So I just went and donated blood again and durring the recovery period it occured to me that it takes quite a bit of work for your body to regenerate that lost blood volume and the actual blood cells. Regrowing that many cells seems like it would be fairly energetically intensive. So how many calories does producing all those new blood cells actually consume? Is there even a way to know that?
Eat the snacks before donating. That way your blood glucose spikes and you ‘burn’ maximum calories.
Grow a wizard beard. That way you have crumbs and can eat while donating too.