It is not the case, despite public-health recommendations to the contrary, that carbohydrates are required in a healthy human diet. Most nutritionists still insist that a diet requires 120 to 130 grams of carbohydrates, because this is the amount of glucose that the brain and central nervous system will metabolize when the diet is carbohydrate-rich. But what the brain uses and what it requires are two different things. Without carbohydrates in the diet, as we discussed earlier (see page 319), the brain and central nervous system will run on ketone bodies, converted from dietary fat and from the fatty acids released by the adipose tissue; on glycerol, also released from the fat tissue with the breakdown of triglycerides into free fatty acids; and on glucose, converted from the protein in the diet. Since a carbohydrate-restricted diet, unrestricted in calories, will, by definition, include considerable fat and protein, there will be no shortage of fuel for the brain. Indeed, this is likely to be the fuel mixture that our brains evolved to use, and our brains seem to run more efficiently on this fuel mixture than they do on glucose alone. (A good discussion of the rationale for a minimal amount of carbohydrates in the diet can be found in the 2002 Institute of Medicine [IOM] report, Dietary Reference Intakes. The IOM sets an "estimated average requirement" of a hundred grams of carbohydrates a day for adults, so that the brain can run exclusively on glucose, "without having to rely on a partial replacement of glucose by [ketone bodies]." It then sets the "recommended dietary allowance" at 130 grams to allow margin for error. But the IOM report also acknowledges that the brain will be fine without these carbohydrates, because it runs perfectly well on ketone bodies, glycerol, and the protein-derived glucose.)
Whether a carbohydrate-restricted diet is deficient in essential vitamins and minerals is another issue. As we also discussed (see page 320–26), animal products contain all the amino acids, minerals, and vitamins essential for health, with the only point of controversy being vitamin C. And the evidence suggests that the vitamin C content of meat products is more than sufficient for health, as long as the diet is indeed carbohydraterestricted, with none of the refined and easily digestible carbohydrates and sugars that would raise blood sugar and insulin levels and so increase our need to obtain vitamin C from the diet. Moreover, though it may indeed be uniquely beneficial to live on meat and only meat, as Vilhjalmur Stefannson argued in the 1920s, carbohydrate-restricted diets, as they have been prescribed ever since, do not restrict leafy green vegetables (what nutritionists in the first half of the twentieth century called 5 percent vegetables) but only starchy vegetables (e.g., potatoes), refined grains and sugars, and thus only those foods that are virtually without any essential nutrients unless they're added back in the processing and so fortified, as is the case with white bread. A calorie-restricted diet that cuts all calories by a third, as John Yudkin noted, will also cut essential nutrients by a third. A diet that prohibits sugar, flour, potatoes, and beer, but allows eating to satiety meat, cheese, eggs, and green vegetables will still include the essential nutrients, whether or not it leads to a decrease in calories consumed.