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My vampires eat normal food too, but if they don’t drink blood they get really hungry for it. If their hunger goes on for too long they become unhealthy and weak. I figure that blood of mammals contains a chemical or compound that their body is unable to create on its own. They crave the blood because they need the chemical, doesn’t mean they aren’t gluttonous for blood or peruse it even when they’re not hungry. I’m just having a hard time researching what they need from human blood.

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    $\begingroup$ ??? Every substance which is made by the human body is transported from the place of manufacture to the place of use by the blood stream. There is no other transportation network in the human body. So that I don't get what you mean by having a hard time researching what they need from human blood; obviously, sugar and spice and all things nice -- insulin, adrenaline, noradrenaline, dopamine, melatonin, etc. etc. etc. $\endgroup$
    – AlexP
    Mar 6, 2023 at 16:53
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    $\begingroup$ Perhaps hemoglobin iron? $\endgroup$ Mar 6, 2023 at 20:29
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    $\begingroup$ In the original concept, they weren't sparkly and driven by the need to survive, but diabolical and driven by the lust to destroy the living and spread despair to those they could not send to hell immediately. Why? That devolves into the silly and unanswerable theological question of "what is evil, truly?". If Stoker focused on blood-sucking, it might have been because of a then-new medical science of hematology, or maybe he was just a kinky freak. The Eastern European mythology doesn't make it so explicit, just that this predatory consumption didn't cause immediate death. $\endgroup$
    – John O
    Mar 6, 2023 at 20:47

5 Answers 5

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Iron: Blood is rich in iron, an essential mineral that plays a crucial role in many bodily functions, including the production of red blood cells. If a vampire's body cannot synthesize iron, they may crave blood to obtain it. Iron deficiency can lead to weakness, fatigue, and anemia, which could explain why a vampire becomes unhealthy and weak without blood.

Proteins and amino acids: Blood contains many proteins and amino acids that are important for building and repairing tissues, transporting molecules, and regulating bodily functions. If a vampire's body cannot synthesize these molecules, they may crave blood to obtain them.

Vitamins and minerals: Blood also contains many vitamins and minerals that are essential for health, such as vitamin B12, vitamin D, calcium, and potassium. If a vampire's body cannot synthesize these nutrients, they may crave blood to obtain them.

Neurotransmitters: Blood contains several neurotransmitters, including dopamine, serotonin, and norepinephrine, which play a critical role in regulating mood, behavior, and other bodily functions. If a vampire's body cannot synthesize these molecules, they may crave blood to obtain them.

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  • $\begingroup$ Other than the neurotrans, regular food is easier to get all that. Iron, for example, can be obtained by eating red meat, or even by using a cast iron frying pan. Neurotransmitters are complicated molecules. If the vamp's digestive system passes them into its blood then they will get other complex molecules and have a blood reaction from the wrong blood type. $\endgroup$
    – Boba Fit
    Mar 6, 2023 at 21:58
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Vitamin D

The vampires have a skin allergy to sunlight, and so can't get vitamin d the usual way. As such, they drink blood to get all their nutritional needs.

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  • $\begingroup$ There is precious little vitamin D in human blood; the human body cannot store it efficiently, so all that's made is either used or pissed out. If they need vitamin D supplements they need to eat fatty fish such as salmon (80 grams per day), or take cod liver oil (half a teaspoon per day). Human blood won't help. $\endgroup$
    – AlexP
    Mar 6, 2023 at 16:59
  • $\begingroup$ Vitamin d is definitely a bonus but I’m th8nking of something different $\endgroup$ Mar 6, 2023 at 17:22
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    $\begingroup$ Could you define what different thing you are thinking of? $\endgroup$
    – Nepene Nep
    Mar 6, 2023 at 17:27
  • $\begingroup$ Lots of vitamin D in beef liver. Don't need to get the humans pissed off for that. If they need just huge amounts of it they can go for carnivore liver (bear, wolf, shark, etc.) but be mighty careful of the dose. Carnivore liver contains high amounts of vitamin A, which can be a big problem. $\endgroup$
    – Boba Fit
    Mar 6, 2023 at 21:55
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Live Red Blood Cells

As other answers have pointed out, all the same basic chemicals and elements present in human blood can also be found in equal or greater quantities somewhere else in nature. But maybe it's not a thing in human blood that vampires need so much as the blood itself.

Vampire biology is physically incapable of producing its own red blood cells; so, they need to intake live red blood cells from humans to fill this need. Any other blood will not be compatible and initiate an immune response; so, it has to be human, and old, cooked, or frozen* cells don't count because the vampire needs live cells to replace his own dying cells.

The vampires can't actually drink the blood because that would would destroy the cells in the digestive tract, instead the fangs are hollow or have adjacent sphincters somewhere in the mouth that allows the vampire to directly transfuse the human blood with his own. This said, nearly all vampires will have an AB+ immune system allowing them to receive and use blood safely from virtually all human donors. A vampire without AB+ blood would be "allergic" to certain humans, and could get very sick if they tried drinking the wrong blood.

* The blood donation industry sometimes freezes cells with cryoprotectants, allowing them to be thawed and still alive, but these need to be properly filtered of the cryoprotectants before being transfused. So, if your vampire steals frozen blood, and does not have the proper equipment to prepare it, then it could be mildly toxic for him.

How often do they need to feed

The average red blood cell lives about 120 days meaning that when a vampire feeds it will get an equal distribution of cells with 0-120 days left in them. A person generally starts to notice the signs of blood lose at about 100 ml which is about 2% of your total blood volume. If vampires are similarly effected, then they should start to get hungry about every 2-3 days. Since significant dizziness and fatigue set in at about 14% blood lose, a vampire should start to feel significantly impaired after not feeding for about 2-3 weeks. And, since 40% blood lose is fatal, a Vampire should "starve" to death after about 7 weeks without blood.

If your vampire is consuming donor blood, count how long your vampire has eaten as equivalent to when the blood was harvested. So, if you have a vampire who raids a blood bank, the blood should range from about 1-35 days old. So, even if he drinks it all, he may still feel like it's been 18 days since he's feed. A smart vampire will only steal and drink blood that was harvested within the past couple of days, but even that could leave him feeling hungry.

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amino acids

your vampire's digestive system is a mess and they have trouble extracting any amino acids from most proteins so they require hemoglobin to get them iron would be the second best answer, their bodies may only be able to absorb iron from organometallic sources so if they don't drink blood or eat raw meat they get anemia.

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  • $\begingroup$ Okay your comment helped me figure it out, vampires constantly feel like they’re starving. Even if they eat food they still feel hungry, but blood lets them feel full somehow. This ties into vampire folklore as it’s said they love a good meal and that’s one way to get them to leave you alone. $\endgroup$ Mar 7, 2023 at 15:33
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You would need to invent something, or make it spiritual. There's no chemical in human blood which you cannot get from a butcher's shop.

I suppose you could say they need to get the blood itself. Like the plasma and blood cells specifically. that it's absorbed directly into their bloodstream, and it needs to be human because pig or sheep's blood isn't compatible, but frankly that seems less likely to me than that they just need soul juice.

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