Though glucose is a primary fuel for the brain, it is not, however, the only fuel, and dietary carbohydrates are not the only source of that glucose. If the diet includes less than 130 grams of carbohydrates, the liver increases its synthesis of molecules called ketone bodies, and these supply the necessary fuel for the brain and central nervous system. If the diet includes no carbohydrates at all, ketone bodies supply three-quarters of the energy to the brain. The rest comes from glucose synthesized from the amino acids in protein, either from the diet or from the breakdown of muscle, and from a compound called glycerol that is released when triglycerides in the fat tissue are broken down into their component fatty acids. In these cases, the body is technically in a state called ketosis, and the diet is often referred to as a ketogenic diet. Whether the diet is ketogenic or anti-ketogenic—representing a difference of a few tens of grams of carbohydrates each day—might influence the response to the diet, complicating the question of whether carbohydrates are responsible for some effect or whether there is another explanation. (Ketosis is often incorrectly described by nutritionists as “pathological.” This confuses ketosis with the ketoacidosis of uncontrolled diabetes. The former is a normal condition; the latter is not. The ketone-body level in diabetic ketoacidosis typically exceeds 200 mg/dl, compared with the 5 mg/dl ketone levels that are typically experienced after an overnight fast—twelve hours after dinner and before eating breakfast—and the 5–20 mg/dl ketone levels of a severely carbohydrate-restricted diet with only 5–10 percent carbohydrates.)

For fifty years after William Banting publicized William Harvey’s prescription for a carbohydrate-restricted diet in 1863, the primary clinical disagreements were on the role of fat in the diet. Banting’s original prescription was a high-fat diet, but then it was modified by Harvey himself and by the German clinicians Felix von Niemeyer and Max Oertel into lower-fat, higher-protein versions, and by Wilhelm Ebstein into a version featuring still more fat. “The fat of ham, pork or lamb is not only harmless but useful,” Ebstein wrote.

The notion of a carbohydrate-restricted diet based exclusively on fatty meat was publicized after World War I by the Harvard anthropologist-turned-Arctic-explorer Vilhjalmur Stefansson, who was concerned with the overall healthfulness of the diet, rather than its potential for weight loss. Stefansson had spent a decade eating nothing but meat among the Inuit of northern Canada and Alaska. The Inuit, he insisted, as well as the visiting explorers and traders who lived on this diet, were among the healthiest if not the most vigorous populations imaginable.

Among the tribes with whom Stefansson lived and traveled, the diet was primarily caribou meat, “with perhaps 30 percent fish, 10 percent seal meat, and 5 or 10 percent made up of polar bear, rabbits, birds and eggs.” The Inuit considered vegetables and fruit “not proper human food,” Stefansson wrote, but they occasionally ate the roots of the knotweed plant in times of dire necessity.