Have you seen a baby lately? I’m talking about a newborn, fresh out of the womb. They’re fat. And cute. But mostly fat. Packed with stored energy prior to birth in the third trimester, the fatness of human babies is unprecedented in the mammal world. While the newborns of most mammal species average 2 to 3 percent of birth weight as body fat, humans are born with a body fat percentage of nearly 15, surpassing the fatness of even newborn seals.
Why is this so? Because humans are born half-baked. When a healthy human baby emerges from the womb, she is born physically helpless and with an underdeveloped brain. Unlike most animals at birth, a newborn human is not equipped with a full catalogue of instincts preinstalled. It is estimated that if a human were to be born at a similar stage of cognitive development to a newborn chimp, gestation would be at least double the length (that doesn’t sound fun —am I right, ladies?).
By being born “prematurely,” human brains complete their development not in the womb, but in the real world, with open eyes and open ears—this is probably why we’re so social and smart! And it is during this period of rapid brain growth, what some refer to as the “fourth trimester,” that our fat serves as an important ketone reservoir for the brain, which can account for nearly 90 percent of the newborn’s metabolism.
10 Now you know: baby fat isn’t just there for pinching. It’s there for the brain. In the context of a “normal” Western diet that’s rich in carbohydrates, significant ketone production is inhibited the vast majority of the time. 11 This is because foods that are high in carbohydrates elicit an insulin response from the pancreas, and ketosis is brought to a grinding halt whenever insulin is elevated. Suppression of insulin, on the other hand, either by fasting or by eating a diet very low in carbohydrates, drives ketogenesis. Let’s explore these two routes to ketone creation