How many calories does a skeleton have? Unveiling the surprising truth

December 21, 2025 •

4 min read

According to a 2017 analysis of human nutritional value, the skeleton of an average adult contains over 25,000 calories from fat and protein. This astonishing figure reveals the surprising biological truth behind the question of how many calories does a skeleton have, with the energy primarily stored within its bone marrow.

Quick Summary

A human skeleton contains a significant number of calories, not from the mineralized bone tissue itself, but from the fatty bone marrow stored in the cavities of the bones.

Key Points

Source of Calories: A skeleton's calories come almost entirely from the fat in its bone marrow, not the mineralized bone tissue itself.
Significant Energy Reserve: An archaeological calculation estimated an average adult skeleton to contain over 25,000 calories from its marrow, proving it's a major energy store.
Marrow Types: Yellow bone marrow is the primary site for fat storage, while red marrow produces blood cells. The proportion of yellow marrow increases with age.
High Caloric Density: Bone marrow is extremely calorie-dense due to its high fat content, with some animal marrow samples showing nearly 800 calories per 100 grams.
Mobilized for Energy: The body can metabolize the fat stored in yellow bone marrow to provide energy during times of caloric deprivation.
More than Just Calories: Bone marrow also contains beneficial nutrients like protein, vitamins (B12), and minerals (iron), not just fat.

The Misconception vs. The Biological Reality

When most people think of a skeleton, they envision a dry, calcium-rich structure with no nutritional or caloric value. The reality is far more complex and fascinating. While the dense mineralized outer layer of bone is primarily for support and protection and offers minimal calories, the soft, spongy tissue inside—known as bone marrow—is a dense source of energy. The misconception arises from viewing bones as inert, lifeless objects rather than metabolically active organs that are a crucial part of the body's energy storage system.

The Primary Source of Calories: Bone Marrow

The secret to a skeleton's caloric content lies in its bone marrow. There are two main types of bone marrow: red and yellow. Red bone marrow is responsible for producing blood cells, while yellow bone marrow is primarily for energy storage, containing a high concentration of fat cells, or adipocytes. As we age, the amount of red bone marrow in our long bones is gradually replaced by yellow bone marrow, increasing the skeleton's capacity for energy storage.

This fat storage is not just a passive process. The body can mobilize the stored lipids in yellow marrow during periods of energy deprivation, making the skeleton a reserve tank of fuel for the body. This is different from the body's other fat depots, like subcutaneous fat, as bone marrow adipocytes interact directly with the bone metabolism and hematopoietic system.

The Nutritional Breakdown of Marrow

The high-fat content of bone marrow is the main driver of its caloric density. The exact nutritional profile can vary depending on the source, but analyses show it is extremely calorie-dense. For instance, some examples show marrow consisting of almost 97% fat by macronutrient content. Beyond just fat, bone marrow also provides other nutrients, including:

Proteins: A small amount of protein is present, including collagen.
Vitamins: Marrow contains various vitamins such as B12, riboflavin, and vitamin E.
Minerals: Small amounts of iron and phosphorus are also found within marrow.
Fatty Acids: It is a source of healthy fats like conjugated linoleic acid (CLA), which has been linked to anti-inflammatory effects.

How Researchers Calculate Skeletal Calories

The notable figure of 25,000+ calories for a skeleton originated from a specific 2017 study by archaeologist James Cole, which calculated the nutritional value of an entire average human body based on its various tissues. The calculation was part of a larger research effort to understand the viability of nutritional human cannibalism during the Paleolithic era. Cole's analysis broke down the calories by body component, attributing a significant portion of the total human caloric value to the skeleton, specifically accounting for the fat and protein within the bone marrow. This calculation revealed that while a human is a caloric source, it is not as efficient a source as many large hunted animals, suggesting cannibalism was likely more for social or ritualistic reasons than pure caloric gain.

Comparison: Calorie Density of Skeletal Components

To understand the difference, a comparison of the different parts of the skeleton can be useful. The compact, dense outer layer of bone is composed of a mineralized matrix, while the interior is spongy bone filled with marrow.


Component	Calorie Source	Primary Function	Caloric Density (Approx.)
Compact Bone	Minimal (collagen)	Support, Protection	Very Low
Bone Marrow (Yellow)	Primarily Fat	Energy Storage, Hematopoiesis	Very High (e.g., 785 kcal/100g in animal marrow)
Bone Marrow (Red)	Fat, Protein	Blood Cell Production	Low to Medium

The Storage and Mobilization of Bone Marrow Fat

As a crucial energy reservoir, the fat stored in yellow bone marrow is an evolutionarily significant adaptation. The body can quickly access and metabolize this fat when other energy sources are depleted, such as during starvation or prolonged exercise. This energy is a lifeline, sustaining vital bodily functions and proving that bones are far from dormant. The entire process is regulated by complex endocrine signals that control fat deposition and release from the marrow. For further reading on this fascinating biological function, the National Institutes of Health has highlighted the role of marrow fat in metabolic pathways.

Conclusion

So, how many calories does a skeleton have? The answer is not zero, as the common perception might suggest. Instead, the number is substantial, with the caloric value residing almost entirely within the fatty tissue of the bone marrow. The hard, mineralized bone tissue serves its structural purpose, while the inner marrow acts as a dynamic energy reserve, ready to be called upon by the body when needed. This biological function, revealed by scientific analysis, reinforces that the skeleton is a living, active organ, not merely a static framework.

Frequently Asked Questions

The calories are stored in the bone marrow, which is the soft, fatty tissue found in the central cavities of bones. The hard, outer bone tissue has very little caloric value.

While it's impractical to consume an entire mineralized skeleton, humans can and do consume the calories from bone marrow, often in dishes like bone broth or by cooking the marrow itself.

The body can mobilize the stored fat in yellow bone marrow during periods of energy deprivation, such as during starvation, to provide a critical source of fuel.

Bone density, which refers to the mineral content of the bone tissue, is separate from the caloric content of the marrow inside. While they are related in overall bone health, the number of calories is tied to the amount and type of marrow, not the density of the mineralized bone itself.

Bone marrow fat is an evolutionarily significant energy reserve. It is a strategically placed fuel source that can be accessed during times of extreme caloric need, and these fat cells interact uniquely with the body's metabolism.

Yes, many animals have skeletons that contain calorie-rich bone marrow. The practice of consuming marrow from animal bones, particularly from larger mammals, is common in many cultures.

Bone broth's calorie content can vary depending on how it's made, but it is not a high-calorie food. While it leaches some nutrients from the bones and marrow, most of the fat and calories remain behind, leaving a broth that is typically low in calories but rich in minerals and amino acids.