That time one study completely changed my mind about gut microbes

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That time one study completely changed my mind about gut microbes


That time one study completely changed my mind about gut microbes


Have you ever seen a new study that goes against what you know; and think “No way, this is BS!”?

I hope so!

Skepticism of new information is very important. Just about all of the headlines that appear on the news, in the media, and online talk about new studies.

And it’s very rare that a new study whacks all of the previous knowledge on its head.

Very rare.

Today, I’m going to talk about one of those very few times that a study actually changed my mind about something. And I know a thing or two about health science.

It’s OK to change your mind. But not for just any new headline, story, or idea. It’s OK to change your mind when solid information comes by.

Here’s my story about a time when one study completely changed my mind about gut microbes.


What I know (knew) about gut microbes and probiotics


Like you, I’m not a gastroenterologist, nor do I have a PhD in microbiology. But, as a science- and holistic-trained nutritionist, I know a bit about the gut microbiome. In fact, over and above the little I learned in school about it, I’ve personally researched and written a few health articles on the topic:

Here are a few things I know about the gut microbiome:

  • The gut microbiome is not just made up of 100s of types of bacteria, but other microbes like viruses & yeasts too.
  • Probiotics, on the other hand, are friendly microbes that you can eat, drink or supplement with. They don’t hang out in your gut for long (they pass right on through within a couple of days).
  • Gut microbes and probiotics love to eat fibre, known as “prebiotics.” (scientists are just so creative with names, no?)
  • Antibiotics can cause clostridium difficile (C. diff) diarrhea. So, I always recommend Bio-K+ probiotics (it’s one of the few that have actual clinical studies that prove prevention of antibiotic-associated diarrhea).
  • The gut microbiome is vast and diverse, and still mostly a mystery. But, new technology has made it a hotbed of research now.(1, 2) Both for resident gut microbes, as well as probiotics.
  • We haven’t even defined what a “healthy” gut microbiome is yet.
  • Gut microbes make vitamins B & K, and some fatty acids. They can help with digestion and absorption of nutrients because of their many actions in the gut. We are learning a lot more as research continues.
  • There are new terms popping up in the scientific literature:
      • Microbiome-gut-brain-axis;

    (this is really common now)

    • Liver-gut microbiota axis; (3)
    • Microbial-mammalian metabolic axis; (4)
    • Gut microbiota-bone axis; (5)
    • Immunity-diet-microbiota axis; (6)
    • Not to mention the number of gut axes that don’t even include the word “microbiome!” (like the axes between the gut and the kidneys, lungs, pancreas, etc.)
  • That there are 10x more “gut bugs” in me than “me” cells in me.


Maybe . . .


What this new study said, and why it changed my mind (but most studies don’t)


Last year I came across this study: “Revised Estimates for the Number of Human and Bacteria Cells in the Body.”(7)

That really piqued my interest because everyone says that we have 10x more gut bugs than human cells.

Everyone. Seriously, everyone. Every. Single. Person. Regardless of how “science-based” they are. E-V-E-R-Y-O-N-E.

But, where did that number originally come from? And, is it even correct?

Well, this new study clearly explains where the number came from, and sought out to confirm it.

Here’s what they did in 2016:

“The aim of this study is to critically revisit former estimates for the number of human and bacterial cells in the human body.”

This means the researchers tried to accurately count the number of human cells and gut microbes.

Here’s what they concluded:

…”Our analysis also updates the widely-cited 10:1 ratio, showing that the number of bacteria in the body is actually of the same order as the number of human cells”…

They found that the actual number was about the same as the number of human cells.

Not 10x more; the same!

So, they’re saying this well-known, oft-quoted 10:1 ratio, is actually closer to 1:1!

Woah! Wait a second. Is this BS, or real?

So, of course, I read the study. #nerdalert

In the first paragraph of their introduction they say:

…”the ubiquitous statements regarding 1014–1015 bacteria residing in our body trace back to an old back-of-the-envelope calculation.”

