As I write, this interview with mathematician Carson C. Chow is the number-one most-emailed story on the New York Times Web site. Chow, a researcher at the National Institute of Diabetes and Digestive and Kidney Diseases, had no experience in the health sciences before he came to study the problem of why so many Americans are overweight. "I didn't even know what a calorie was," he says.

This kind of outsider's perspective can be invaluable when attacking a problem as difficult and entrenched as the epidemic of obesity in the U.S. Chow relates the story of starting work at the institute—a division of the National Institutes of Health—and finding a mathematical model created by a colleague that could predict "how body composition changed in response to what you ate." The problem, as Chow describes it, was that the model was complicated: "hundreds of equations," he told the Times. "[We] began working together to boil it down to one simple equation. That's what applied mathematicians do."

And what did Chow's simple model reveal about the nature and causes of obesity? Basically, that we eat too much. "The model shows that increase in food more than explains the increase in weight." Food in, fat out. Simple enough to be captured in a single equation.

Unfortunately Chow's outsider's perspective on the obesity crisis isn't really an outsider's perspective at all: it is the physicist's perspective. Physicists have a long history of marching into other sciences with grand plans of stripping complex phenomena down to the essentials with the hope of uncovering simple fundamental laws. Occasionally this works. More often, they tend to overlook the very biochemistry at the heart of the process in question.

Chow's conclusion is not just obvious—it's a tautology. Because for Chow, a calorie is just a unit of energy. Eat more calories than you burn, and the energy must go somewhere. That somewhere is fat cells. The conclusion is built into the assumptions.

But perhaps a calorie is not just a calorie. Perhaps, as some prominent researchers argue, the body processes calories from sugar in a fundamentally unique and harmful way. According to this hypothesis, we're not getting fat because we're eating more. We're getting fat because of what we're eating more of. The biochemistry that explains why this would happen is complex—certainly difficult to include in a computer model—but that doesn't make it wrong.

Ultimately experiments will decide if this hypothesis is true, or if it is not true, or if it is true but just one part of a nuanced understanding of obesity that includes biochemistry, microbiology, neurobiology, politics, economics and much more. The obesity crisis isn't rocket science. It's complicated.