There are, in very broad terms, two types of scientific advance worth writing about. Some findings tug back nature’s veil to reveal something new about the ineffable mysteries of life and the universe around us. And others, more prosaically, promise to make us stronger or faster or thinner or less gassy.

That distinction is what kept coming to mind as I followed the excitement and subsequent backlash surrounding a new study in Nature Medicine from researchers at Harvard University about a “performance-enhancing microbe” isolated from the poop of Boston marathoners. Did this story belong in the first category or the second category? It’s an important question, because I suspect your reaction to the study will depend in large part on how you classify it.

The gist is as follows: for a week before and after the 2015 Boston Marathon, the researchers collected (more or less) daily stool samples from 15 runners and 10 non-running controls. An analysis of the microbes in their poop identified one particular strain, Veillonella atypica, whose levels spiked in the days after the marathon in those who raced. Even before the race, this microbe was more common in the runners than the non-runners, though the difference wasn’t statistically significant.

This bit of the story (though not the specific name) was already public, having been revealed in a press release in 2017 (and, for that matter, nicely described in this Outside feature last year). Veillonella, it turns out, breaks down lactic acid—which, as that press release put it, “can lead to muscle fatigue and soreness.” To confirm that lactic acid (or, to use the physiologically correct term, lactate) circulating in your blood can make it into your intestine where Veillonella can get at it, the researchers injected lactate labeled with a special carbon isotope into the tail veins of mice. Sure enough, the labeled lactate turned up in the mouse colons, bolstering the idea that the microbes in your gut can help soak up lactate so that you can run farther and faster.

The only problem, as physiologist Mark Burnley swiftly pointed on Twitter, is that this idea is based on a thoroughly outdated understanding of what makes you tired. Lactate doesn’t cause fatigue or soreness; it’s simply a byproduct of hard exercise. Even in the unlikely event that your colon microbes could lower your overall lactate by a meaningful amount—unlikely because the amount of blood flowing to your gut is severely reduced during hard exercise, Burnley added—that wouldn’t make you faster.

But there’s a bit more to the story: it may be that it’s what happens to the lactate that matters. Veillonella transforms lactate into (among other things) a short-chain fatty acid called propionate. Could it be that the addition of propionate, rather than the subtraction of lactate, is what’s helpful?

To test this idea, the researchers turned once again to mouse experiments. Let’s just state, for the record, that mice are not humans. You’d be a fool to assume that results you see in mice will automatically translate to humans. Still, as long as you keep that in mind, it’s possible to gain useful preliminary insights from mouse experiments.

The most attention-grabbing experiment was a straightforward performance test. A total of 32 mice were divided into two groups. Half of them received a daily dose of Veillonella, while the other half received a daily dose of another microbe that doesn’t process lactate. Five hours after receiving the microbes, the mice ran to exhaustion. After a week, the two groups switched microbes and repeated the process for another week. The results were significant: when the mice received Veillonella, they ran, on average, 13 percent longer than when they received the other microbe.

The next stage was to figure out whether propionate itself could be the reason for the enhanced performance. In this case (sorry to be graphic), the researchers simply introduced propionate via the rectum into the mice’s colons, where it could be absorbed into the bloodstream. When compared to a rectal insertion of plain saline, the propionate once again boosted run-to-exhaustion time by a similar amount. That supports the idea that it’s the propionate itself that is the performance-booster.

That’s the main gist of the argument. I’m skimming over some of the ancillary details, because there are a bunch of other sub-experiments. For example, they recruited another 87 ultramarathoners and Olympic Trials rowers, and replicated the finding that levels of Veillonella in your poop increase after hard exercise, presumably because you’re generating lots of lactate that Veillonella likes to eat. But the main flow of logic is: athletes have more Veillonella (either because they exercise a lot, or because they’re born that way); Veillonella turns lactate into propionate; propionate makes you (or, at least, mice) faster for unknown reasons.

You may notice that there quite a few leaps in this chain of logic. For example, it’s far from clear (and not statistically significant) that athletes actually do have more Veillonella than non-athletes, as science journalist Ed Yong, the author of a 2016 book on the microbiome, pointed out on Twitter. The sample sizes are small, and the effects are small—“But sure,” he added caustically, “start a company.”

And there’s the rub. Three of the paper’s authors have indeed started or hold equity in a company called FitBiomics. According to the company’s website, they are “sequencing the microbiome of elite athletes to identify and isolate novel probiotic bacteria for applications in performance and recovery... [and] purifying these novel probiotics and commercializing as ingredients to disrupt consumer health and nutrition and cater to the next generation athlete.”

In other words, what I initially read as a Type I story (isn’t it amazing how the body works?) is actually a Type II story (have we got a pill for you!). Back in 2017, discussing the company’s impending launch, the first author of the new paper, Jonathan Scheiman, said: “I would like to think that a year after we launch, we could have a novel probiotic on the market.” And to be honest, that goal-focused approach makes me read the results with a bit more skepticism. It doesn’t automatically mean the results are wrong. But they didn’t happen to stumble on a neat microbe with potential performance-boosting effects; they were looking for one. And the problem with that kind of science is that you often find what you’re looking for, even if it’s not really there.

My new book, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance, with a foreword by Malcolm Gladwell, is now available. For more, join me on Twitter and Facebook, and sign up for the Sweat Science email newsletter.