After 4 or so days without carbs your body will go into ketosis, which is when it starts using your stored fat for energy. However, it takes until about the 6th or 7th day of no carbs before your body has burned enough fat to make your body feel normal. Days 4-6 are absolutely awful. Terrible headaches, feeling sick, having no energy. No one will tell you the no carb diet doesn't work, though. It's a great short term diet.