I'm no expert but let me give this a shot lol. Ketosis simply means that our bodies are using fat for energy. When you're not eating the body will go straight to the fat reserves for its source of energy since it's not getting any energy from glucose (type of Sugar taken from the foods we eat) so it is necessary to be in ketosis stage when fasting. Ketosis is a sign of fat being burned for energy. That usually kicks in on the 3rd day/takes anywhere from 3 to 5 days to see appetite suppression but can happen sooner or later; really depends on the person. Hope that was a bit helpful. Take care :-)