The law says that the jailing cannot be to punish you but rather to get you to talk to the grand jury. In legal terminology, this means that the intent of the jailing must be coercive rather than punitive. This distinction matters very little when you are actually in jail. If you are a vegetarian or a vegan, request the kind of food that you can eat. Put your request in writing.
Many jails now provide vegetarian meals.
Be prepared to be harassed by the guards or other prisoners. So beware of talking to people that you didn't know from before. You are doing a brave act. You are there because you believe in a cause, and you have not been forgotten. Many people are working to get you out, so stay strong.