After the nitrogen-containing head is chopped off, converted to urea, and sent on its way, the tails are used to make glucose (a process called gluconeogenesis, which literally means “making new sugar”) or ketones, both of which can be used for energy, as we’ll see below. Proteins are typically a minor part of the daily energy budget, providing around 15 percent of our calories each day. But they are a vitally important emergency energy supply if we’re starving, a bit like setting the furniture on fire to heat your house.
