The sewage of fat cities like Little Rock and Toledo is easy to distinguish from that of skinny ones like Denver and San Diego.
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If someone were to ask you what distinguishes skinny cities from fat ones, you might think of the prevalence of fast-food joints, the average length of automobile commutes, or the relative abundance of parks and jogging trails. But there’s also another, more underground factor: their sewage.

Researchers with the University of Wisconsin-Milwaukee collected raw sewage samples from the intakes of municipal wastewater treatment plants in 71 cities around the country. Their results, published last month in mBio, the American Society for Microbiology’s open-access journal, showed that the microbial content of that sewage predicted each city’s relative obesity with 81 to 89 percent accuracy.

The finding actually isn’t all that surprising, says lead author Ryan Newton, a visiting professor at UWM’s School of Freshwater Sciences. Other studies