Cells require energy, but they can’t take energy directly from the food you eat. Your body has transporters that help the energy source cross over into the cell – like a bouncer for your cells.
What you eat can determine how your cells receive energy. When you eat carbs, your body produces glucose, which means you need glucose transports (gluts). However, if you’re in ketosis, your liver is producing ketones. Monocarboxylate transporters (MCTs) transport the ketones where they need to go. This process allows your body – especially your brain, heart, and muscles — to receive energy even if you’re fasting.
Hokin, Lowell E. “Metabolic Pathways, Third Edition: Metabolic Transport, Volume VI.” ScienceDirect, Academic Press, 1972.