How fuel sources shift as a fast lengthens
In the hours after the last meal, the body relies mainly on circulating glucose and the liver's glycogen stores, which in an average adult hold roughly the energy needed for less than a day of normal activity. As those stores draw down over the first day or so, the liver increasingly makes new glucose through gluconeogenesis, using substrates such as glycerol, lactate, and amino acids.
By later stages, typically beyond two to three days in many people, ketone production rises substantially as fat-derived fuel partly spares glucose for the brain. The exact timing varies widely with body composition, activity, prior diet, and individual physiology, so any single hour marker is an approximation rather than a fixed threshold.
Why prolonged fasting is not for everyone
Extended fasting changes electrolyte handling, hormone levels, and protein turnover, and it can be unsafe for people who are pregnant, have diabetes, take glucose-lowering or blood-pressure medication, have a history of eating disorders, or are underweight.
This diagram describes a general physiological sequence for educational purposes. It is not medical advice, and prolonged or repeated fasting should only be undertaken with guidance from a qualified clinician.