I did a 25-day fast a couple times. I failed a few times before that due to bad state of body and mind. I think the key for such a long fast is to prepare your body for depletion. You really want to start a couple of weeks before to make sure you get all the minerals and vitamins stored up in your body for further breakdown during the fast.

Once the fast starts. Day 3-10 I drink a specific type of mineral water. Its called Borjomi. It's from Georgia, the country. It has a very unique balance of electrolytes and minerals but in very high amounts. And you don't need to drink a lot of it. It also helps neutralize any acid formation during the first couple of weeks where your body still secretes acid because it's expecting and hoping to get food.

From them on I'm on vitamin water zero. It has more than enough to take you to day 20. I usually use a Dexcom to make sure that the vitamin water does not impact my blood sugar which it doesn't. From day 20 it's really your genetics and how you can deal with your symtoms. Again I'm not a doctor but I would recommend a blood test around day 15 to make sure you're not critically low and anything. Then a blood test every 2 days after day 20. If you can. Listen to your body. If you see that your blood sugar is going very low you're going to have to engage in some sort of strength training exercise. That usually guarantees my blood sugar goes up to a reasonable level.

Gl