Why Fasting Can Make You Feel Cold
Experiencing body chills or a general sensation of coldness during fasting is a well-documented phenomenon. It is not a sign that something is wrong, but rather an indication that your body is undergoing a series of planned metabolic changes to adapt to a reduced caloric intake. The primary goal of these adaptations is to conserve energy when external fuel from food is not available.
The Metabolic Slowdown
When you stop eating, your body no longer needs to expend a significant amount of energy on digestion. This process, known as the thermic effect of food, is a source of internal heat. Once digestion stops, this internal heat production ceases. Simultaneously, to conserve energy, your basal metabolic rate (BMR) decreases. Your body essentially shifts into an 'energy-saving mode,' which results in lower overall heat generation. This decrease in metabolic activity is a direct contributor to a drop in core body temperature and the sensation of feeling chilly.
Shift to Ketosis and Fat Adaptation
As a fast progresses and your body depletes its glycogen (stored glucose) reserves, it transitions to burning fat for fuel, a metabolic state known as ketosis. This shift in primary energy source has several implications for body temperature. While the body becomes very efficient at utilizing ketones for energy, this initial adaptation phase can sometimes lead to reduced thermogenesis, or heat production, from cellular mitochondria. This can make you feel colder, particularly in the early stages as your body is not yet fully accustomed to using fat as its primary fuel source. For many, the cold sensation diminishes as metabolic flexibility improves with continued fasting.
Hormonal and Circulatory Changes
Several hormonal shifts occur during fasting that influence temperature regulation:
- Thyroid Hormones: Levels of certain thyroid hormones, which regulate metabolism, can fluctuate during fasting. This can affect your metabolic rate and, consequently, your body's ability to regulate its temperature.
- Stress Hormones: Increased levels of norepinephrine and glucagon signal the body to mobilize stored energy, but they also impact temperature regulation.
- FGF21: Changes in the hormone FGF21, which plays a role in energy balance, have been linked to increased cold sensitivity during fasting.
Additionally, during fasting, blood flow is often redirected away from the extremities (hands and feet) toward vital organs to keep the core body temperature stable. This redistribution of blood can make your hands and feet feel particularly cold.
Low Blood Sugar (Hypoglycemia)
For some individuals, especially those new to fasting, blood sugar levels can drop low enough to cause hypoglycemia, a common symptom of which is a cold sweat or feeling of chilliness. While this is a normal adaptive response, it can be unsettling. It is more common with more restrictive fasts and generally improves as the body becomes more efficient at using ketones for energy.
Practical Ways to Counter Fasting Chills
- Increase Warm Fluid Intake: Drinking plenty of warm, non-caloric beverages like herbal tea or black coffee can help raise your internal temperature and promote circulation without breaking your fast.
- Layer Clothing: Dressing in layers gives you the flexibility to add or remove clothing as your body temperature fluctuates throughout the day. Wool, silk, or polypropylene are excellent materials for retaining heat.
- Incorporate Light Activity: Gentle exercise, such as walking or stretching, can stimulate your metabolism and increase heat generation without overexerting your body.
- Stay Hydrated and Mind Electrolytes: Proper hydration is critical for temperature regulation. Imbalances in electrolytes like sodium, potassium, and magnesium can also affect your body's nerve and muscle functions, which are critical for maintaining warmth. Ensuring adequate intake during your eating window, and potentially supplementing during a prolonged fast, can be helpful.
Fasting Stages and Thermoregulation
This table outlines the differences in thermoregulation during the two primary stages of a typical fast.
| Feature | Early Fasting (Glycogen Depletion) | Fat-Adapted State (Ketosis) |
|---|---|---|
| Primary Fuel Source | Stored glycogen (glucose) from the liver. | Fat stores, converted into ketones for energy. |
| Metabolic State | Body starts slowing metabolic rate to conserve energy as fuel becomes scarce. | Body becomes more efficient at utilizing ketones, but overall heat production may still be lower than when consuming calories. |
| Body Temperature | May drop noticeably as the thermic effect of food ceases and BMR slows. | Remains slightly lower than during a fed state, but the body learns to compensate. Extremities may still feel cold. |
| Shivering Risk | Higher risk, especially if blood sugar drops too low. The body might shiver to generate heat. | Lowers as the body adapts to fat-burning. The cold sensation is more of a persistent chill than intense shivering. |
Is it Unhealthy? What to Look For
For most healthy individuals, feeling cold while fasting is a temporary and normal response. It simply means your body is adapting to a different energy source. However, there are instances where persistent cold intolerance could signal an underlying issue that requires medical attention. These include nutrient deficiencies, low thyroid function, or anemia, which can be exacerbated by fasting. If you experience any of the following, consider consulting a healthcare professional:
- Prolonged or severe cold intolerance that does not improve over time.
- Extreme fatigue, dizziness, or loss of concentration.
- Significant and persistent mood changes, such as irritability or anxiety.
- Sudden or dramatic weight loss, especially of muscle mass.
- History of disordered eating or other pre-existing medical conditions.
Conclusion
Yes, it is entirely possible and common for fasting to cause body chills. This occurs as a natural physiological response where the body slows its metabolic rate and shifts energy sources to conserve fuel during periods of caloric restriction. Understanding this process allows individuals to manage the symptom effectively through strategies like staying hydrated, layering clothing, and engaging in light exercise. While a temporary chill is normal, paying attention to your body's signals and consulting a medical professional if you have persistent or severe symptoms is important to ensure your fasting practice is safe and healthy.
Disclaimer: This content is for informational purposes only and does not constitute medical advice. Consult a healthcare professional before starting any new diet or fasting regimen, especially if you have pre-existing health conditions.
Authority Link
For more detailed information on the metabolic pathways involved during fasting, you can refer to the National Center for Biotechnology Information (NCBI) for peer-reviewed studies on the topic: Physiology, Fasting - NCBI Bookshelf.
