A liter of whole mammalian blood contains roughly 600 to 900 kilocalories, with the vast majority of that energy coming from protein rather than fat or sugar. That puts blood somewhere in the caloric range of whole milk, though the nutritional profile is nothing alike. The number shifts depending on the species, the animal’s health, and how the blood is processed, but the core story is the same: blood is a dilute, protein-heavy fluid that offers far less energy per swallow than most solid tissues.
Where the Calories Actually Come From
Blood is about 78 to 80 percent water by weight, which immediately limits how energy-dense it can be. Of the roughly 20 percent that remains, protein dominates. Hemoglobin alone accounts for the bulk of it, typically running around 120 to 170 grams per liter of whole blood in mammals. Plasma proteins like albumin and immunoglobulins add another 30 to 45 grams per liter. At around four kilocalories per gram, all that protein adds up to something in the neighborhood of 600 to 750 kilocalories per liter just from amino acids.
Fat and carbohydrate contribute surprisingly little. Blood glucose in a healthy mammal hovers around one gram per liter, worth roughly four kilocalories. Circulating lipids add perhaps another 30 to 70 kilocalories per liter depending on the animal’s fed state. So the caloric profile of blood is overwhelmingly one-note: it is a protein solution with trace amounts of everything else. This has real consequences for any creature trying to live on it, as we will see with vampire bats.
Not All Blood Is Created Equal
The caloric content of blood shifts meaningfully between species. A comparative study of porcine, chicken, and duck blood found that pig blood had the highest protein and fat content of the three on a dry-weight basis, followed by duck and then chicken blood. Chicken blood, on the other hand, had notably more carbohydrate than the others.1PubMed Central. Comparative Study on Compositions and Functional Properties of Porcine, Chicken and Duck Blood These differences are not trivial for food scientists or farmers using blood as an ingredient, but they also illustrate a broader point: the phrase “calories in blood” does not have a single fixed answer. Species, diet, and the animal’s physiological state all move the number around.
When blood is dried and processed for industrial or agricultural use, the protein concentration climbs dramatically. Dried blood meal can reach up to 80 percent crude protein by weight, making it a potent feed supplement.2PubMed Central. Dried Blood-Rumen Content Mixtures as Sustainable Poultry Feed: A Review on Nutritional, Economic, and Environmental Potential Removing the water effectively concentrates all that hemoglobin and albumin into a calorie-dense powder. Per gram, dried blood meal packs roughly three to four kilocalories, which is comparable to most other high-protein animal feeds.
How Blood Compares to Other Tissues
If you are wondering how blood stacks up against the foods you actually eat, the short version is: it is far less calorie-dense than solid meat and enormously less dense than fat. A liter of blood, at roughly 700 to 800 kilocalories, contains less energy than a similar volume of lean ground beef and a fraction of the energy in the same volume of rendered fat or vegetable oil. This is almost entirely because of the water content. Blood is a transport fluid, not a storage tissue. It evolved to carry oxygen, nutrients, and immune cells around the body, not to serve as an energy reserve.
Muscle tissue stores meaningful amounts of glycogen and intramuscular fat on top of its protein. Adipose tissue is essentially a lipid battery, packing around nine kilocalories per gram of pure fat. Blood has none of that concentrated storage. It carries glucose, but in tightly regulated amounts, typically less than a teaspoon’s worth dissolved per liter in a healthy mammal. Circulating lipids are in transit, not warehoused. The result is that blood is one of the least calorie-dense tissues in the body, beaten only by fluids like lymph and cerebrospinal fluid.
Why Vampire Bats Need So Much of It
Vampire bats are one of only three mammal species that survive exclusively on blood, and their biology reveals just how challenging a blood-only diet really is. Because blood is so protein-heavy and so low in fat and carbohydrate, vampire bats have essentially rebuilt their metabolism around amino acids as a primary fuel source. A study exercising common vampire bats on treadmills found that they burned amino acids from their blood meal as their dominant energy substrate, with the labeled meal accounting for as much as 60 percent of the oxidized fuel at peak usage.3PubMed Central. Vampire bats rapidly fuel running with essential or non-essential amino acids from a blood meal The bats did not distinguish between essential and non-essential amino acids either, burning both at similar rates. For most mammals, amino acids are a backup fuel used mainly during starvation or extreme exercise. Vampire bats have made them the main event.
The genetic consequences of this dietary specialization run deep. A genomic study of common vampire bats identified multiple gene losses tied specifically to the demands of a blood diet. Some losses related to reduced insulin secretion and limited glycogen stores, which makes sense when your food contains almost no sugar and you rarely need to manage glucose spikes. Others reflected the biased nutrient composition of blood or the unique pathogens that blood-feeding animals encounter.4PubMed Central. Gene losses in the common vampire bat illuminate molecular adaptations to blood feeding A gene involved in gastric physiology was also lost, consistent with the vampire bat’s unusual stomach, which functions more like a stretchy holding tank for liquid meals than a standard digestive organ.
The practical upshot of all this is that a vampire bat has to drink a substantial volume of blood every night just to stay alive. They typically consume about half their body weight per feeding, roughly 20 to 30 milliliters for a bat weighing 30 to 40 grams. If they fail to feed for two consecutive nights, they can starve to death. Blood is dilute enough, and the bat’s metabolic demands high enough, that there is almost no margin for error.
The Problem With Living on Blood
Vampire bats illustrate the extreme version, but the fundamental challenge applies broadly: blood is a difficult food. The caloric return per unit of effort is low compared to eating solid flesh, seeds, fruit, or insects. Beyond the raw calorie math, blood presents several nutritional headaches for any animal trying to subsist on it.
