Er? i assume looking at the time, you have already done this, i would say you should up your fluids and i would say have something more today/tomorrow. Giving blood can kickstart weight loss anyway and really its not about today, as you will give you 450ml of blood today but it will be the next few days when your body is going to be replacing that blood volume properly and i would suggest that whilst you might have the reserves to replace the calories from "stores" you might struggle to supply the other components that your body needs to refill that system.
Though on a personal note, well done with the blood giving! something like only 1 in 25 ppl do it, 4%! and yet we all expect there to be enough blood for operations and for emergencies so full credit for being one of the few
