The most common question I am asked by people who’ve heard about the microbiome: should I take a probiotic? Which one?

The global probiotics market is worth $34 billion, of which at least $6 billion is supplements, according to a May 2016 report by BCC Research.


The International Scientific Association for Probiotics and Prebiotics (ISAPP) defines probiotics as “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host”. Experts reserve the term “probiotic” for supplements, as opposed to foods that happen to contain live, healthy microorganisms, but that distinction is lost on most people, who just want to be sure they consume enough “good” bacteria.

Perhaps this is a good time to keep up my regular rant about why I don’t like the term “good” or “bad” when it comes to microbes. Everything depends on context: it’s possible to have too much of a good thing, just as it’s possible to have too little of some bad things. Nature knows how to manage a delicate balance and it’s foolhardy to pretend we know all the consequences of a major change in either direction. Martin Blaser, in his excellent book Missing Microbes, reminds us of the native proverb: “Elk are there to feed the wolves; wolves are there to keep the elk strong.” You may not want to get rid of everything, nor would you want to fill yourself up with too much of anything.

The Promise

Among unhealthy people, there is evidence that, under a doctor’s care, probiotics can help with antibiotic-associated diarrhea and similar conditions in children or among people recovering from C. difficile infections. so I’m pretty sure this experiment is safe for me.

The Letdown

A recent scientific review of all well-done studies of probiotics among healthy people couldn’t find evidence that probiotics made much difference compared to a placebo in randomized controlled trials.


Like anything you put into your body, you can’t just assume it’s all upside.

Presumably you’re reading this because you are convinced that microbes have a powerful affect on the body, perhaps as powerful as prescription drugs, yet you wouldn’t consider taking random prescription drugs just to see what happens. The billions of microbes you send into your gut is in a concentration and quantity far greater than anything you’d get from nature. Please remember that.

Here’s an analogy: let’s say scientists discover a breed of parrot that is found in abundance in healthy ecosystems in Costa Rica, so they decide to introduce it to Yellowstone Park. They dump thousands of live parrots all over the park and when they count the overall species diversity the following day, they note with pride that the experiment worked: Yellowstone is now home to a new species, one that is associated with healthy ecosystems! Unfortunately, upon testing again a week later, they learn that the parrots are gone. What happened? You and I can laugh at the idiots who thought they could transplant a tropical species into Wyoming, but maybe that’s exactly what you’re doing if you try to introduce a new species that is not adapted to your microbiome. It may show up in a couple of early gut tests, but if it disappears soon thereafter, was it helpful at all? In the parrot example, it may actually be harmful if it served as food to dangerous predators.

Fortunately, the body is pretty robust and it’s harder to deliberately change the microbiome

Which probiotic brands

References lengthy list of other resources Clinical Guide to Probiotic Products

Expert Consensus Document published by the peer-reviewed scientific journal Nature (2014)

Recent articles in the popular press

The data-heavy web site FiveThirtyEight recently did a week-long series on Gut Science, including a detailed survey of what’s known about probiotics. Their conclusion: “There’s no evidence in humans, however, to support taking probiotics just to generally improve your gut health.”

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