Feeling a persistent chill when abstaining from food is a common experience during periods of voluntary food restriction. This sensation is a direct result of the body implementing an energy-saving survival strategy. When fasting, the body interprets the lack of incoming calories as a signal of scarcity and begins to conserve internal resources. The resulting drop in body temperature and increased cold sensitivity is a normal, temporary physiological adjustment to this shift in energy availability.
How the Body Changes Energy Sources
The sensation of cold begins as the body transitions away from its primary fuel source, glucose derived from recently eaten food. During the initial hours of a fast, the body first utilizes its stored form of glucose, known as glycogen, primarily held in the liver and muscles. Once these glycogen stores are significantly depleted (typically taking around 12 to 24 hours), the body must look elsewhere for energy.
The system then initiates lipolysis, the breakdown of stored body fat into fatty acids to fuel most tissues. Concurrently, the liver begins ketogenesis, converting these fatty acids into ketone bodies that can be used as an alternative fuel for the brain. This switch from a glucose-dependent state to a fat-burning state represents the body prioritizing energy efficiency. This metabolic change sets the stage for adjustments in heat production.
The Link Between Metabolism and Body Temperature
The primary reason for feeling cold during fasting is a deliberate reduction in the Basal Metabolic Rate (BMR), the energy required to keep the body functioning at rest. The body’s internal heat generation is a byproduct of metabolism, and slowing down the BMR reduces energy expenditure. Fasting can significantly reduce the Resting Metabolic Rate (RMR) as an adaptive response to caloric restriction.
This reduction is part of a broader energy-saving response known as adaptive thermogenesis, where the body decreases its heat output. The heat generated by metabolizing stored fat is less immediate than the heat produced from processing carbohydrates, contributing to a lower overall thermal output. Furthermore, the hypothalamus-pituitary-thyroid (HPT) axis downregulates during fasting, leading to a decrease in the active thyroid hormone triiodothyronine (T3). Since thyroid hormones regulate BMR and non-shivering thermogenesis, their reduced levels directly contribute to the feeling of cold.
Another mechanism involves the circulatory system, which redistributes blood flow away from the extremities toward the core organs to conserve heat. This survival mechanism maintains the temperature of vital organs but results in a noticeable sensation of coldness in the hands and feet. The sympathetic nervous system (SNS), which normally stimulates heat production, experiences suppressed activity during fasting, further dampening the body’s ability to generate warmth.
Why Some People Feel Colder Than Others
The degree to which a person experiences cold sensitivity while fasting is not uniform and depends on several individual factors. The duration of the fast plays a role, as the metabolic slowdown and hormonal changes become more pronounced during longer periods of abstinence (e.g., a 48-hour fast compared to a 12-hour fast).
Body composition is another variable; individuals with a lower body fat percentage have less natural insulation, making them more susceptible to external cold. The magnitude of the adaptive thermogenesis response also varies between people, meaning some individuals naturally reduce their BMR more dramatically than others under the same conditions. Pre-existing nutritional status can also be a factor, as deficiencies in minerals like iodine, selenium, or zinc can negatively impact thyroid function, which is already suppressed during fasting.
Simple Strategies to Manage Coldness
While feeling cold is a sign that the body is effectively adapting to the fasted state, there are practical steps to mitigate the discomfort. One effective strategy is to drink warm, non-caloric fluids, such as herbal tea or vegetable broth. Consuming warm liquids helps to raise internal body temperature from the inside out.
Wearing warm layers of clothing, especially covering the extremities, helps to counteract the blood flow redistribution that causes cold hands and feet. Light physical activity, such as a short walk or performing household chores, can generate metabolic heat and temporarily increase core temperature. Taking a warm bath or shower provides immediate relief by introducing external heat to the skin and increasing overall comfort.