“For me, a calorie is a unit of measurement that’s a real pain in the rear.”

Bo Nash is 38. He lives in Arlington, Texas, where he’s a technology director for a textbook publisher. He has a wife and child. And he’s 5’10” and 245 pounds—which means he is classed as obese.

In an effort to lose weight, Nash uses an app to record the calories he consumes and a Fitbit band to track the energy he expends. These tools bring an apparent precision: Nash can quantify the calories in each cracker crunched and stair climbed. But when it comes to weight gain, he finds that not all calories are equal. How much weight he gains or loses seems to depend less on the total number of calories, and more on where the calories come from and how he consumes them. The unit, he says, has a “nebulous quality to it.”

Tara Haelle is also obese. She had her second son on St Patrick’s Day in 2014, and hasn’t been able to lose the 70 pounds she gained during pregnancy. Haelle is a freelance science journalist, based in Illinois. She understands the science of weight loss, but, like Nash, doesn’t see it translate into practice. “It makes sense from a mathematical and scientific and even visceral level that what you put in and what you take out, measured in the discrete unit of the calorie, should balance,” says Haelle. “But it doesn’t seem to work that way.”

Nash and Haelle are in good company: More than two-thirds of American adults are overweight or obese. For many of them, the cure is diet: One in three are attempting to lose weight in this way at any given moment. Yet there is ample evidence that diets rarely lead to sustained weight loss. These are expensive failures. This inability to curb the extraordinary prevalence of obesity costs the United States more than $147 billion in healthcare, as well as $4.3 billion in job absenteeism and yet more in lost productivity.