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Vampires are a very common creature in various fantasy stories, they drink blood and often human food poisons them. Forgetting for the time being the additional rumours/fiction such as sunlight, crosses and immortality could such a being sustain itself from drinking blood alone?

Obviously human blood tends to contain traces of almost everything found in the body, this is why it's such a good place to start to test for diseases/deficiencies but does it provide EVERYTHING the body needs to sustain itself?

Reasonable evolutionary adaptions can be assumed.

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The simple answer is yes it's possible, if they have a suitably developed physiology. Vampire bats, leeches, mosquitoes and more are proof of that. There are complications that make it less practical than you might want though.

Lets start from a Vampire Bat (as a mammal it seems a good starting point for comparison). From wikipedia:

A typical female vampire bat weighs 40 grams and can consume over 20 grams (1 fluid ounce) of blood in a 20-minute feed. This feeding behaviour is facilitated by its anatomy and physiology for rapid processing and digestion of the blood to enable the animal to take flight soon after the feeding. The stomach lining rapidly absorbs the blood plasma, which is quickly transported to the kidneys, and on to the bladder for excretion.A common vampire bat begins to expel urine within two minutes of feeding. While shedding much of the blood's liquid facilitates flight takeoff, the bat still has added almost 20–30% of its body weight in blood. To take off from the ground, the bat generates extra lift by crouching and flinging itself into the air. Typically, within two hours of setting out in search of food, the common vampire bat returns to its roost and settles down to spend the rest of the night digesting its meal.

So this suggests that they consume half their body-weight in blood every day and digest it. It also suggests that being near a feeding vampire would be unpleasant as they need to quickly dispose of the liquid waste from their feeding.

The maximum a human can lose without serious risk is 3 litres. 1 litre is the maximum than can be lost without side effects.

If we go with the "half your body weight" formula then even a lightweight 50kg vampire would need to consume 25kg of blood. Since 1kg of blood is the approximate weight of 1 litre the vampire would need 25 litres of blood per night.

That's draining to death 5 people or partially draining 25 people. Per night!

Now if the vampire was not flying and/or had a lower metabolic rate it might have lower energy requirements. If rather than scaling a vampire bat up we looked at a human we can see that your average adult human needs 2000 to 3000 calories per day. A litre of blood contains 600 calories according to some Google searches that are going to look interesting in my browser history. We're going to assume that there are enough raw materials present as well as the calories to allow any missing resources to be synthesized.

That gives us a lower figure of 4 to 5 litres needed per day. That's more manageable but still seems like a hard thing to hide. We're now talking 1 death or 5 living victims per night.

It's seems far more likely the vampires would go after herd animals such as cows than after humans. Cows contain more blood, tend to be more exposed at night, and will ask fewer questions.

The mess made from expelling the waste will also be less likely to be noticed in the middle of a field than in someone's bedroom!

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    $\begingroup$ The calories don't scale the way you suggest. Vampire bats have a very high metabolic rate (due to small size and flying). Blood has around 900 calories per litre. As a human, you'd only need 2-3 litres to get enough calories, it is vitamins, like vitamin C, which are in short supply, but a suitable metabolism (which synthesised anything necessary) could fix that. $\endgroup$
    – Neil Slater
    Oct 13, 2014 at 13:09
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    $\begingroup$ The reason of getting rid of the waste soon is, from the Wikipedia text you quoted: "to enable the animal to take flight soon after the feeding." I guess that's because of the weight of it. A non-flying vampire would therefore not have an equally pressing need to get rid of it. $\endgroup$
    – celtschk
    Oct 13, 2014 at 13:18
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    $\begingroup$ @celtschk Considering they need to be able to move fast if discovered I'd suggest they still have a pressing need to not be carrying so much extra weight. Less so if consuming the blood one litre at a time admittedly. $\endgroup$
    – Tim B
    Oct 13, 2014 at 13:19
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    $\begingroup$ @PaulZ It wouldn't bother my girlfriend, I'm more wondering how the company I'm contracting for at the moment would react :p $\endgroup$
    – Tim B
    Oct 13, 2014 at 18:14
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    $\begingroup$ @Baldrickk Your comment is hopefully meant as a thought experiment, but I still feel it needs a "kids don't do this at home" sticker: if you drink 4-5 liters of water at once, you'll drop dead due to water toxicity. Or, if you avoid death, you'll still suffer. $\endgroup$
    – rumtscho
    Oct 13, 2014 at 21:03
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Addition to TimB's answer:

Expelling the waste might also be taken care of differently - if the vampire were able to digest the blood quickly enough, it could simply drink and return. This would require a powerful blood filtration system, much more so than human's, but it might be doable. As soon as the blood is mostly stripped of its nutrients, it can be returned back to the victim.

