Deep Extended Fast
Ninety-six hours. This is beyond what most people ever attempt. It is also where the body's adaptive intelligence is on full display.
What's Happening
Four days in, the body has fully adapted to running on stored fat and ketones. Basal metabolic rate has slightly decreased as an adaptive response. Protein catabolism is possible if fat stores are limited, though growth hormone and ketones work to protect lean mass. Sodium, potassium, and magnesium management is critical at this stage.
How You're Feeling
Most people report reduced physical energy but exceptional mental calm at this stage. Hunger is largely absent — the appetite suppression from ketones and adapted hormones is at its strongest. This is the phase where fasting stops feeling like deprivation and starts feeling like a different relationship with energy entirely.