It depends. If youre talking about dryfasting, its normal to feel a bit off.
How long are you into the fast?
If youre a beginner, dont force it if you really feel awful. And no you should never keep pushing if you feel extremely awful.
What are you feeling exactly?
If you decide to quit, end the fast healthily and try again in a week! Dont go back to eating shit directly, that probably will destroy your goal for dryfasting in the 1st place.
If it’s mild I continue so I can tell my body its ok to be a bit thirsty or hungry, don’t make a big deal out of it. This way it gets accustomed to it and can go longer each time. I stop when it’s more than mild though, the thirst is huge or the hunger is hitting different then I stop.
This method let me go 5 days dry fasted with zero to mild discomfort.
Either a very fast heart rate or a very slow heart rate. Before you start the fast get a baseline. When sitting around at your computer posting to Reddit make sure to measure your heart rate.
If your HR goes plus or minus 25% from that baseline for the same activity level, break the fast. For example, if it’s 75 beats per minute when not fasting but is 75 + (25% 75) = 94 or higher then break it. Or if it’s slower than 75 - (25% 75) = 56 then break it.