Depleting liver glycogen levels over 11-12 hours of fasting initiates ketogenesis, where the body burns fat for fuel and produces ketone bodies.

“It takes around, I think, 10 to 12 hours...for your liver glycogen to deplete and fatty acids get immobilized, they go to the liver, you start to make beta-hydroxybutyrate and other ketone bodies which then get transported to other tissues and are used for energy in the brain, or they act as signaling molecules.”

“Whereas in humans, it's obviously, depending on exercise. [Dr. Mattson]: So, 10 to 12. [Dr. Patrick]: Ten to 12. Right. Yeah.”

“It typically takes at least 10 hours to deplete the glucose in the liver. So if a person eats breakfast, lunch, and dinner, and then has a snack around 8:00 or 9:00, you know, they may get up and eat breakfast and have not depleted the energy in their liver and have not switched to using fat.”

“when they see intermittent fasting, intermittent fasting is an eating pattern. It's not a diet. Oftentimes, in kind of the lay press and so on, intermittent fasting will be lumped in as a diet. But it's not a diet. It's an eating pattern. A diet is what you eat and how much you eat. Intermittent fasting is an eating pattern that includes intermittent periods of not eating, sufficient to deplete the glucose stores in the liver and cause a switch to the use of fat from your fat cells and the ketones produced from those fat cells.”