I fast, not intermittent fasting but real fasting, 48 hours or 72 hours or longer depending on social obligations, during parts of the year. I might be the only one who does this so I hope my testimony helps you out.

During these fasts I have water and nothing else. If I feel inclined to exercise or if I'm peeing excessively I will take electrolytes in the form of powder in water, but that's it. In the morning I'll have black coffee. So technically this is called a "dirty fast".

Initially it sucks. The first day in particular can be a bit hazy. Tip of tongue syndrome, tendency to hyper focus on a single thing, etc. It is objectively terrible.

After about 24 hours it becomes a very smooth feeling and you feel mentally agile. It might be placebo but it feels like more mental agility than when not fasting.

After about 48 hours it becomes peaceful on top of all that. Forgiveness comes easier, anger comes slower, you will feel like you should do less harm, things like that. It's a bit of a trip. Hunger is still there, might even be roaring with hunger at this point. Sense of smell is greatly sharpened.

After 72 hours you no linger are hungry. It's scary. There is just no desire to eat other than a psychological worry that you're missing whatever vitamin you're recently obsessed with. You might ask yourself "what's happening to my nicotinamide riboside reserves" and other such questions. However, physiologically there is not much hunger at all, and it is a bit scary because you know that you need food so it's strange and makes you mistrustful of your impulses to experience a disconnect between instinct and need.

Beyond 72 hours it stays the same, you've evolved beyond needing food and bowel movements and are closer to machine, you can get a lot done and you will have more time than you know what to do with. If you're someone who enjoys indulging in earthly pleasures you might be extremely bored.

Disclaimer the following contains information about fasting, it is not qualified medical advice as I am not a doctor nor am I claiming that it is safe:

My "advice" to someone who has just started is to take it slow, primarily for medical reasons. Electrolytes aren't exactly safe to tinker with. If you have a heart condition I would say straight up don't do any extended fasting. However if you want to know how it feels like to fast then work your way up to an extended fast, try an all day fast then resume regular eating for 3 days then try again. Then work up from there. Don't become obsessive about whether one calorie from sugarless gum breaks a fast, focus on the discipline required to fast through meals and let that discipline strengthen other aspects of your life. Also, take a very thorough accounting of your own health. If fasting causes you to lose minerals, you should be able to notice it right away. Take careful care of your teeth and nails so you can see any potential warning signs before they become issues. If anything goes wrong just resume eating, there is always tomorrow. And remember that water is your friend but if it is absorbing too much electrolyte out of your body then use powdered electrolytes to stay balanced.

Lastly, in general, and in stark contrast to the above, I do NOT recommend fasting to most people unless they hunger for a challenge that requires immense self-discipline. You can get plenty of metabolic health benefits from IF or OMAD without nearly as much discomfort. You can say "I'm fasting" and simply skip breakfast or dinner. It's almost a no-brainer especially if your only goal is weight management.

Source: me about to be 72 hours fasted