resistant starch. The Journal of nutrition 141, 883-889 (2011).
- Souza da Silva, C., Bolhuis, J.E., Gerrits, W.J., Kemp, B. & van den Borne, J.J. Effects of dietary fibers with different fermentation characteristics on feeding motivation in adult female pigs. Physiology & behavior 110-111, 148-157 (2013)