Ketosis and the brain. Babies can go into deep ketosis in a couple hours whereas  it might take an adult two days. Ketones are so valuable for brain development and function that the brain prioritizes usage of ketones in the bloodstream. Ketones are made from fat. Probably why human babies are born fat, a feature kind of exclusive to our species.

Insulin researcher Ben Bikman does a deep dive into fat storage. Where it is stored and why.

https://www.youtube.com/watch?v=UEjkD4weTzM&t=654s

Fat cells can turn glucose into fat with the influence of insulin.