I would drink water for 5 days. It's not too difficult. The hardest part is the boredom. I'm quickly reminded how much of my life revolves around food. Farmers market, grocery shopping, happy hours, cooking dinner, work lunches, events, etc. You suddenly have all this free time. For me, I just play a lot of golf to take my mind off of it.

If you want an easy version, look up fast mimicking diet by Dr. Longo. Prolon sells premade kits.

This was my experience as well. The longest I've gone on a water/coffee fast is 7 days, and after the initial 24 hours of hunger subsides, the biggest factor was psychological. I learned that I often use food to relieve boredom, and it's honestly a struggle for me to find a replacement activity which is equally satisfying.

What do you do to get back to eating food normally?

How do you mean? I just start eating as per usual. I often find that the portion I need to feel full is reduced, but I eat the same foods I did before the extended fast.

Have you tried fasting for 20 hours and only eating during a 4 hour window? That seems to work well for me. The first few days I was hangry but pretty used to it now. Biggest change in my lifestyle was switching to black coffee in the morning as you can't consume any calories during fasting. This is the paper that inspired me to do this:

https://www.nejm.org/doi/full/10.1056/nejmra1905136

How far did you get?

In my experience, the first day is by far the most difficult. I find that it's most effective to start fasting on days where I am going to be occupied with something, since I don't think about food when I'm active.

I'd be miserable if I attempted to fast on a day where I was lounging around watching TV.

Second day is usually where it falls apart for me. Part of the issue is the rest of my family is not exactly helping ( 'hard to abstain from sweets when you are knee-deep in 'em' type situation ).

I will be trying again soon, but this time around I think I will need to properly plan it ( initial attempts were basically spur of the moment things ).

I have the same question. Specifically, for a fast lasting longer than 24 hours, is any supplement taken? What side effects occur?

I did Paleo in 2012 for several months with success. However, I have tried Keto/low carb again several times with less success. Specifically with Sated meal replacement. I get muscle cramps and other side effects that I assume are due to changes in the diet.

I completed a 7 day water fast about a month ago. For the first time in my life managed to break 70kg/154lbs barrier(30M,5'5) lost about 3.5kg/7.5lbs during that week. First day or two are tough due to cravings, best way to counteract that is to supplement with multivits, electrolytes and nutritional yeast, dissolved in water(for vitamin B). Once ketosis is in full gear, you will get used to it (around day 3) and feel an amazing euphoric energy. Sleep is another thing to factor in, sleep quality will be impacted during the first couple of days so brace yourself.

Also be careful when refeeding[1], I had bone broth to wake my digestive system slowly, then moved onto complex food within hours.

Would I do it again? absolutely!!

[1]https://en.wikipedia.org/wiki/Refeeding_syndrome