Diet is one of the most important factors shaping health, but it’s surprisingly difficult for researchers to figure out what people eat.

Researchers’ usual tools, like food frequency questionnaires or asking people to recall what they ate the day before, are tedious and expensive, said Christopher Gardner, a nutrition scientist at Stanford. Such efforts depend on people being candid and accurate; people forget about 15 to 20 percent of what they ate after a few days.

It’s one of the reasons that nutrition research is so difficult and why the best evidence comes from small metabolic chamber studies, Dr. Gardner said. In such studies, researchers essentially “incarcerate someone,” he added. “They can’t leave. You cook the food for them, you slide it under the door, you know if they didn’t finish it.”

But a new study, published today in the Proceedings of the National Academy of Sciences, points to a treasure trove of nutrition data: sewage. Using wastewater in North Carolina, researchers searched for traces of food DNA that had survived digestion. For less than a penny a person, they identified hundreds of foods that had been consumed — like chocolate, lettuce, turkey, blueberries, black pepper — and could even infer the socioeconomic profile of a community.

During the Covid-19 pandemic, wastewater monitoring became a familiar tool in public health, helping cities track viral surges without testing every resident. But scientists are increasingly using this same infrastructure to measure illicit drug use, antibiotic resistance, stress hormones and, now, diet.

With such data, researchers could assess if nutritional needs are being met in a community and how policy changes, like food stamp cuts, alter eating behavior in near real time. But the technology also raises tricky questions around privacy, surveillance and risks from potential commercialization.

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