Did you catch that? This number of gut microbes was an estimate calculated on a piece of paper! And it was published. Presumably because it seemed legit at the time by the researchers and peer-reviewers. And there was no other evidence to support or deny it. It was the first estimate. And once it’s peer-reviewed and published, it’s out for the whole world of health researchers to see (and criticize, and try to replicate).

Yes, this number seemed legit at the time. Back in 1977!(8)

Does this mean we shouldn’t believe health studies because scientists don’t know what they’re talking about?

Absolutely not!

What it means is summed up in a famous quote by Maya Angelou:

I did then what I knew how to do. Now that I know better, I do better

Keep doing research. Keep criticizing research. Keep trying to learn, prove, and disprove hypotheses until there is enough evidence to call it a “theory.” Otherwise known as a FACT.

Keep doing research. Keep criticizing research. Keep trying to learn, prove, and disprove. #science Share on X

In fact, they weren’t the first to criticize this number. Back in 2014(9):

“Science rights itself by critical reanalysis of facts. Most scientists are hypercritical and very resistant to new claims in the absence of unscrutinized data. But once in a while some fake “fact” gets into the literature that is difficult to expunge. A recent case in point: the frequent assertion that the number of cells in the human microbiota is ten times as numerous as the number of cells in the human body”

This discussion should make clear that the assertion of 10-fold more microbiota than human body cells was not based on hard facts. Scientists may have repeatedly cited this assertion because of laziness. Alternatively, scientists may have suspended criticality because this ratio dramatically conveys the idea of the human as ecosystem, and suggests important physiological roles for human microbiota. Even if these ideas are valid, we should be clear that the 10-fold ratio was crudely estimated and what should be cited is a range of values with appropriate qualifiers and primary references. The new American Academy of Microbiology report, “FAQ: Human Microbiome,” partly accomplishes this.”

So, how did the scientists re-do the calculations more accurately in 2016? How do we know we should trust their numbers, and not keep quoting the 1977 ratio of 10:1?

“We begin our analysis by revisiting the number of bacteria through surveying earlier sources, comparing counts in different body organs and finally focusing on the content of the colon. We then estimate the total number of human cells in the body, comparing calculations using a “representative” cell size to aggregation by cell type. We then contrast the cell number distribution by tissue type to the mass distribution. In closing, we revisit the ratio of bacterial to human cells and evaluate the effect of gender, age, and obesity.”

Is this now a “fact?”


But, it sure seems to be a more accurate estimate. And even better research can change that number again.

The bottom line for me is that this one study completely changed my mind about the number of gut microbes we actually have. It rarely happens. But it did.

this one study completely changed my mind about the number of gut microbes we actually have. #gut Share on X

NOTE: If you want a detailed review how to evaluate health studies, I walk through how to do it in these two posts. Bonus, the study I use as an example actually looks at gut microbes. 🙂


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About health studies . . .


Scientific studies are designed to learn about ourselves and our world. Science is not a belief system, it’s a method of investigation. What make scientific studies different from our own personal opinion or experience is the massive effort to reduce bias. Over the course of several centuries, the methods have been getting stronger.

For example, I talk about the first clinical trial, done in 1747 right here. It was an effort to treat scurvy. Scurvy ravaged navy operations in Europe and sometimes half of the mariners would have symptoms.

This first clinical study:

  • Was randomized (thanks Dr. James Lind!). Randomization helps to ensure the groups of people being studied are approximately the same.
  • Was not “blinded.” Both the people who participated, as well as the physician running the study knew what they were eating. Of course, blinding helps to measure the placebo effect. And this is a key reason why most nutrition/dietary studies cannot be blinded, and are therefore “observational,” and not “experimental.” Hence the saying, “Correlation does not equal causation.” Observational studies find correlations; whereby experimental studies can find causation.
  • Did not test many biomarkers. No blood tests were done. Because we didn’t have the biochemistry knowledge or lab techniques back then.
  • Was done before we ever even knew about nutrients! #thanksscienceforteachingusabouthealth

Back then, they also didn’t use the complex statistical analyses to look for confidence intervals and type I or II errors, there was no ethics review prior to doing a clinical trial. There was also no clinical trial registry, peer-review, nor, declaration of funding sources and competing interests.

