IF is not a diet as it doesn't dictate what to eat, and how much to eat. IF is simply an eating window that can support any 'typical' diet. The most common forms of IF will be 16:8 (16 hour fast, 8 hour eating window), 20:4 (20 hour fat, 4 hour eating window) or 5:2 (two days per week eating under 500 calories - ouch). The fasted window = no calories. Water is fine and so is any liquid that contains zero calories.