Feeling cold while fasting is a common experience when restricting food intake. A lack of incoming energy triggers complex metabolic and hormonal adjustments as the body prioritizes survival. This cold sensation is a measurable physiological response to energy conservation. The mechanisms behind this chilling effect center on reducing heat production and prioritizing core warmth.
Metabolic Rate Reduction and Energy Conservation
The primary reason for feeling cold during a fast is a deliberate reduction in the body’s overall energy spending, governed by the Basal Metabolic Rate (BMR). The BMR is the minimum energy required to maintain basic functions like breathing, circulation, and cell production while the body is at rest. When the body detects a lack of food, it enters a state of conservation, perceiving a temporary resource scarcity.
This “conservation mode” manifests as a measurable decrease in the BMR, which translates directly to less heat being produced internally. Studies show that the resting metabolic rate can drop significantly during an extended fast. This reduction ensures that stored energy reserves are stretched to last longer, effectively turning down the body’s internal thermostat to conserve fuel.
Fuel Source Transition and Heat Generation
The body’s shift in energy source also contributes to the drop in internal heat production, a process called thermogenesis. Normally, the body relies on glucose, and the metabolic pathways used to process glucose generate a substantial amount of waste heat as a byproduct.
When fasting progresses beyond 12 to 24 hours, the body depletes its glycogen stores and initiates a metabolic shift to burning stored fat for energy. This fat-burning process, which involves breaking down fatty acids and producing ketone bodies, is metabolically cleaner and generates less heat. The pathways for utilizing fat and ketones are inherently more efficient at converting fuel to usable energy than carbohydrate processing, meaning less energy is wasted as heat. This reduction in heat production is a major contributor to the chilled sensation.
Hormonal Regulation of Body Temperature
Metabolic changes are tightly controlled by the endocrine system, which adjusts hormone levels to influence body temperature. The thyroid gland governs the basal metabolic rate and heat production through thyroid hormones. Specifically, the active form of thyroid hormone, triiodothyronine (T3), often shows reduced levels during fasting.
Lower circulating T3 levels signal the body to slow down metabolic processes, which is a direct mechanism for decreasing heat generation. The body’s response to cold is also mediated by catecholamines, such as norepinephrine, which stimulate heat production. During fasting, the activity of these catecholamines may also be reduced, further limiting the body’s ability to generate warmth.
Peripheral Circulation and Fluid Balance
Beyond the core metabolic and hormonal changes, two physical factors exacerbate the feeling of coldness, particularly in the extremities. The body prioritizes the warmth of its vital organs through peripheral vasoconstriction. This process narrows the small blood vessels in the skin, especially in the hands and feet, shunting warm blood away from the body surface and toward the core.
This redirection of blood flow conserves heat internally but leaves the limbs feeling colder. Mild dehydration is also common during fasting if water and electrolyte intake are not maintained. Adequate fluid balance is necessary for efficient blood circulation and temperature regulation, and its impairment can further hinder the body’s ability to maintain temperature.