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My main character is based on the basic concept of a vampire. That she needs blood to survive.

She was genetically engineered to digest ‘blood’ instead of eating. She still needs to drink water. Where she varies from a vampire is that it isn’t real blood, it just mimics the idea of it, so it’s basically a synthetic replica with different doses of nutrients suited to her etc.

Although, she does eat some human food and drink now and again when she fancies and she smokes. This is possible because she still has all working internal systems just like a human etc.

But where would she get her energy from? Because a human eats food for energy to perform basic functions. Would it be possible to obtain it from her ‘blood’?

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    $\begingroup$ Would she not get her energy from the synthetic blood? $\endgroup$
    – F1Krazy
    Nov 16, 2017 at 8:46
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    $\begingroup$ "She still needs to drink water." She never drinks... ... wine. $\endgroup$
    – Joel Coehoorn
    Nov 16, 2017 at 14:45
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    $\begingroup$ Why did you mention her smoking? Does it affect her diet somehow? Should we assume she doesn't do other things you haven't mentioned? $\endgroup$
    – Muuski
    Nov 16, 2017 at 16:13
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    $\begingroup$ "New" vampires are solar-powered, right? That's why they sparkle in the sun? /s ;-) $\endgroup$
    – Xen2050
    Nov 17, 2017 at 0:27
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    $\begingroup$ I'm confused, what makes her a vampire? She drinks a synthetic fluid that's presumably made in a lab somewhere, not in a person. Or did someone genetically engineer humans to use 'blood' instead of blood for her to feed on them? $\endgroup$
    – alexgbelov
    Nov 17, 2017 at 1:23

9 Answers 9

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From the calories in blood

According to fitbit.com, 100 g of lamb's blood has 75 calories. I'd imagine the USDA hasn't done any studies on human blood, them not being vampires and all.

To get 2000 calories a day, you would need about 2.6 kg of blood, which is about 2.6 liters. A human has about twice that, so a vampire would need to kill and drain a person every two days, which sounds pretty reasonable and in line with the vampire mythos.

Also consider that you only need to burn 2000 calories if you are doing normal human activities for 16 hours a day. If vampires are into sleeping in their coffins, which I hear they are, they might be much less active and require less calories. On the other hand, turning into a bat and exerting super strength sounds pretty calorie intensive, so lets call that a wash.

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  • $\begingroup$ It would be hard to get away with killing a person every two days and not get caught if you are not constantly travelling and even then it would be tough. For instance there are only about a hundred murders in all of London a year, so you, personally, are looking at more than doubling the murder rate in a major city and hoping it will slip under the radar. $\endgroup$
    – John Meacham
    Sep 24, 2018 at 12:58
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If you were to live only on the blood of your "prey," as a predator you'd need a lot of it.

Just look at it this way: A regular predatory animal spends its energy resources on hunting prey and eating them entirely, including consuming the blood within the flesh. A predator like that mainly survives on the flesh, as the blood does not contain enough calories to sustain its body and behaviour.

Though blood contains lots of nutrients, it will not replace the energy a predator spends when it hunts. Wasting all the energy that could be gained from the flesh of already-hunted prey would be extremely inefficient.

The only living beings that live on blood are insects and small animals. And they live off of much larger animals. A human could not easily find something to suck blood from without killing or fighting it. It would have much higher energy expenses and would be very inefficient. Killing the prey and eating it, including the blood, is a lot more efficient.

Some science

The body needs 5.3MJ to 5.7MJ to replace a litre of blood. So we can say that's the maximum amount of energy it contains. The body's production of it is not 100% efficient and energy is lost during production. Also, no digestive system is 100% efficient.

A calorie need of 2000 kcal (8.4MJ) per day is a good guess for a woman.

So you would need at least 1.5 Litres of blood per day (calculated with maximal energy - though unrealistic because of energy losses).

If you think that such an amount of synthetic blood is realistic, then it works. But that amount is unlikely to be achieved in a more natural way without killing anybody.

Your synthetic blood could be made to contain more calories so she would need less. But that's up to your story.

