try electrolyte supplement bulk powders do 1 1g serving and is like 100g so 100 servings are you eating or taking anything else while fasting?
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it might be keto flu aswell so its something you just hae to ride out what makes it easier if before fasting you go on keto diet
Maybe you could try this more gentle way of fasting if you have a glucose monitor. You only eat when your blood sugar reaches a target point. So you would track your average over a day and then set a trigger level to eat a meal. Looks like they’ve set it 0.5 below the daily average for this example person in this article. So you fast until you reach that target. Then eat. Then fast again. You gradually reduce your target over the days. https://optimisingnutrition.com/how-to-use-your-glucose-meter-as-a-fuel-gauge/