What is the Main Source of Stored Energy in the Body?

December 24, 2025 •

3 min read

An average human's body can store an estimated 131,600 calories as fat, compared to only about 2,400 calories stored as glycogen. While glycogen provides a fast-acting, short-term energy reserve, the body's primary and most concentrated form of stored energy is fat, or adipose tissue.

Quick Summary

The body primarily stores energy as fat (adipose tissue) for long-term reserves and as glycogen (stored carbohydrates) for short-term, readily available fuel. Fat offers a highly concentrated energy source for sustained activity and survival, while glycogen is used for immediate needs like intense exercise.

Key Points

Fat is the main storage source: Adipose tissue (body fat) holds the majority of the body's stored energy for long-term use and survival.
Glycogen is for quick energy: Stored primarily in the liver and muscles, glycogen provides a readily available, short-term source of fuel for immediate needs and high-intensity exercise.
Fat is more energy-dense: With approximately 9 calories per gram, fat stores more than twice the energy of glycogen per unit of weight.
Storage is efficient with fat: Unlike glycogen, which is bound to water and bulky, fat is stored in a concentrated form with minimal water, making it an efficient energy reserve.
The body prioritizes energy use: During exercise, the body uses glycogen first before tapping into its more abundant fat stores for sustained activity.
Both sources are crucial for health: Glycogen maintains stable blood sugar and fuels intense activity, while fat is essential for endurance and energy security during food scarcity.

The Dual-Storage System: How the Body Manages Energy Reserves

The human body has evolved a sophisticated system to store and access energy, adapting to varying levels of physical activity and food availability. This system relies on two primary storage molecules: glycogen for short-term energy and lipids (fats) for long-term reserves. Understanding the roles of these two energy sources is key to grasping how metabolism works and why the body prioritizes them differently based on its needs.

Glycogen: The Quick-Access Energy Source

Glycogen is a multibranched polysaccharide of glucose that serves as the body's immediate energy bank. It is made and stored primarily in the liver and skeletal muscles.

Liver Glycogen: The liver stores approximately 100 grams of glycogen, which is primarily used to maintain a stable blood glucose level, especially during periods of fasting or overnight. When blood sugar drops, the pancreas releases the hormone glucagon, which signals the liver to break down glycogen and release glucose into the bloodstream. This ensures that organs like the brain, which rely almost exclusively on glucose for fuel, have a constant supply.

Muscle Glycogen: Muscles store a much larger quantity, about 400 grams, but this energy is for the muscle's own use. Muscle cells lack the enzyme necessary to release glucose into the bloodstream, so their glycogen stores are used as a local fuel source for muscle contraction. This allows for a rapid burst of energy during intense exercise, like sprinting or weightlifting.

Lipids (Fat): The Long-Term Energy Depot

While glycogen is vital for immediate energy, lipids represent the body's main source of stored energy for sustained, low-intensity activities and survival during starvation.

High Energy Density: Fat is a far more energy-dense molecule than glycogen, providing approximately 9 calories per gram compared to glycogen's 4 calories per gram. This means the body can store more than twice the energy in the same amount of mass.
Efficient Storage: Fat is stored in adipose tissue with very little water content, whereas each gram of glycogen is bound to several grams of water, making it significantly bulkier. This makes fat a more efficient and compact storage solution for the body's long-term energy needs. An average person carries tens of thousands of calories in fat reserves, enough to last for weeks.
Utilization: During low-intensity or long-duration exercise, like a marathon, the body shifts from using glycogen as a primary fuel to burning fat. The fat is broken down into fatty acids, which are then transported to cells and converted into ATP, the cell's energy currency. The process is slower than using glycogen but provides a steady, prolonged energy supply.

The Importance of Both Glycogen and Fat

The dual system of storing both glycogen and fat provides the body with metabolic flexibility. Glycogen is readily available for quick, high-intensity demands, while fat acts as a large, lightweight, and efficient reserve for sustained needs. This strategy is an evolutionary advantage, allowing our ancestors to survive periods of food scarcity and to perform physically demanding tasks.

Comparison Table: Glycogen vs. Fat


Feature	Glycogen (Carbohydrates)	Lipids (Fat)
Storage Location	Liver and muscles	Adipose tissue throughout the body
Energy Density	~4 kcal per gram (with water)	~9 kcal per gram (without water)
Storage Capacity	Limited (approx. 2,400 calories)	Expansive (approx. 131,600+ calories)
Mobilization Speed	Very fast	Slower (requires more complex breakdown)
Primary Use	High-intensity, short-duration activity; maintaining blood sugar	Low-intensity, long-duration activity; long-term survival
Water Content	High (binds 3-4 grams of water per gram)	Low (hydrophobic)

The Body's Priority in Energy Use

During exercise, the body typically uses available glucose and glycogen stores first before relying more heavily on fat. This is because breaking down glucose for energy is a faster process. Only after the readily available carbohydrate reserves are diminished does the body increase its use of stored fat. This is often described by endurance athletes as "hitting the wall" or "bonking" when their glycogen stores become depleted.

Conclusion

In summary, while glycogen offers a crucial, quickly accessible source of carbohydrates for immediate energy needs and maintaining blood sugar, the definitive main source of stored energy in the body is fat. Its high energy density and compact storage make it the ideal long-term energy depot for sustaining life over extended periods. This sophisticated two-pronged system ensures the body is prepared for both short bursts of intense activity and prolonged periods without food, showcasing a remarkable example of metabolic efficiency.

Human Kinetics - The Body's Fuel Sources

Frequently Asked Questions

Fat provides more energy per gram than glycogen. Fat stores approximately 9 calories per gram, while glycogen stores about 4 calories per gram.

Glycogen serves as a rapidly accessible, short-term energy reserve. Liver glycogen helps maintain stable blood glucose levels, while muscle glycogen provides fuel for muscle contraction during exercise.

Fat is a much more efficient energy storage medium. It is more energy-dense and less bulky than glycogen, which is bound to significant amounts of water. This makes fat ideal for long-term, compact energy storage.

During low-intensity, long-duration exercise, the body primarily uses fat as its fuel source. This conserves the more limited glycogen reserves for when they are needed for higher-intensity activity.

Although the liver has a higher concentration, the majority of the body's total glycogen is stored in the skeletal muscles, simply because muscle mass accounts for a larger portion of total body weight.

When needed, the body breaks down stored fat (triglycerides) into fatty acids through a process called lipolysis. These fatty acids are then transported to cells to be burned and converted into ATP for energy.

The body cannot efficiently convert fat back into glucose to be used by all tissues, such as the brain. This is why a minimum intake of carbohydrates is needed to prevent the body from breaking down protein for glucose production.