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    $\begingroup$ +1 for the actual energetics, but I would not want to drink a litre and a half of blood a day. I can barely manage that much tea, and I’m English! $\endgroup$
    – Joe Bloggs
    Nov 16, 2017 at 9:45
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    $\begingroup$ Your answer deals with natural blood and its nutritional and energy supplementation. This is excellent in and of itself, but the question is about synthetic blood as a source of food. My guess it would have to be something like a normal meal put through a blender. $\endgroup$
    – a4android
    Nov 16, 2017 at 12:53
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    $\begingroup$ "The body needs 5.3MJ to 5.7MJ to replace a litre of blood. So we can say that's the amount of energy it contains." - no, sadly not really. See for ecample here - it' more like 2.5 MJ than 2.3 - human body is not perfectly effective factory, not even close. Never assume biological processes to be lossless. $\endgroup$
    – Mołot
    Nov 16, 2017 at 13:45
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    $\begingroup$ @Molot you are right, but i was just giving a rough estimation. Since i did not now the exact answer i was doing an upper bounds calculation. You are right that i did not mention that. I'll edit it accordingly. $\endgroup$
    – ArtificialSoul
    Nov 16, 2017 at 13:49
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    $\begingroup$ @Baldrikk: Mmmm.. a 1.5l stein of warm, slightly congealed blood on top of my normal water consumption. Lovely. $\endgroup$
    – Joe Bloggs
    Nov 16, 2017 at 16:33
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If the artificial blood has all the nutrients a human body needs, you already have your energy.

It's not different from only eating Soylent or any other of those full meal replacements.

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    $\begingroup$ nutrients does not equal energy. nutrients is what your body needs to keep its biochemistry intact. But unless you also get something to sustain your body on (carbs, fats) you'll end up malnourished nonetheless and die from it. $\endgroup$
    – ArtificialSoul
    Nov 16, 2017 at 9:03
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    $\begingroup$ @ArtificialSoul There are a lot of data about people only eating Soylent for months without problems. The major con is acostuming to only ingering liquid. $\endgroup$
    – Malkev
    Nov 16, 2017 at 9:30
  • $\begingroup$ i added a bit of calculations to my answer. If you have 1.5 litres of blood a day it can work. Or less if you increase the calories of the synthetic blood in some way. $\endgroup$
    – ArtificialSoul
    Nov 16, 2017 at 9:33
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Milk is a liquid food that can provide complete nourishment for mammals. I went through an eggnog phase myself. I guess that makes me an eggnog vampire! Happy Holidays!

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"She still needs to drink water." - It's funny how many times this trope can be found in fiction, considering how much water the blood itself actually contains (roughly half of what makes up blood is water). If anything it should be the exact opposite: a vampire should need blood to survive and also non-liquids to supplement the diet. And this actually holds true for real-world mammal hemophages (I'm explicitly excluding insects here) such as the vampire bat. If your setting doesn't allow for magic, then you should make your vampire supplement her diet with normal food. That would actually be quite original, I think (I'm a fan of vampire stories and have yet to find one where normal food would be just as necessary for a vampire as blood is).

Edit: couldn't find a source for my statement that the vampire bat needs non-liquids in its diet, so please take it with a grain of salt

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  • $\begingroup$ The US Army Survival manual does not consider blood as an adequate source for water - actually, it states it is food, which needs additional water to digest. $\endgroup$
    – Michael Schumacher
    Nov 17, 2017 at 13:17
  • $\begingroup$ Interesting, but it seems counter-intuitive. Does it say anything as to what role additional water plays? $\endgroup$
    – eugene2k
    Nov 17, 2017 at 19:49
  • $\begingroup$ Do you have a source of information for vampire bats requiring non-blood food? I've always seen them classified as obligate hemotophages. Granted, the pups start out on milk, but they're weened by their mother gradually introducing regurgitated blood into their diet, no solid food involved. $\endgroup$
    – Cowrie
    Dec 13, 2017 at 1:56
  • $\begingroup$ Unfortunately, I couldn't find where I picked up that piece of knowledge from, so I may be wrong. Thanks for asking for proof. $\endgroup$
    – eugene2k
    Dec 14, 2017 at 6:54
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Blood (real one for sure, so presumably also "synthetic" one) has a very high amount of nutrients in it.

There are several (real) creatures feeding exclusively on it.

There's no difficulty in "getting energy" from it.

P.S.: We eat to get a lot of things, including building materials, vitamins, oligo-elements etc., not just "energy" (all of them can be found in blood).

