CALORIES HUMAN BLOOD or HUMAN BLOOD CALORIES: The standard U.S. blood donation is 500 ml and weighs a tad under 1.2 pounds. Nutritionists tell us that losing one pound is equivalent to losing 3,500 calories. Does this mean that donating blood is a good way to start a diet? The answer is "No," because blood is mostly water, and thus not 3,500 calories per pound. To cut to the chase - a donation contains roughly 450 calories. There is an energy cost involved in synthesis of replacement cells and non-cellular (plasma) proteins, which means a replacement total would be higher. Details of the math:
As an aside here, for most people only 20 calories' worth of glucose (sugar) is circulating in the blood at any given moment, with the exception of after a meal, when there is a bump up for a few hours.. Muscle and other organs are constantly adding or withdrawing glucose to maintain this constant amount. For people with diabetes, excess sugar is poorly removed from circulation, leaving the sugar molecules to inappropriately attach to proteins, i.e., glycosylation, causing cumulative damage. Measuring blood sugar gives a real-time picture. Measuring hemoglobin A1C - glycosylation of a blood component - assesses long-term control.
All in all, the calories removed in a unit of donated blood are less than the equivalent of one normal-sized meal. The actual energy cost of making new blood will be higher than this calculated calorie content, as synthesis is not 100% energy efficient. But keep in mind that even without a blood donation, the body replaces about 1% of red blood cells every day. So the excess energy needed to replace a donation of roughly 10% of one's blood supply is not large, and takes place over four to eight weeks.
During prolonged vigorous exercise it is possible to deplete the easy to access stores of energy in the form of glycogen - a precursor for glucose - and not be able to metabolize fat or protein fast enough to keep up with demand. The transition from fueled to empty is quick and the consequences dramatic. In the U.S. the condition is referred to as 'bonking,' or 'bonked.' Marathon runners used to call it 'hitting the wall.' When circulating blood sugar drops, the brain, which relies solely on blood sugar, crashes. Recovery is slow, because even if the athlete stops moving and rests, any sugar being made by muscles or liver tends to get used locally before it gets to the brain.
Calvin Trillin, renowned food writer, oft told of his frustration at being in a Chinese restaurant in the U.S. and not knowing what the choices meant on the Chinese-language parts of the menu. When he tried pointing at the entries the waiter typically replied "You no like." So if you see 血豆腐 on the Specials menu, likely, you no like. It translates as pig's blood tofu.