cleverjewishpun – I’ll give you my medical opinion, if you do my Simchos for free. Just kidding -unless you really want to.
Ok, First thing -try taking the Kalei Tzom before the fast. After Havdallah, since a lot of the weakness is due to dehydration -the fluids have to be replaced. The best is something like Pedialyte, then things like Gatorade.
What I usually do is have something like Lox & Cream Cheese on a bagel with OJ. The last two fasts, I drank a bottle of Gatorade first. These foods/drinks contain what your body needs – carbs, protein, sodium and potassium.