Flickr/Phoebe Baker

What can a city's waste reveal about the health of its population?

The Centers for Disease Control and Prevention have a few different ways of measuring obesity in the U.S., including phone surveys, in-home interviews, and physical exams of nationally representative slices of the population.

None of these methods involve poop.

It's an unsurprising fact, probably, but one that bears noting only because some scientists believe it may soon change—or at least, that it should. In a study recently published in the journal mBio, researchers from the University of Wisconsin and the Marine Biological Laboratory in Woods Hole, Massachusetts, laid out the case for sewage as a public-health tool, arguing that human waste may be a more efficient way of measuring obesity at the population level.

To prove their concept, the authors examined more than 200 sewage samples taken from treatment plants in 71 U.S. cities, using genetic sequencing to identify which bacteria in the mix came from human feces (an average of around 15 percent across the samples). A city, like each of its individual inhabitants, has its own unique microbial signature that tends to stay constant over time; by looking at the bacterial makeup of a city's sewage, the researchers found, they were able to predict its obesity rate at a accuracy rate between 81 and 89 percent.

"This is a way to generate information about the public without infringing on any individual's privacy," says Murat Eren, one of the study's co-authors and a researcher at the Marine Biological Lab. "In a natural way, we access the microbiome of a given community."

The mBio study is one of a handful of projects in recent months that have mined a city's waste for information about the bodies that created it. If the old journalism cliché—three examples equals a trend—holds any truth, then it's probably fair to say that sewage is having something of a moment. In January,The Boston Globe highlighted the joint effort of the Cambridge Public Health Department and the Massachusetts Institute of Technology to take a "sewage snapshot" of the city, a test case for the MIT team's larger goal of analyzing waste for population-level information on illegal drug use, diet, and the prevalence of certain bacterial and viral diseases. And in November, The New York Times profiled Jean Carlson, a New York University geneticist collecting sewage from each of New York's neighborhoods to "create a genetic map [that] highlight[s] the city's microbial diversity across different districts."

"It's a preventative public-health question. We need to know the baseline to know how the baseline changes," Carlson told the Times. Eren makes a similar argument: "By looking at the changes in microbial composition, we can come out with some early warning signals" about how a community's health may be changing over time, he says. In discovering what normal sewage looks like, researchers can understand when and why it strays from the norm.

In some ways, the sewer-as-lab is a more community-minded extension of people's growing fascination with what their own waste can do. Companies like uBiome have customers send in stool samples for an in-depth analysis of their microbiomes; fecal transplants are catching on as a viable treatment option for certain gastrointestinal conditions. Here, now, is the democratization of waste research, a chance for each of us to make our own contributions to science (ideally at least once every three days, doctors say). In fact, the average person will flush around 360 pounds of science per year.

"There are traces of all of us in the sewers—and those will tell a different, more collective story," a member of the MIT team told the Globe. Or, put another way: It's a scientist's world, and we're all just pooping in it.

This post originally appeared on The Atlantic.

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