We’ve got the whole thing backward, he argues. The overweight are not lazy hogs who eat too much and exercise too little. The thin are not virtuous and disciplined. Rather, all of us are fulfilling a fixed biological mandate, just as growing children are. Our bodies have a nonnegotiable agenda, and our behavior evolves to make that agenda happen, he writes: “Eating in moderation and being physically active (literally, having the energy to exercise) are not evidence of moral rectitude. Rather, they’re the metabolic benefits of a body that’s programmed to remain lean.”

In other words, you don’t haul your body off that couch and out to the gym; your body hauls you.

Meanwhile, “those who get fat do so because of the way their fat happens to be regulated,” Mr. Taubes writes. “A conspicuous consequence of this regulation is to cause the eating behavior (gluttony) and the physical inactivity (sloth) that we so readily assume are the actual causes.”

The actual causes, he argues, with a great deal of observational and experimental data to support his points, are the array of regulatory enzymes and hormones that move fuel, in the form of fat and sugar molecules, in and out of storage depots around the body.

And the only one of these hormones under even a smidgen of voluntary control is insulin. At this point Mr. Taubes merges onto the narrative highway traveled by all low-carb advocates: The body’s insulin levels are largely determined by ingested carbohydrates, and for some people the high-carb foods that stimulate insulin secretion and cravings for more high-carb foods are, in this worldview, just so much poison.

So that apple  a filling package of fiber and vitamins to the Weight Watchers folks  is just a serving of fructose to Mr. Taubes. Fructose is the problematic sugar our bodies turn to fat the most readily, and if you are programmed to be fat, an apple will make you that much fatter.

Mr. Taubes draws an analogy to cigarette smoking: Not every long-term smoker gets lung cancer  in fact, only a minority do  but among people with lung cancer, smoking is by far the most common cause. “In a world without cigarettes, lung cancer would be a rare disease, as it once was,” he writes. “In a world without carbohydrate-rich diets, obesity would be a rare condition as well.”

How to account for the fact that in virtually all head-to-head comparisons of various diet plans, the average long-term results have invariably been quite similar  mediocre all around? The party line holds that backsliding is universal. Mr. Taubes makes much of the addictive effect of carbohydrates: once you taste them you never forget them.