I have looked into the possible reasons for this in answer to a query from a member and thought it would be worthwhile putting them here in the FAQ.
There are two reasons for getting headaches during the first couple of fasts:
One is not drinking enough. Food contains a lot of water so by not eating we are not taking in as much water. One symptom of dehydration is headache. You need to drink far more than you think in order to compensate for the lack of water you are having in food.
The second reason is low blood sugar. Although the body has a great system for swapping between glucose as a fuel to glycogen stores and eventually to fat stores, when we are overweight the body is reluctant to swap to fat burning (this is called metabolic inflexibility). This is at least partly due to high insulin levels which act to prevent the fat stores being mobilised. Because the body is slow to swap to fat burning, it fails to compensate for the lack of glucose in the blood (by producing ketone bodies through fat burning), the resulting low blood sugar combined with no ketone bodies leaves the brain short of fuel and triggers a headache/migraine. I think this may also be a contributory factor to those headaches that we sometimes wake up with...we all fast while we sleep (unless we sleepwalk to the fridge and have a midnight snack) and often we start fat burning overnight, perhaps when we wake with a headache we didn't manage to make the switch to fat burning.
After the first fast, the body will change more easily to fat burning, and over the next few fasts the headaches/migraines will disappear. Of course this applies to the other low blood sugar symptoms as well.
Ketone bodies, which are produced during fat burning and which the brain uses instead of glucose when glucose is in short supply, have been shown to be very beneficial to the brain and enhance nerve growth as well as damping down over-excited nerves as occurs in epilepsy and in migraine. In fact, before the dawn of anti-epileptic medicine, a ketogenic diet (i.e., low carb/high fat) was used to help control epilepsy.
So to avoid migraines you need either to keep consuming carbohydrates and risk attendant obesity problems or to try to enter a state of ketosis. Fasting will do this, but you do have to weather the migraines/headaches. A possible way to help the transition into fat burning would be to eat a low carb diet at least on the day before a fast and during the fast and to do some exercise while fasting. This encourages the switch.
http://www.fastday.com/fasting/tips-fas ... n-fasting/