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    $\begingroup$ Hmm, according to your link, the common vampire bat drinks about a quarter of the blood in a cow in a year, which is about ten liters (pdf warning). On one hand, that's not a lot, but on the other they weigh at max 40 grams. Extrapolating from that, we're talking roughly two hundred liters a day for a human-sized creature. $\endgroup$
    – HAEM
    Nov 16, 2017 at 14:43
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    $\begingroup$ Extrapolation is not always meaningful. A common sparrow eats daily more than it's weight worth of seeds (highly energetic food). Wikipedia states: "Shrews need to consume 200 to 300% of their body weight in food each day in order to survive". $\endgroup$
    – ZioByte
    Nov 16, 2017 at 16:48
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    $\begingroup$ @HAEM No that means 80kg of vampire bats can drink 2000 cows/year or less than 5,5 cows day or ~213 liters. But since a small flying critter needs to eat a order o magnitude more food than a human tha fall in the order of 5~20 liters/day (still a lot of blood) $\endgroup$
    – jean
    Nov 16, 2017 at 19:24
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In addition to obvious and very good answers based on the fact that the blood is actually the thing which distributes energy throughout the body in form of lipids and sugars, You can, if You will, go more wacky. Let's see (pardon my tongue in cheek tone, would you? ;) )...



1. The vampire consumes the victim's soul along with the blood.

First, we have to assume that a soul actually carries some free energy. Since it takes between 15 to infinity years to create an adult human (sadly, most people never really develop) and throughout all those years a LOT of energy is consumed by the brain - which in turn is the machinery in which the software called soul resides - i think it's a safe assumption. Further more, we can bring up an old STEM joke: Exam question: Is hell exo or endo thermic?

Having been convinced that soul carries some thermodynamically usefull energy, we are now safe to say that a vampire with ability to devour it along with blood could get energy it needs from it. And think about all the other ploot hooks this brings!

PROS: Lots of possible plot hooks.
CONS: You can end up with a bit too many voices in Your head if You retain those souls and dont devour them completely.



2. In far cyberpunk dystopian gloomy raining future, humanity has perfected and miniaturized fusion reactors to pocket sizes - and vampires use them.

So, the basic idea is that all the energy calculations presented in other answers assume your normal average door-to-door biochemistry. But there is a loot of room down there, and lot of energy too. If You'd care to check, You'd find that biomolecules contain a LOT of hydrogen. So, screw normal biochemistry, trash glycolysis and throw mitochondria to a bin of history. Your vampire has a biochemical cycle which rips any and all hydrogen atoms it can find in Your food and provides them to a miniature fusion reactor in your chest, which produces all the energy You'll ever need.

PROS: You're cyber-vampire-tony-stark
CONS: Staking You could trigger a nuclear meltdown? Hm, maybe that's a pro too...



3. Thanks to combination of really eldritch, ancient astral magic and possibly some Quantum Field Theory You either have magnetic monopoles or naked singularities in a catalytic center of a digestive enzyme

Fusion is nice, but let's go for E=mc^2. By a combination of exotic physics and magic You literally turn ALL matter of the blood You've drank into energy. Just like that, a molecule comes in, a lot of very very energetic gamma photons come out, then You just harvest them.

PROS: You are better than sun in terms of energy production efficiency.
PROS: You probably could shoot gamma rays
CONS: A horde of physicists stalks You all the time to make measurements and possibly kidnap You and make their lab rat
CONS: You leak photons. A real lot of them. You're sparkling.

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From your description i'd say with her syntetic blood she would be feed(have energy).

Us humans actually don't digest 100% of our food so she could "eat" normal food just for the taste or to blend in but at the end of the day, it would have no nutritional effect.

Alcohol should have effect on her(coz it's not broken down in our body either it just sticks around in our blood until it gets eliminated) and same for smoking and most drugs.

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Make the synthetic blood closer to gasoline in chemical structure and energy content. A gallon of gasoline has 31,500 calories, so for a 2,000 calorie daily diet, your vampire would only need about 8 ounces (1 cup). To justify their enhanced speed and strength, you might want to double that consumption level to 4,000 calories/1 pint per day.

A pint a day keeps death at bay.

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    $\begingroup$ But but but but the environment... isn't there something in Paris accords against high octane vampires? :( $\endgroup$
    – Maciej
    Nov 17, 2017 at 9:27

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