But Segal argues that their study packs extra statistical punch despite including just 20 volunteers. That’s because it’s a crossover trial—each participant ate both breads in turn, and so could be compared against themselves. If the sourdough bread changed any of the clinical variables by just 5 to 10 percent compared to the white bread, the team would have spotted that difference—and they didn’t. They also showed that overall, eating bread did change a variety of factors including cholesterol, iron, and calcium levels. It wasn’t that eating bread did nothing; it was that eating sourdough wasn’t radically different to eating white.

Why? “One possibility is that the two breads induce exactly the same effect,” says Segal. “The more intriguing possibility is that each bread induces different effects in different people.” And that’s what his team found in the new study. The team could tell who was whom based on their microbiome, and they used their data to develop an algorithm that could look at a person’s gut bacteria and accurately predict how they’d respond to different breads.

Again, Roberts is skeptical. “This is wildly over-extrapolating,” she says. “It’s more likely that people liked the two breads more or less, and so were eating more or less before the testing days. That would influence their glycemic response. There might have been a personalized response to the breads, but this study doesn’t tell us whether it was a methodological error or some real individual fingerprint.”

Segal says that his team gave each volunteer a fixed amount of bread to eat during breakfast, calibrated so that each serving had the same amount of carbohydrates. They also logged all their meals on a smartphone app. Based on these recordings, it seems like the volunteers were sticking to their instructions, and weren’t just shunning one bread or another based on their preferences.

That being said, people have a tendency to lie when logging their own meals, and such self-reports are notoriously inaccurate. A single study like this also cannot refute a large body of epidemiological evidence showing that people who eat more whole-grains tend to have lower risks of cancer, heart disease, type 2 diabetes, and other conditions. If you assume that these effects are causal, rather than just correlational, it may be that you need to eat a lot of whole grains, over long periods of time, to experience any benefits. Alternatively, large population-wide studies might mask the fact that only some people would benefit from these foods.

That’s what Elinav and Segal want to know. They’re acting as scientific consultants for a company that was set up on the back of their research, and that analyzes the microbiome to offer personalized dietary advice for controlling blood-sugar levels. They’re also setting up a year-long experiment to see if their recommendations can improve people’s health in the long-term.

This, they say, is the future of nutrition. After all, advising people to eat more sourdough because it officially has a lower glycemic index than white bread would have been misleading for half the volunteers in their experiment. Instead of offering universal dietary recommendations, or categorizing foods as “healthy” or “unhealthy,” the science of nutrition might need a more personal touch.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.