Does This Phylum Make Me Look Fat?

We would all love a simple weight-loss plan. Beyond carbs and fats, some studies have hinted that a key group of gut microbes — from the phylum Firmicutes — might be more common among people who are overweight.

Thinner people, these studies suggest, might have more bacteria from the phylum known as Bacteroidetes. Maybe we just need to reestablish a Bacteroidetes-favoring gut to more easily lose weight, some people have said. A stack of diet books has already jumped on the notion.

But a computer analysis led by Katherine Pollard, a biostatistics professor at the University of California, San Francisco and a senior investigator at the Gladstone Institute, recently sifted through an ocean's worth of detailed data that's been collected on gut bacteria. Her analysis found no link between Firmicutes and obesity, after all.

If there's any real link between gut microbes and the size of your gut, it's not just the phylum of bacteria that matters, or maybe even the species, according to Pollard's results. A host of other factors that scientists are only beginning to understand probably contribute, too, she says.

Pollard discussed her team's analysis this month at a meeting of scientists in Seattle.

The technology for investigating the microbiome has been improving fast. Researchers can now detect and identify many of the different species of bacteria in your gut — not just phylum, class, order, family or genus.

But, that's created a new problem, Pollard says. It's one thing to identify the whole cast of microbial characters, and quite another to figure out what the microbes are actually doing in there. The same species of bacteria might act differently in my gut than in yours — and the technology used in these big studies doesn't detect that level of detail.

"What matters to the human host," Pollard says, "is not the Latin names of the microbes" but what each microbe produces — whether that's anti-inflammatory molecules, vitamins, or factors that speed the conversion of extra calories we eat into body fat. (The scientists published a formal description of some of their data-crunching finds late last year in the journal PLOS ONE.)

"Individual microbial species can have widely variable genomes," says William Anton Walters, a researcher in molecular biology and genetics at Cornell University. Any given bacterium may share less than half of its genes with another member of the same species. These genetic differences, rather than phylum or even species designations, Walters says, "could explain the differences between the obese and lean gut microbiota."

Greg Gloor, a professor of biochemistry at the University of Western Ontario, agrees that Pollard and her team are on the right track, as they analyze massive amounts of data of all sorts, looking for trends. The results indicate "that there is more strain-level variation in the microbiome than we have appreciated," Gloor tells Shots in an email.

We know, for example, that some strains of E. coli can be harmless to humans, but that others can sicken or even kill us. Why should we expect other strains of gut bugs to be more uniform? "Bacteria are very diverse," Gloor says.

Gut bacteria may yet be found to play an important role in the extra pounds that plague some people, the scientists say. Walters points to work from studies of fecal transplants that have shown there are meaningful differences "in the gut microbiome of lean and obese individuals." The trick, he says, will be in figuring out more precisely what those differences are.

Katherine Harmon Courage is a contributing editor for Scientific American and a freelance journalist. Her next book, about the microbiome and diet, will be published by Random House in 2016. Follow her on Twitter: @KHCourage