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A number of animals, including mosquitos, leeches, ticks and so on, feed on blood. And yet, at least in mammals, most stored nutrients can be found in body fat as fat, muscles and liver as glycogen and digestive system as undigested nutrients, leaving very little in the blood itself. Normal blood sugar before a meal is 70-99 mg/dL, which at the high end means blood contains roughly 3.8 kilocalories per liter. This means that to gather a single kilocalorie from blood glucose, it would take 26 mL of blood, more than any leech could reasonably hold. Estimating the mass of a leech at 20g and scaling down human caloric needs linearly, 2500 kilocalories per 70 kg, yields a leech caloric requirement of 0.714 kilocalories per day. By this estimation, a leech would need to feed at least once per day to meet its energy needs, and yet data from the American Museum of Natural History states that a leech need only feed once every few months. The same math holds for other bloodsucking animals.

So what in blood do bloodsucking animals actually feed on?

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    $\begingroup$ What does the blood consist of, is the real question. And you seem rather focused on the calories. Yes calories are important for energy, however, our body needs fat, proteins, minerals etc to function. Also the caloric requirement, how did you scale that down? Simply by mass? What about other variables like organs, mobility, neural activity? A huge part of our caloric intake is used by our brain $\endgroup$ – user27740 Nov 26 '16 at 9:16
  • $\begingroup$ There are advantages, too, for blood-sucking. Blood is liquid. So needs no cut, chew macerate etc process and small effort for digestion. It is easy to collect a little amount of blood from the host and then flyaway. but for tougher tissues it is not so swift. Might they leave more nutritious options for these advantages? $\endgroup$ – Always Confused Nov 27 '16 at 3:59
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Lets find out how much calories are there in the blood. I am using this site for main reference, but will also add more references where needed. But before that, we should know that one donation of blood is about 1 pint (or 450 ml) blood1. But here, I calculate with volume 500 ml.

  • First calculate calories for plasma. Blood is denser than water by six percent2, 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% protein3, to get to grams of protein. Finally, multiply that result by four to get protein calories. Here, considering 55/100, I get 81.62 calories.

  • 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/dl4). 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 1 calorie.

  • For plasma calories from fat, use 0.5 grams per 100 ml5 of plasma, times hundreds of mls of plasma, times nine - for calories from grams of fat - to get to calories. 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 women6, 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. Hemoglobin is only 90% of the dry weight of RBCs7. 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. I get 353.28 here.

  • Finally, estimate calories for the other types of cells. Of the 500 ml blood donation, roughly one percent will be white blood cells and platelets8. 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 9 calories per gram.

Add everything up. The total should be approximately 425 calories for women and 460 for men in 500 ml blood i.e. even 10 ml of a man's blood contains about 9.2 kcal (there's no definite answer about how much blood a leech drinks at a time). Now, as you say in your question that a leech requires about 0.714 kcal, it means that a leech can survive for almost 13 days by sucking just 10 ml of a man's blood (if they have to metabolize everything present in blood (except, of course, water)), which justifies your statement.

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  • $\begingroup$ I see my error was assuming the only value in blood is in the glucose, and ignoring everything else $\endgroup$ – TheEnvironmentalist Aug 29 '17 at 20:36
  • $\begingroup$ @theenvironmentalist yeah, but since the parasite has got only that much amount as its sole source of energy, it would definitely do everything to extract as much energy as possible ;) $\endgroup$ – another 'Homo sapien' Aug 31 '17 at 4:40

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