Fat is the way that humans and all other mammals store energy. We do this to provide for ourselves during periods of nutritional shortfall, when there isn’t enough food or when food sources are irregular. One of the reasons such stores are so important for humans is that we have an extremely demanding organ that requires a lot of energy: our brain.
A human baby’s brain is massive relative to its body size and is estimated to use around 50 to 60 percent of a baby’s energy budget. That means if there are any shortfalls in energy or if an infant’s nutrition is poor, there can be serious consequences. As such, babies have large energetic reserves in the form of fat deposits that they can use if nutrition is inadequate. High fat at birth is particularly useful for humans, who go through a sort of fasting period after birth while waiting for their mother’s breast milk to come in; the first milk, or colostrum, is packed with protein, vitamins, minerals, and immune boosting antibodies but is lower in sugar and fat content than regular breast milk.
On top of needing to provide for their big, energetically expensive brains, human babies also require energy for growth and for staving off illness. As I mentioned, they continue to grow their fat reserves through the first 4 to 9 months of postnatal life. Interestingly, it is at this stage in their development that infants begin to experience two major issues: an increase in exposures to pathogens that can make them sick—crawling around on the ground, putting literally everything in their mouths—and marginal nutrition. During this phase, the nutrition that mom provides through breastfeeding isn’t enough and has to be supplemented with specially prepared, nutritionally dense foods. While some of us can now acquire manufactured baby foods designed to do just that, such shortcuts weren’t available for the majority of human history. Between increasingly complex nutritional needs and the demand for energy necessary to fight off illness, human babies use their baby fat reserves as an essential energetic buffer for these transition periods, allowing them to feed their brains and continue their growth.
So my pudgy tummy didn’t offer warmth, but I guess my mom was right about one thing: Baby fat isn’t so bad after all.