It never occurred to me that it could be the source of fuel that causes the weakness. I have always assumed it was the lower blood pressure/slowing metabolism.
I just did a search to find more info on this aspect of Water Fasting but was unable to find anything specific. Dehydration and low salt levels are the main causes of low blood pressure when fasting.
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Symptoms of low blood pressure: excessive sweating, weakness, tiredness, lack of energy, dizziness, especially on getting up from sitting position, pale appearance and feeling faint. Causes: Too little fluid intake decreased salt intake.