Water fasting. And I suggest to start anywhere from 10-12 hours and increase slow from that. But you can also consume plain tea or just plain black coffee. Juice fasting is not technically 'fasting' since it will provoke an insulin response via glucose intake. We want to keep insulin very low here and have glucagon active (opposite action to insulin). Please note that you might not feel optimal for a week or more on a fast since the body needs some time to adapt. Warning: This should be done by otherwise 'healthy individuals' with no recognized illnesses.