What is the Origin of Fasting Tolerance?

The term is rooted in metabolic endocrinology and is a fundamental concept derived from the study of starvation and survival mechanisms across human evolutionary history. In a modern context, it is applied to assess an individual's metabolic health in the context of therapeutic fasting protocols like intermittent fasting. The origin of the body's adaptive response is the rapid sensing of nutrient scarcity by the central nervous system.

What is the Mechanism of Fasting Tolerance?

The mechanism is governed by a swift, coordinated hormonal shift involving the pancreatic and adrenal axes. Decreased nutrient intake causes a dramatic downregulation of insulin secretion and a reciprocal, rapid upregulation of counter-regulatory hormones like glucagon, norepinephrine, and growth hormone. This hormonal cascade stimulates hepatic glycogenolysis and gluconeogenesis to stabilize blood glucose, concurrently activating lipolysis in adipose tissue to release free fatty acids for subsequent hepatic ketogenesis.

Fasting Tolerance ∞ Area

Fasting Tolerance

Meaning

Fasting Tolerance is the physiological capacity of an organism to successfully transition to and maintain metabolic homeostasis during periods of caloric deprivation. It reflects the body’s adaptive ability to switch from using glucose as its primary fuel source to relying on stored fat and the production of ketone bodies. High tolerance is clinically characterized by stable blood glucose levels and minimal subjective distress during the fasting period. This capacity is a key indicator of metabolic flexibility and insulin sensitivity.