Our lives have become so modern and convenient that sometimes it's hard to imagine what it was like when food was not fast, easy, and readily available. But our ancestors had a very different experience, often going for long periods without eating. As such, their bodies had to be prepared and evolved to be able to withstand harsh conditions and long periods of time without food.
They did this through metabolic flexibility. And to be clear, metabolic flexibility is the body's ability to shift back and forth between using glucose (from carbohydrates) and fatty acids (from fat) for fuel.