Could a Fecal Transplant Help You Lose Weight?

by Mike Miesen

You have an organ that weighs about three pounds and is critical to your day-to-day functioning. It’s kind of mysterious, and scientists aren’t quite sure how it works, but it’s pretty clear that you wouldn’t want to be without it.

The human brain is about three pounds and fits that description perfectly, but it’s not the organ I’m referring to. Scientists are just beginning to understanding the vast influence of the “virtual organ” that is your “gut microbiome” – the 100 trillion (with a T) bacteria, viruses, and fungi that reside in your stomach and intestinal tract. It’s thought to affect nearly every part of you, from how you feel to how you think and act[1]. And yes, estimates are that it weighs about three pounds.

New research indicates that transplanting a thin person’s microbiome into a fat person may, with dietary changes, help him or her lose weight. The implications for an increasingly-fat world are remarkable, and point to the gross-sounding but misunderstood fecal transplant as a potential future weight-loss tool.

To carry out the study, researchers at Washington University, in St. Louis, found human twins who were “discordant for obesity” (in English: one was obese, the other wasn’t), and took fecal samples from each; these were introduced to otherwise-identical germ-free mice. Now, there were mice with the obese fecal matter (let’s personify this group as “Obbie”) and mice with the lean fecal matter (“Lenny”); this matter made its way to the intestinal tracts of each, and colonized them with bacteria, viruses, and fungi.

Initially, Obbie and Lenny were kept separate and allowed to eat as much low-fat mice food as they wished. While Obbie didn’t eat any more than Lenny, he was fatter; Lenny had approximately the same fat to total weight ratio as before, while Obbie had a higher ratio. The authors surmise that Lenny’s microbiome broke down and fermented the food more effectively than Obbie’s microbiome did, and as a result, Lenny’s body got rid of more of the food (he pooped out more).


(After the initial experiment: meet Obbie, on the left, and Lenny, on the right (source).

Next, Obbie and Lenny were put in the same cage. Because mice eat poop (I prefer the sanitized scientific jargon of “mice are coprophagic”), by sharing a cage, Obbie and Lenny were essentially sharing a microbiome. They were fed the same low-fat mouse food as before.

Here’s where things get weird: this time, when Obbie and Lenny ate the same amount of food, they also had the same amount of fat. They looked and weighed the same.



(The newly-thin Obbie, with still-thin Lenny (source).)

What changed? Lenny’s microbiome “infected” Obbie’s: it was as if the two microbiomes went to war, and Lenny’ microbiome won, with Obbie’s intestinal tract as the spoils. Obbie, with a mostly-new microbiome, lost fat and lost weight.

A similar effect was seen when the Obbie and Lenny were placed in the same cage with a germ-free mouse: it picked up Lenny’ microbiome, too. It seems that, somehow, either mice are predisposed to accepting the thin microbiome, or the thin microbiome was born a winner.

But here’s where things get even stranger.

The researchers wanted to see if Lenny’s microbiome would dominate Obbie microbiome when a more “American” diet was eaten (i.e., higher in saturated fats and lower in fruit and vegetable consumption). If Obbie and Lenny ate nothing but Big Macs and French fries, would Lenny’s microbiome still win the war?


(Obbie on the left, after his microbiome fought off the invading thin microbiome. It was “fortified” by the high-fat diet.)

No; it lost. It was like the high-fat diet fortified Obbie against Lenny’s invading microbiome, and Obbie’s microbiome stayed “fat.”

Before getting to the potential implications of the study, let’s take a moment to recognize that these are studies of mice, not humans. Additional research is critical and necessary to our understanding of how human microbiomes work and if the causal mechanisms are even similar.

But let’s speculate a bit, just for fun. Something in Lenny’s microbiome – a specific bacteria or the interaction of groups of bacteria, maybe – caused him to stay thinner than Obbie, even when eating the same amount of food. If Lenny and Obbie were humans, could we coax Lenny’s whole microbiome into Obbie?

Probably. Readers of this blog know one way to do it: a fecal transplant!

In layman’s terms: a donor’s poop is collected, analyzed, diluted, and then introduced to a recipient’s colon via a nasogastric tube. The procedure has been shown to effectively cure clostridium difficile, or c. diff, a nasty hospital-acquired infection.

And now, there’s a glimmer of hope that a similar procedure could, along with a low-fat diet (no more Big Macs!), promote weight loss. In the future, a fecal transplant (or a probiotic super pill) could be one component of weight-loss treatment for obese individuals.

The authors of the study seem hesitant to recommend fecal transplants, preferring instead to better understand which bacteria are causing this effect, so that they can offer it as a standalone treatment in pill form. This attitude is probably driven by equal parts scientific circumspection and financial motivation: a drug that actually led to significant and long-term weight loss would be a blockbuster, and could make the researchers very, very wealthy.

Fecal transplants, on the other hand, aren’t likely to bring untold wealth to pharmaceutical companies or the researchers, since they’re usually done with a donor’s poop.

As one doctor not involved with the study noted in a New York Times write-up, isolating the specific causal mechanism “could take many decades,” and obesity is an immediate concern for individuals and the federal health budget.

Fecal transplants are regarded as safe, so why not start with them now?

Well, for starters: we don’t know if this actually happens with humans, too. Even if it does, it’s unclear who would benefit from the treatment: only those who are very obese? Slightly overweight? We’re don’t know yet.

But we can all agree on one thing: discussing fecal transplants is fun!

Just not during a meal. Sorry, roommates.


Mike is a healthcare consultant turned aid worker turned traveler (currently: East Africa) and freelance journalist. Follow him on Twitter @MikeMiesen or subscribe to the blog.


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