What Happens When You Fast: Metabolic Shifts, Autophagy, and Hormonal Benefits

This podcast episode by Dr. Ekberg debunks the common misconception that fasting leads to energy depletion, instead highlighting the body's sophisticated backup mechanisms for maintaining a constant energy supply. It details the metabolic journey the body undergoes during fasting, starting with the immediate use of glucose, followed by the breakdown of stored glycogen for the first 12-16 hours. As glycogen stores deplete, the body transitions to gluconeogenesis, producing glucose from scratch, initially from recycled byproducts like lactic acid and pyruvate, and later from amino acids and glycerol, which are derived from protein and fat respectively.

The episode further explains the crucial shift towards fat burning and ketone production as glucose becomes less available. Ketones emerge as an alternative and efficient fuel source for the brain, while glucose is spared for tissues like red blood cells and the renal medulla that lack mitochondria and cannot utilize fat directly. Dr. Ekberg clarifies that the fear of muscle loss during fasting is largely unfounded, as the body is metabolically intelligent. He explains that early gluconeogenesis from amino acids is limited and often uses 'junk' protein, and that significant increases in growth hormone and autophagy during fasting actively promote muscle sparing and prioritize structural tissue over fuel.

Beyond energy metabolism, the podcast emphasizes the profound health benefits of extended fasting. These include a significant increase in growth hormone (up to five-fold after two days), which aids in fat burning and muscle building, and enhanced autophagy, a cellular recycling process. Furthermore, fasting boosts Brain-Derived Neurotrophic Factor (BDNF), described as 'Miracle Grow for the brain,' which supports brain repair and rewiring. After 48-72 hours, there's a tremendous increase in stem cell production, crucial for body repair and healing, alongside improved immunity.

Dr. Ekberg provides practical recommendations for incorporating fasting into a healthy lifestyle, differentiating between maintenance goals and reversing conditions like obesity, cancer, or autoimmunity. He suggests a combination of a low-carb, high-fat diet with intermittent fasting protocols (16/8, 18/6, OMAD), monthly 36-42 hour fasts, and quarterly or biannual longer fasts (3-10 days) for deeper repair and stem cell activation. The core advice stresses the importance of a gradual approach, individual customization, and listening to one's body to achieve metabolic flexibility and optimize health outcomes.

"the body has these amazing mechanisms a backup to provide a constant supply of energy pretty much no matter what happens"

"for the first 12 to16 hours after a meal if we don't eat again then glycogen is going to be broken down and it's going to provide most of the glucose for the body for the brain and the tissues"

"As time goes on the body is going to start looking for another source it's going to start making glucose from scratch that's called gluconeogenesis"

"now is when the ketones really kick in because that's the alternative fuel for the brain"

"when I talk about true gluconeogenesis that way I look at it is when the body makes glucose from something that was never glucose and now it's making it from amino acids or glycerol"

"already after about twelve hours we have a significant increase in growth hormone and this keeps growing over the next week so that after two days we already have a five-fold increase in growth hormone"

"the body is way too smart to be burning protein for fuel when there is any fat available"

"BDNF which together with growth hormone is like Miracle Grow for the brain"

"once we get past the 48 into 72 hours now the stem cell production which is involved in body repair and healing increases tremendously"

"most importantly don't rush into it do this gradually learn how your body functions learn how it feels and then you move on from there"