Wednesday, September 15, 2010

Calories in Human Blood

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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:

Blood tests always include hematocrit - the percent of blood that is cells versus the percent that is liquid, determined by spinning a tube of blood in a centrifuge to pack the cells to one end. Whatever percentage that is not packed cells is plasma. Men's blood has a lower plasma content (55%) than women's blood (60%) because men have more red blood cells, i.e., a higher hematocrit than women.

First calculate calories for plasma. Blood is denser than water by six percent, so multiply that 500 ml donation times 1.06 to get to grams (because calorie content will be calculated from grams). Multiply that by 55/100 for men or 60/100 for women to get to grams of plasma. Now, multiply that by .07, because plasma is only 7% protein, to get to grams of protein. Finally, multiply that result by four to get protein calories. Answer in the 80’s? Good.

Now for sugar in plasma. Normal fasting blood sugar is around 0.09 grams per 100 ml of plasma (more familiarly expressed as 90 mg/dl). Multiply the amount in grams time 2.75 for men or 3.00 for women, i.e., equal to how many hundreds of mls of plasma there are in 500 ml of blood, and then times four to get to calories. Result should be one calorie. Blood is not sweet!

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.   

For plasma calories from fat, use 0.5 grams per 100 ml of plasma, times hundreds of mls of plasma, times nine - for calories from grams of fat - to get to calories. The 0.5 g/100 ml covers triglycerides and fatty acids in lipoproteins but not cholesterol, as cholesterol is not metabolized for energy. Result is roughly 12 calories.

Next comes calories from red blood cells. RBCs are mostly water and protein, the protein almost entirely hemoglobin. Hemoglobin does not conform to the rule of four calories per gram because the heme portion of the molecule is not metabolized for energy. Use average hemoglobin values of 160 g/L for men or 140 g/L for women, multiplied by 0.5 to get to the blood donation volume of 500 ml, multiplied by 0.96 to correct for using only the non-caloric heme portion of the hemoglobin molecules, then multiplied by four to get to calories. But wait! Hemoglobin is only 90% of the dry weight of RBCs. Multiply one-tenth the amount of the hemoglobin weight in 500 ml times six (see below for why six) and add that to red blood cell calories.

Finally, estimate calories for the other types of cells. Of the 500 ml blood donation, roughly one percent will be white blood cells and platelets. Figure five grams, times one-third to get to dry weight, times six, to get to an estimated 10 calories. The "times six" is because some of the dry weight is protein or carbohydrates, which both have four calories per gram, and some is fat, which is nine calories per gram.

Add everything up. The total should be approximately 425 calories for women and 460 for men. Anemia would result in a lower value. Use of EPO or NESP, drugs which unethical athletes can use to increase their red blood cell count and thus their oxygen transporting capacity, would result in a higher value.

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.   

BLOOD AS FOOD: Blood, human or otherwise, can not be considered a nutritionally complete or balanced food for humans, as it is lacking in essential fatty acids and sufficient carbohydrates, has too much protein, and does not provide sufficient vitamins, minerals or fiber. Vampire bats get by, but to do so they have to consume more than a third of their body weight in blood every night. And pee a lot. As for vampire calorie requirements, if vampires have the same caloric needs as (living) humans then it would take five units a day (a night?) to keep a vampire in caloric balance. But given that vampires have no resting metabolism calorie requirements (they’re dead, remember?) calorie needs are probably much lower.

Non-human blood is featured in many traditional dishes from around the world. Blood sausage combines blood with grain and spices. Note that boudin blanc the popular Cajun sausage uses pork livers and hearts, rice and spices, but not blood. The boudin noir version uses blood. Blood is used as a soup ingredient in many cultures. In Eastern Asia, blood is allowed to congeal in shallow trays and then cut into squares. This is called “blood tofu” and used in many stir-fry dishes.

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.