I gave blood today, and as I was sitting there when the vampires were taking all my blood, I had a thought cross my mind…
How many calories does the body use when you give a pint of blood? I figure you would loose calories from sugar in the blood you give, plus the extra calories required to make the blood in the first place.
So, how many?
I have also wondered about this but never took the time to do the calculations. As I have a test tomorrow, this seems like a good way to put off studying.
A unit of blood is 450 mL.
A typical blood glucose would be 100 mg/dL, so a unit would contain 450 mg of glucose. The energy density of carbohydrates is 4 kcal/gram, so that is 1.8 calories.
For the protein content of the red blood cells, I’ll just use the hemoglobin since that is by far the most common protein. A normal hemoglobin concentration would be 15 g/dL of blood, or 67.5 grams in a unit of blood. The energy density of protein is also 4 kcal/g, so hemoglobin contributes 270 calories.
Blood plasma is approximately 55% on blood volume. Total serum protein is 60-80 grams per liter of plasma, so that contributes 17.325 grams of protein per unit of blood or 69.3 calories.
Normal serum triglyceride (fat) levels are under 150 mg/dL serum. At that level, a unit of blood contains 371.25 mg of triglycerides. Since the energy density of fats is 9 kcal/gram, triglycerides contribute 3.34 calories.
I can’t find a figure for the energy density of cholesterol, but I imagine its calorie contribution is less than that of triglycerides. I’m also ignoring the contribution of the lipid membranes of the red cells, which is probably negligible as well.
Adding those figures up (and assuming I got all my calculations correct), I get 344.44 calories per unit of blood.
In this column Cecil gives a value of 600 calories per unit, so I’m not sure who is correct. It is possible that somebody has measured it but I cannot find the source. I am also having trouble thinking of a source for the other 250 calories for which my calculations have not accounted.
Here’s an old thread on the question. I think you’ll be interested in my post there.
You may be down a few hundred calories when you get off the table but you’ll probably put them all right back on before you walk out the door since you’re supposed to sit down and have some fruit juice, cookies, crackers…
Hmmm, my guess would be even higher than Hirundu82’s figure. “Completed” proteins that your body has already worked hard to produce isn’t nearly the same thing as a hamburger headed into your stomach for digestion in terms of caloric potential. So, if you were to eat that blood it might be worth 600 calories, but to create it, I bet you put in significantly more work.
Also, in the old thread, KP argues that your body doesn’t compensate for the blood loss by increasing blood production, but I would disagree. I don’t have time to pull cites right now, but I’m pretty confident that a major loss of blood volume will be compensated for by increased EPO and in turn more RBC production.
So, if you only eat a cookie, I still think blood donation would take a significant chunk of calories.