In a nutshell, we’ve gotten better at objectively learning about the world (including ourselves) using the scientific method.

We've gotten better at objectively learning about the world using the scientific method. #science Share on X


It’s OK to change your mind with new (solid) information


I’ll always remember the quote that says,

It's ok to have an open mind, just not open enough that your brain falls out

I don’t know who said it, because on the first page of Google, six people are listed as the author!

Is it OK to be skeptical of new information? Hellz ya! Is it OK to change your mind with new information? Only when that info is clearly better quality than what we knew before.

This example is one of just a precious few times where a new study changed my mind. And I read dozens of studies each week when I’m researching and writing blog posts for my clients.


Signing off and toasting: To cautious and intelligent updating of health knowledge!


Over to you


What do you think? Does this make you feel like you can be skeptical of new studies, but still be open to changing your mind? Do you feel confident that, to change your mind, you need some pretty solid science? Do you have any comments or questions?

Let me know in the comments and I’ll personally reply!




(1) Arnold, J.W., Roach, J., M. Andrea Azcarate-Peril, M.A. (2016).  Emerging Technologies for Gut Microbiome Research. 24(11), p887–901. DOI:

(2) NIH Human Microbiome Project 2. About the Human Microbiome

(3) Yip, L.Y., Aw ,C.C., Lee, S.H., Hong, Y.S., Ku, H.C., Xu, W.H., Chan, J.M.X., Cheong, E.J.Y., Chng, K.R., Ng, A.H.Q., Nagarajan, N., Mahendran, R., Lee, Y.K., Browne, E.R. & Chan, E.C.Y. (2017). The Liver-Gut Microbiota Axis Modulates Hepatotoxicity of Tacrine in the Rat. Hepatology. doi: 10.1002/hep.29327.

(4) Chilloux, J., Neves, A. L., Boulangé, C. L., & Dumas, M.-E. (2016). The microbial-mammalian metabolic axis, a critical symbiotic relationship. Current Opinion in Clinical Nutrition and Metabolic Care, 19(4), 250–256.

(5) Villa, C.R., Ward, W.E., & Comelli, E.M. (2017). Gut microbiota-bone axis. Crit Rev Food Sci Nutr. 57(8):1664-1672. doi: 10.1080/10408398.2015.1010034.

(6) Brandsma, E., Houben, T., Fu, J., Shiri-Sverdlov, R. & Hofker, M.H. (2015). The immunity-diet-microbiota axis in the development of metabolic syndrome. Curr Opin Lipidol. 26(2):73-81. doi: 10.1097/MOL.0000000000000154.

(7) Sender, R., Fuchs, S. & Milo, R. (2016) Revised Estimates for the Number of Human and Bacteria Cells in the Body. PLoS Biol 14(8): e1002533.

(8) Savage, D.C. (1977). Microbial ecology of the gastrointestinal tract. Annu Rev Microbiol. 31:107-33. PMID: 334036 DOI: 10.1146/annurev.mi.31.100177.000543

(9) Rosner, J.L. (2014). Ten Times More Microbial Cells than Body Cells in Humans? Microbe. 9(2):47.  DOI: 10.1128/microbe.9.47.2


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I’m Leesa Klich, MSc., R.H.N.
Health writer – Blogging expert – Research nerd.

I help health and wellness professionals build their authority with scientific health content. They want to stand out in the crowded, often unqualified, market of entrepreneurs. I help them establish trust with their audiences, add credibility to their services, and save them a ton of time so they don’t have to do the research or writing themselves. To work with me, click here.

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