The 36-hour prolonged fast is divided into five phases that reflect profound transformations in human metabolism. In the first phase, between 0 and 12 hours, the use of glycogen stored in the liver and muscles predominates. The body gradually reduces insulin and increases growth hormone, paving the way for subsequent changes.
The second phase, from 12 to 18 hours, corresponds to the so-called "metabolic shift." The body begins to deplete its glycogen stores and activates enzymes that facilitate fat oxidation. At this point, ketone body production begins, and hunger gradually decreases.
Between 18 and 24 hours, the third phase occurs: deep ketosis and activation of autophagy. The brain begins to feed primarily on ketones, which translates into greater mental clarity. At the same time, autophagy increases up to fourfold, eliminating damaged proteins and cellular waste, while growth hormone levels increase to protect muscle mass.
The fourth phase, from 24 to 30 hours, marks the peak of metabolic transformation. The body reaches its peak efficiency in fat burning and cell regeneration. Neuronal autophagy intensifies, reducing accumulations associated with neurodegenerative diseases. Furthermore, the neurotrophic factor BDNF increases, stimulating new brain connections.
Finally, in the 30- to 36-hour phase, metabolic flexibility and mitochondrial biogenesis are consolidated. The body prepares to resume food intake with highly optimized insulin sensitivity, ensuring efficient nutrient utilization. Breaking the fast with a light, nutrient-dense meal, such as broth or steamed vegetables, is recommended to maintain the benefits obtained.
This process not only promotes fat loss and muscle preservation, but also strengthens metabolic health, optimizes brain function, and reduces markers of inflammation. The benefits can extend up to 72 hours after the fast ends, consolidating a "metabolic reset" with a comprehensive impact on the body.
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