The key is to already be in ketosis, so you experience zero carb withdrawal "keto flu" symptoms or else you'll feel like shit most likely 2 out of the 3 days you fast. If you're already in ketosis prior to doing a fast you'll feel like you're on rocket fuel, your ketone levels will be optimal, you'll feel crazy mental focus like you've never felt before, I've swam for 4 hours straight once during a fast just because my energy levels were through the roof.