From what I know, after someone gives blood they’re offered cookies or juice. What’s the point of this? Does raising blood glucose in some way ameliorate the reduced volume of blood now in the body?
Just a guess:
Less blood means less glucose going to the brain. Hence, people can feel faint.
Eating a cookie would help get that glucose up, at least to prevent a big downward shift.
That’s about it. Any thing to raise the blood sugar level to make up for that lost in the blood donation.
Back when I worked in a blood donation center, it was really the juice we were concerned about getting into someone, not so much the cookie.
Assuming a healthy human being, the glucose loss in the that pint of blood is something the body can easily handle. What you want to do is get the blood VOLUME up again, which you do by drinking fluids - such as the ubiquitous blood donation orange juice. The cookie is, in part, a delaying tactic - it helps convince the folks to sit down (which cuts down on the fainting and allows the body a little more time to adjust to the blood loss) and drink their fluids (gotta wash the crumbs down).
We had some regular donors who’d bring their own food, which certainly didn’t have to be cookies. Some brought their own preferred drinks, too. They use cookies on the general public mainly because just about everyone will eat them, they can be left on a plate for awhile without going bad, and are individual serving size without the need for cutting tools and the like.
So my own preference for cookies plus water is OK? Some of the blood donation ladies won’t let me have water, and try to make me drink OJ or apple juice in the belief that I need the sugar. I’ve always figured that I’m getting enough fluid from the water and enough sugar from the cookies.
Some folks back home talk about offers of stout being given after a donation on the “pint for a pint” basis. Is this just nonsense, would having an alcoholic drink after a donation be good/bad/indifferent for a person?
In the US, the American Red Cross SPECIFICALLY instructs you NOT to drink alcohol for a period of time following the donation, they suggest several extra glasses of nonalcoholic liquid.
That said, (and I am certainly not advocating such behavior), I’ve no problem with having a few cocktails after donating, as I’ve never had any issues with the donation process, or the blood volume makeup (5’9 230#, so I’ve plenty of volume). I do drink the extra fluids, but a few drinks following the donation is so much more “efficient” compared to my “full blood volume” normally. 
There are many folks out there that do have problems with the donation process, so certainly the advice they give is good advice. I just like to live dangerously!
-Butler
Yeah, that should be OK.
My problem is that I’m allergic to orange juice (among other things). When I donated I usually brought my own grapefruit juice along. But there were a couple times when I just had water. Or had water in addition to juice. Really, the important thing is fluid - except things like alcohol and caffeine which can interfere with fluid uptake. I still drank tea after donating, but I made sure to drink a lot of non-caffeinated fluids in addition. A single alcholic drink later in the day is unlikely to harm you - but it could make you considerably more intoxicated than usual. Heavy drinking is definitely not a good idea, nor would mixing drinking and driving - which isn’t a good idea at any time. You’re better off skipping alcohol entirely for a few days after donating.
A lot depends on the individual person. I never felt lightheaded, faint, or queasy after donating. Some people do. As a general rule the bigger/heavier you are the less likely you are to have problems, but we had 90 year old ladies barely able to make the lower weight limit who practicaly tap-danced their way out the door and six foot four 200 lbs young male athletes who’d pass out two or three times during the donating and immediately afterwards.
For most healthy adults you could probably get away with just resting for a bit before resuming sedentary activities without the snack… it’s just that the donation folks are erring on the side of caution. You really do need to replace that fluid, so they’re giving you a head start on the process. Donating blood does put some stress on the body. If you’re healthy it’s not a problem, but it still is some stress and there’s no point in adding more stress on top of it.
Where I work, when they have the blood drives they also offer raw vegees and fruit, paid for the by the company. Nothing wrong with good nutrition, either.
Remember that many times the folks working the cookie table are volunteers, and not necessarially medically trained at all. They are just as likely to have odd notions as anyone else.