Iron is the most obvious one. Hemoglobin is packed with iron, and a blood meal delivers far more of it than most organisms can safely handle. Iron overload damages tissues, so blood-feeding animals have evolved elaborate mechanisms to dump or sequester excess iron quickly. Vampire bats excrete iron at remarkable rates, essentially starting to urinate out the water and mineral excess within minutes of feeding, concentrating the caloric payload while shedding the dangerous surplus.
There is also a near-total absence of certain vitamins and fiber. Blood contains very little vitamin C, almost no vitamin E, and effectively zero dietary fiber. A human trying to survive on blood alone would face deficiency diseases relatively quickly, even if the protein and calorie numbers looked adequate on paper. The Maasai and other pastoral cultures that incorporate raw cattle blood into their diets do so alongside milk, meat, and other foods, not as a sole source of nutrition.
Blood in Human Cuisine
Humans have used animal blood as an ingredient for thousands of years, and these dishes exist in nearly every food culture that raises livestock. Black pudding in Britain, boudin noir in France, morcilla in Spain, sundae in Korea, czernina in Poland, blodpudding in Scandinavia: the list is long and varied. In almost every case, blood is mixed with fat, grain, onions, spices, or other fillers before being cooked, because blood alone is not a particularly pleasant or satisfying food.
From a nutritional standpoint, cooked blood dishes are a genuinely good source of protein and bioavailable iron. Heme iron, the type found in hemoglobin, is absorbed much more efficiently by the human gut than the non-heme iron found in plant foods. This is one reason blood-based foods have historically been valued in cultures where iron deficiency was common, particularly among women of reproductive age and growing children. A serving of blood sausage can provide a significant fraction of the daily recommended iron intake.
The caloric content of finished blood dishes varies enormously because of what gets added. A plain glass of fresh blood might contain around 70 to 80 kilocalories per 100 milliliters, almost all from protein. A slice of black pudding, by contrast, might run 250 to 350 kilocalories per 100 grams because of the added fat, oatmeal, and other ingredients. The blood itself is the nutritional backbone, but the additional ingredients dominate the calorie count in the finished product.
Blood as Animal Feed and Industrial Ingredient
Outside of human kitchens, blood from slaughterhouses represents an enormous source of protein that the livestock industry has been recycling for decades. When you dry blood and grind it into blood meal, you get a product that is roughly 80 percent protein by weight, making it one of the most protein-dense animal feed ingredients available.2PubMed Central. Dried Blood-Rumen Content Mixtures as Sustainable Poultry Feed: A Review on Nutritional, Economic, and Environmental Potential It is commonly added to poultry feed to boost growth performance and feed efficiency.
Blood meal is also used as a nitrogen-rich organic fertilizer in gardening and agriculture. Its protein breaks down in soil into ammonia and then nitrate, feeding plants. This is a reminder that the caloric value of blood is not just relevant to eaters; it represents stored chemical energy and nitrogen that can cycle through ecosystems in multiple ways. A liter of raw blood washed down a drain is a waste disposal problem precisely because it is so rich in organic matter. Blood’s biochemical oxygen demand is extremely high, meaning microorganisms will consume enormous amounts of dissolved oxygen breaking it down, which is why slaughterhouse wastewater needs careful treatment.
Other Blood-Feeding Animals
Vampire bats get the headlines, but they are far from the only animals that have evolved to feed on blood. Mosquitoes, ticks, leeches, bedbugs, tsetse flies, vampire finches, lampreys, and several species of parasitic catfish all consume blood to varying degrees. The caloric math differs for each of them because of scale. A mosquito takes roughly two to three microliters of blood per meal, which amounts to a tiny fraction of a calorie. For an insect that weighs a milligram or two, though, that is a substantial meal, potentially doubling or tripling the mosquito’s body weight.
Female mosquitoes are the ones that bite, and they do it primarily for the protein, not the energy. The amino acids and iron in blood are needed to produce eggs. Between blood meals, mosquitoes fuel their day-to-day flight and metabolism with plant sugars from nectar. This dual-fuel strategy is a telling sign of blood’s limitations as an energy source: even a tiny insect that has evolved to feed on blood still relies on sugar for its routine energy needs. Blood provides raw materials for reproduction, not efficient locomotion fuel.
Ticks take a different approach, gorging themselves on a single massive blood meal that can increase their body weight by a factor of 100 or more. A fully engorged tick may weigh half a gram, having consumed roughly half a milliliter of blood over the course of several days. That blood sustains the tick through its next developmental stage or reproductive cycle, sometimes for months. Ticks are effectively converting a dilute, protein-heavy fluid into slow-release energy, which is possible because their metabolic rates are astonishingly low compared to warm-blooded animals.
How Much Energy Is in Your Own Blood
An average adult human has about five liters of blood circulating at any given time. Using the rough figure of 700 to 800 kilocalories per liter, your entire blood volume contains something in the range of 3,500 to 4,000 kilocalories. That is roughly equivalent to a day and a half of typical caloric intake for an adult. It sounds like a lot until you consider that your body’s fat stores typically hold 50,000 to 100,000 kilocalories or more, and your muscle protein stores represent another large reservoir. Blood is one of the smallest energy reserves in the body, which is partly why significant blood loss causes such rapid physiological crisis. You are not just losing oxygen-carrying capacity; you are losing fluid volume, clotting factors, immune cells, and the transport medium for essentially every molecule your cells need. The caloric loss from bleeding is, by comparison, the least of your problems.
This also helps explain why blood donation, which removes about 470 milliliters at a time, is calorically trivial. You lose somewhere around 300 to 350 kilocalories worth of protein and other nutrients in a standard donation, roughly the equivalent of a large snack. Your body replaces the fluid volume within a day or so, and the red blood cells within a few weeks. The iron is harder to replace, which is why donation centers enforce minimum waiting periods between donations and why frequent donors sometimes develop low iron stores even when they feel fine.