To keep the take-aways short, here's what I found:

1. Do not attempt to do any sort of exercises that require real exertion (slow walking is fine) while doing fasting of any kind, and I would (given my experience with ketogenic diets) extend this to keto as well, especially when lifting weights. Despite what some liars and/or freaks will tell you, you cannot be fasted or ketogenic and maintain quality performance.

2. The 3 days are probably fine if you do them with a couple of months in between if you have other goals you're working on (like building muscle). Doing them every week will minimize any other fitness goals that you may have.

3. The 5 days and over left me feeling tired, but the tiredness would come in hard waves. For instance it was not uncommon to want to immediately fall asleep at random hours of the day only to feel wide awake at 2am the next day. Just being hungry messes with your sleep, and I suspect (but have not confirmed) that it probably messes with your circadian rhythm.

4. I lost more cumulative weight running two 3-day fasts than I did with one 10-day fast. This was confirmed against both of my 10-days. Most will attribute this to water weight, however in the +5-6 days after the fast ended I was still lighter with the cumulatives.

5. There were only a couple of times where I got that euphoric feeling that many claim to get, and that was only in the first two fasts I did. All subsequent fasts did not produce this.

6. In the last 5-day+ fast I did I experienced some weird neuropathy in both legs and hands, despite being fully hydrated and taking electrolytes. I immediately broke fast and felt better and have not experienced any neuropathy since.

7. I no longer do long term fasts, instead opting for intermittent fasting (16:8) daily for several reasons, coupled with not eating bread or pasta. This seems to have much better and more sustainable results.

The only really interesting thing to say about it is that by day 3 I had entered the calmest period of my life. I'm prone to depression and anxiety, have meditated extensively, weaned off coffee...whatever you can think of apart from hard drugs I have done. Never felt so at peace looking at a wall for hours, total absence of intruding thoughts. Just immersed in a warm pool of grey space.

Only stopped at 5 days because it was affecting my sex life. I know people who have fasted for a month, no big deal once you get going.

To directly answer your question, I didn’t talk to a doctor ahead of time, but I did make sure to drink lots of water during that period.

I remember the hardest day was day 2. For the first day, you “feel” hungry as your body is accustomed to eating, but it is day 2 you are genuinely hungry. However, per the article, after about the third day you simply don’t notice yourself being as hungry anymore.

I noticed then (and notice now when I fast) positive effects on mood, and ironically energy.

The only drawback I have noticed is that the first meal after a fast, especially if a large carbohydrate based one, seems to have outsized effects on drowsiness.

Overall I’ve only been able to do longer fasts when OOO as my cognitive motivation is so low the first few days. Keeping just with tv shows / exercise / nerding out also makes a big difference when OOO and not noticing being hungry cause of work stress and reflexively wanting a snack as motivation to do more work.

24 hours isn't two bad. The second 24 hours is awful. That's when the headaches come. The third 24 hours your body starts stabilizing and it's pretty doable from there. Friends who went longer said it gets bad again in the second week.