Summary
When starting intermittent fasting or fasting in general, it is normal to experience a drop in core body temperature — not just cold extremities. This is part of the body’s adaptation process and does not indicate a problem with fat burning or thyroid function. The sensation typically improves as the body adapts over time.
Key Takeaways
- Feeling cold on keto or intermittent fasting is a normal adaptation response, not a sign something is wrong.
- Core body temperature actually decreases during fasting, not just in the fingers and toes.
- Cold sensation does not mean you are burning less fat — fat burning continues regardless of body temperature.
- Taking a cold shower can actually increase fat burning, disproving the idea that heat is required to burn fat.
- Vitamin B1 (taken as nutritional yeast for a natural source) can help reduce the intensity of cold feelings during the transition.
- The cold sensation improves over time as the body adapts to fasting.
- There is no negative effect on thyroid function from the temperature drop during fasting.
Details
Why Your Body Gets Cold During Fasting
When you begin intermittent fasting — especially in the early stages — your core body temperature naturally decreases. This is part of the metabolic adaptation process the body undergoes when food intake is restricted. It is a systemic response, not limited to poor circulation in the extremities.
Fat Burning Is Not Heat-Dependent
A common misconception is that the body needs to feel warm or “fired up” to burn fat efficiently. Dr. Berg clarifies this is not the case. In fact, cold exposure — such as taking a cold shower — can actually increase the number of fat calories burned. Temperature and fat metabolism operate independently of each other.
How to Reduce Cold Sensitivity
For those who find the cold sensation uncomfortable during the adaptation phase, vitamin B1 is recommended. The preferred form is nutritional yeast, which provides B1 in a natural, whole-food form rather than a synthetic supplement. B1 supplementation can ease the transition and make the adaptation period more comfortable.
What to Expect Over Time
The cold feeling is temporary. As the body becomes more efficient at running on fat for fuel, the temperature regulation improves. Dressing in warmer clothing during the initial adaptation period is a practical short-term strategy.
Thyroid Function Is Not Affected
The drop in core temperature during fasting is not a sign of thyroid dysfunction. It is a distinct physiological response tied to the fasting adaptation process, and should not be confused with symptoms of hypothyroidism.