Returning to fasting man, brain use of βOHB, by displacing glucose as its major fuel, has allowed man to survive lengthy periods of starvation. But more importantly, it has permitted brain to become the most significant component in human evolution. Other secondary adaptations had to be made, particularly in reproduction. The mega brain of H. sapiens and the recent antecedents such as neanderthalensis, erectus, and habilis posed a problem in getting the big head through the pelvic canal, particularly with bipedalism, as the hominoids became hominids, some 7 mya. Bipedalism necessitated narrowing of the pelvic canal for optimal mobility. Speed necessitates limbs close together, whereas limbs far apart result in waddling, i.e., greyhounds versus bulldogs, deer versus turtles, pheasants versus ducks. We are the only primate born facing backward and, more importantly, born obese. Obstetric problems in primates other than humans are essentially unknown (60).
Not well known, however, is the metabolism of the human newborn, which is essentially ketotic. Blood glucose levels fall strikingly in the neonate, and concentrations of βOHB may rise to 2—3 mM. The newborn human brain consumes 60%—70% of total metabolism at birth, nearly half via β-hydroxybutyrate. Fitting in with this pattern is maternal colostrum. It contains much triglyceride and protein, but little lactose, starting man’s entry into society on an Atkins diet (Figure 6)! Lactose gradually increases during the first two to three days of lactation (46), during which time ketosis disappears. Also, humans are born a few months premature compared with our primate cousins. And, again, we are the only primate born fat, probably to furnish the caloric bank for our big brains. We are also the only primate with significant neonatal brain injury (20) due to extreme sensitivity to hypoxia/ischemia (78). Again, this is a penalty for having a big brain!