Hello,
I wonder where the headache from fasting came from. I mean what the biological process behind it.
A guy told me it happen when the digestive system is cleaning himself.
I feel sometimes the same sensation when I’am doing cardio sport. And I wonder if it’s related with low suger in blood?
My guess : Changes in blood pressure and blood vessels due to loss of electrolytes.
When fasting, the body will start burning its glycogen (aka stored glucose) first. Glycogen is stored with water. Burning glycogen releases this water and electrolytes with it, especially sodium.
The common answer for water fasters suffering from headaches is to supplement with electrolytes.
Low blood sugar can cause headaches, but is usually accompanied with other symptomes : cold sweats, anxiety/panic attack, blurry vision, dry/tingling mouth, shaking or nausea. If you are only getting a headache, salt/sodium is the more likely cause
On a related note, caffeine restricts blood vessels. The headaches from caffeine withdrawals comes from relaxed blood vessels, which messes with blood pressure.