The Science Behind Vampires: Bloody Diets

Shiki. Daume, BS Fuji.

I recently finished the gripping and disturbing anime series ‘Shiki’, which features vampire-like beings. I’d recommend it to those who like their entertainment bloody and thought provoking (and full of really bizarre hairdos).

One thought it provoked in me was whether it is feasible to live off blood alone. A few months ago I was reading the Q&A section on the BBC Science Focus site, where people can ask weird and wonderful things for some science boffin to answer, and this particular question was asked. The basic answer is that it isn’t possible, since we wouldn’t be able to extract water from it – because it has the same water and salt content as our own blood, there is no osmotic gradient to cause the water to move into the bloodstream – and it’s also too salty for us. Dehydration and kidney failure would quickly dispatch any wannabe vampire. The rich iron content could also prove toxic, although the author of the article states that 26 litres of blood would have to be imbibed per day to bring about iron poisoning. So obligatory hematophagy, or just eating blood, is not a possibility for humans.

What would it take to be able to live off blood then, and what adaptations would vampires need if they were a real life species? The best way to consider this is to look at existing examples.

The first thing any vampire should do is secure, and then hold onto, the food source. Choosing a sleeping victim is the most practical, but if they were to wake not alarming the food source would be handy. Some fictional vampires have a hypnotising ability to calm down their prey. Vampires could simply have incredible persuasive powers a la Derren Brown, making the victims want to kindly donate their life fluid. Another way would be to inject the victim with a psychoactive substance upon biting. Scopolamine, produced by plants in the Solanaceae family, has been used in a spate of robbings and assaults in South America in the past few years. Those affected by such crimes have no memory of such events, as the substance causes confusion and amnesia by blocking the neurotransmitter acetylcholine. It also reduces aggression and anxiety, leading to passiveness. Because of this, acetylcholine has been used legitimately to treat anxiety problems (as well as motion sickness). Such a chemical, if present in vampiric saliva, could prevent arousing suspicion in the blood bags.

Vampires - vampire bat
Hey there: a vampire bat (courtesy of National Geographic)

Next is to stop your yummy meal from clotting on you. A New Scientist article from 2008 reports research into how vampire bats have evolved to live solely on the red stuff. Their saliva contains the enzyme plasminogen activator (PA), which converts plasminogen to plasmin. Plasmin is another enzyme which breaks down blood clots. Due to this action PA is used to break down clots in conditions such as ischaemic stroke (where the blood flow to an area of the brain is cut off by a clot). Having PA in the saliva would be a must for the vampire.

Having engorged on a blood meal, vampires would then have to effectively extract enough nutrients and also survive the toxic perils which would kill us mere mortals. First off, to sustain an average adult human male with about 2700 calories per day would take approximately three litres of blood. This is a load of fluid. Discounting potential differences in the calorific needs of the undead for now, I’ll assume that vampires would need the same (and maybe more per feed if food were scarce). Providing that there are enough blood donors to provide this volume of blood in one sitting, the feeding vampire would need to get rid of excess fluid pretty quick sharpish. Vampire bats, which have to drink up to 80% of their own body weight in blood, have stomachs lined with many tiny blood vessels that absorb and transport water rapidly from their meals to their kidneys. They then pee whilst feeding. Another lovely feature that Bram Stoker omitted. How about the salt problem? With hormonal control, the vampire bat can alternate between producing copious volumes of dilute urine or small amounts of very concentrated urine to deal with any imbalances. Removing water and toxins this way just leaves behind the protein and nutrients. This mechanism also removes the large quantities of urea that are produced from such a protein-rich diet. Utilising this same mechanism, vampires could subsist solely on salty blood and never need water. And as most vampire lore refers to them being undead and lacking their own blood, they’d probably need all the iron provided by their meals and avoid iron overload poisoning.

The problem with living as a sanguinivore is the lack of energy storing ability. Blood provides energy through protein only; fat and carbohydrate content is negligible. Vampire bats have been found to be incapable of storing fat and can starve within a few days of fasting, so need a blood meal every night. The same would apply to vampires. How this might influence human-vampire relations has been a hot topic in vampire fiction, such as the ‘True Blood’ series of books and the (superior) TV show it inspired, as well as ‘Shiki’. If they followed the lead of the bats, vampires who were well fed could share some of their recent meal with one who had missed out. Tasty.

That’s how vampires might survive off a diet of just human blood. As for other vampire myths such as immortality or UV susceptibility – I’ll have to leave that for some other time. For anyone curious about vampiric theories, Katherine Ramsland’s ‘The Science of Vampires’ would make an interesting read. Especially the part about whether or not vampires could escape detection from our modern day forensics techniques. Food for thought. Meanwhile I’m off for a fry up – suddenly have a weird craving for black pudding.

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