Absolutely Sacristia,
fasting has a habit of resetting and re-balancing the biochemistry of the body, especially metabolism, but for many (including me) it is just getting to that stage of readjustment from glucose-burning to fat-burning that can be the most difficult within the first 3 to 4 days, and the part I hate most of all, as it does affect my emotions and mental outlook.........caused by low blood sugar. Thankfully this is only a temporary stage of the fast.
The problem arises if Hypoglycemia (low blood sugar) is of an ongoing nature in the absence of diabetes and other seemingly endless causes.