In fact, if you also add some enzyme to force the victim to feed more nutrients to their blood, you could get much more then 900 kcal per litre - and in fact, do so repeatedly. While this would prolong the feeding process, it would also mean that he'd be able to feed comfortably on just one person, and without doing harm to the victim, other than certain level of feebleness.

So - induce a "need more energy" state in the victim, which will make it pump more nutrients into its blood, including metabolising fats, filter the nutrients out as quick as possible, and return the blood back. Ideally, in an outright loop, similarly to how human kidneys work, just on a bigger scale. You don't get water toxicity, you don't gain 5kg in weight, you get much more nutrients than from just sucking the victim dry, and you don't kill the victim - win, win, win, win.

In fact, this could even be seen as a symbiotic relationship if you add some beneficial effect of the blood filtration. Perhaps the vampire could also improve the immune system of the victim temporarily (might have evolved to protect the vampire from infections through victim's blood). Maybe it can filter out stuff that can cause harm to the victim, e.g. poision. It could help prevent various diseases related to blood content, e.g. some coronary diseases, artery clotting... At the very least, it could be used to help the host stay slim, by draining a certain portion of his nutrient supply - the ultra-cool vampire diet! In fact, I could easily imagine a society with nobles hosting vampires specifically to show off their wealth etc. They would eat 2-3x as much as "poor" humans, and dispense the extra to their vampire symbiotes, thus staying well in shape.

The symbiotic relationship would thus be beneficial to both the human host and the vampire. Feeding gets easier for the vampire, because it's suddenly a helper, rather than a mugger (think medicinal use of leeches). Humans might get a host of benefits, including improved health and easier weight management. Imagine how much people would pay for a vampire service today, just to get rid of that extra fat! :D

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XKCD suggests that regular consumption of large amounts blood (by humans) would probably lead to iron overload (as well as having other drawbacks).

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    $\begingroup$ That is true for humans, but OP specified that reasonable evolutionary adaptions could be assumed. There is no evolutionary constraint that prevents mechanisms for getting rid of exess iron, there is just no evolutionary pressure on humans to develop such a mechanism. $\endgroup$
    – Taemyr
    Oct 14, 2014 at 10:12
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Went and found a list of human blood components on Wikipedia. Every building block of human life is in there. Is it there in quantities to support a human who functioned on nothing but blood...probably not. Especially considering that a human consuming blood usually gets nausea.

All the major proteins (that I can think of), fats, minerals, sugars, etc etc etc are present in blood which makes sense as the body's pick-up and delivery system.

But you can make the assumption that vampires don't require the same exact nutrients as a human. If they're not biologically different then...well they're not vampires.

So yes, you're sane.

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    $\begingroup$ And of course, "Reasonable evolutionary adaptions can be assumed." $\endgroup$
    – user
    Oct 14, 2014 at 11:02
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The Masai tribe do this - or used to, these days they farm and have a more varied diet. According to Wikipedia they are extremely healthy, so maybe there's something going for it.

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    $\begingroup$ They did that "from drinking blood alone"? $\endgroup$
    – Shokhet
    Oct 19, 2014 at 18:14
  • $\begingroup$ Well they claimed to. But probably they were eating veg and stuff on the quiet... $\endgroup$
    – RedSonja
    Oct 20, 2014 at 13:40
  • $\begingroup$ I think I read as a kid (in Willard Price's Elephant Adventure, written 50 years ago, and which I read about 30 years ago, if I remember right - so not the most reputable cite, I know) that they both milked and blooded their cows, and drank both. But yes, I imagine they also ate the beef, and most likely gathered veggies and hunted, too... ooh, here we go: en.wikipedia.org/wiki/Maasai_people#Diet waytago, Willard Price! "Traditionally, the Maasai diet consisted of raw meat, raw milk, and raw blood from cattle." No wonder he got it right, his uncle Weston went to study the Masai. $\endgroup$
    – Dewi Morgan
    Feb 4, 2015 at 9:30

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