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For me, this went away after a fortnight or so. I also find that if I fall off of my routine, when I restart with a decent fast (72 hours or so) it eliminates this adjustment period. Kicks the body into keto

You may not be getting enough electrolytes, especially salt. Try drinking broth, taking a salt tablet, or adding some salt to your water. For me, the brain fog lifts within half an hour of taking salt.

Also, take multivitamins when dieting.

It could be the water is helping, and not the salt.

Dieting tends to dehydrate me. Burning fat releases a boatloat of stored toxins, and does a number on the kidneys, etc.

not an expert but could be that you're eating too much sugar before fasting, or entering ketogenesis (it goes away if that's the case)