Metabolic Adaptations During Fasting

Early Fasting (0–24 h)

Hepatic glycogenolysis supplies the majority of glucose needed to maintain blood glucose during the first 8–12 h of fasting. 1, 2
The brain consumes roughly 100–120 g of glucose per day and accounts for most of the body’s glucose oxidation in this period. 1, 3, 4
Tissues that are completely glucose‑dependent (e.g., red blood cells, immune cells, transparent ocular tissues, renal medulla, and muscle during anaerobic contraction) obtain ATP solely via glycolysis. 1, 3

Fasting triggers an increase in glucagon and catecholamines and a decrease in insulin, promoting glycogen breakdown. 5, 2
The hormonal shift enables the release of free fatty acids from adipocytes. 5, 2

After glycogen stores are exhausted (≈24 h), gluconeogenesis becomes the principal source of glucose. 1, 2
Amino acids released from muscle protein breakdown (particularly alanine and glutamine) together with glycerol from triglyceride hydrolysis serve as key endogenous substrates for gluconeogenesis. 1, 6
Persistently low insulin levels during prolonged fasting further enhance fatty‑acid release from adipocytes. 5, 2
Hepatic oxidation of fatty acids produces ketone bodies (β‑hydroxybutyrate and acetoacetate), which are utilized by skeletal muscle, cardiac muscle, liver, kidney, and adipose tissue. 5, 2
Physiological ketosis is characterized by plasma ketone concentrations of approximately 0.3–4 mmol/L, distinguishing it from pathological ketoacidosis (>7–8 mmol/L). 7

Both ketone bodies and lactate can safely fuel the brain when plasma glucose is low, progressively replacing glucose as the primary cerebral energy source. 1, 3, 4

Completely glucose‑dependent tissues (red blood cells, immune cells, transparent ocular tissues, renal medulla) generate ATP exclusively via glycolysis. 1, 3
Strongly but not wholly glucose‑dependent tissues (brain) can adapt to utilize ketones during low‑glucose conditions. 1, 3, 4
Tissues not directly dependent on glucose obtain ATP entirely from fatty‑acid oxidation. 1, 3

A rapid decline in plasma glucose can precipitate coma with potentially irreversible neurological damage, although ketones may provide an alternative fuel source. 1, 3
The basal glucose requirement for an adult is estimated at about 2 g · kg⁻¹ · day⁻¹; theoretically, dietary carbohydrates could be eliminated if adequate metabolic adaptation occurs. 1, 3
Providing parenteral glucose during fasting exerts a protein‑sparing effect by reducing the need for skeletal‑muscle protein degradation. 4