I saw another post in the message boards from 2009 asking about the claim of a 600-650 calorie loss from donating a pint of blood - basically the question was if it was true and HOW it would result in such a loss.
I donated blood today and have searched the net about this. I can find no credible resource that supports this claim today. Mayo no longer has it in their FAQs, nor does the American Red Cross. I asked the ARC nurses and they referred me to the national number. In calling that number the person on the other end of the line gave me this is the info:
Every person is different. While it is possible that you may lose calories by donating blood, it is not recommended that it be used as a weight loss or diet regulation tool. Continue to eat a healthy diet. If you are restricting calories, it is not recommended that you go below that calorie range on the day that you give blood. Nor is it recommended that you overcompensate for the potential calorie loss.
Still no word on how this could occur, other than you are taking a form of mass from your body in addition to expending energy over time in the process of regenerating the blood. I also have a question posted on WebMD in case an expert happens to answer. It would seem to involve the knowledge of dieticians and biochemists. Nonetheless, the info I've gotten seems reasonable enough to me.
Personally, 600ish calories is about the equivalent to a workout on the elliptical for me and considering they don't recommend I do a workout like that today I think it all evens out. Did I eat their cookies? Heck yes I did! Is it license to go off the deep end for the rest of the day, week, month.... Nope!
I hope this helps get some good info out there so that you all can make healthy decisions too!