This observation about the physiological nature of obesity was made decades ago, perhaps centuries ago. The most conspicuous examples are animals (as Astwood noted with his “consider the pig” point) and the animal models of obesity that nutritionists and obesity researchers have studied since the late 1930s. Indeed, researchers would occasionally admit that it’s clearly true about animals and animal models of obesity—that some animals get fat effectively independent of how much they eat and even when they eat no more than lean animals—but then somehow reject its relevance to humans on the basis that everyone knows that humans get fat because they eat too much. Their devotion to their energy balance thinking and to its implications was so great that they couldn’t escape it. 

Take, for instance, Jean Mayer, the most influential American nutritionist in the 1960s and into the ’70s. Mayer started his research career at Harvard in the late 1940s and then moved on to become dean of Tufts University. The nutrition school at Tufts was later named after him. As a nutritionist, Mayer got some things right and many things wrong, as scientists often do, even the best of them. He spent the later years of his life arguing that people with obesity get that way because they don’t exercise enough. Our current obsession with physical activity is largely rooted in Mayer’s proselytizing in the 1970s. But at the beginning of his career in the 1950s, he studied a strain of obese mice. “These mice,” he wrote, “will make fat out of their food under the most unlikely circumstances, even when half starved.” 

That’s the nature of overweight and obesity. That’s what it means to have a “compulsory tendency toward marked overweight due to abnormal accumulation of fat.” Mayer’s mice did not get fat by overeating. They got fat by eating. Half-starving them didn’t make them lean. It only made them hungry and slightly less fat. So let’s redefine what we mean by obesity. People with obesity are not thin people who couldn’t control their appetites (for whatever reason, psychological or neurobiological) and therefore ate too much. They’re people whose bodies are trying to accumulate excess fat even when they’re half-starved. The drive to accumulate fat is the problem, and it’s the difference between the fat and the lean. The hunger and the cravings, and then the failures and the sins, as Astwood suggested, are the results. This observation should be blindingly obvious to anyone who has ever had a weight problem, who fattens easily. Those who fatten easily are profoundly different from those who don’t and may have been from the womb onward. Their physiology is different; their hormonal and metabolic responses to foods are different. Their bodies want to store calories as fat; the bodies of their lean friends don’t. In George Bernard Shaw’s play Misalliance, written in 1909–10, his character John Tarleton puts it this way: “It’s constitutional. No matter how little you eat you put on flesh if you’re made that way.” Shaw, via Tarleton, may have been exaggerating slightly, but that’s as good a way to capture the simplicity of the idea as any. If these people want to be relatively lean and healthy, if such a thing is possible, they have to eat differently. There may be foods they cannot eat. Foods that make them fat may not make their lean friends fat.

Gary Taubes. The Case for Keto: Rethinking Weight Control and the Science and Practice of Low-Carb/High-Fat Eating (Kindle Locations 697-700). Knopf. Kindle Edition.