Understanding the Secondary Source of Energy in Our Body

December 25, 2025 •

3 min read

While many believe carbohydrates are the only energy source, up to 95% of all dietary fats are triglycerides that can be stored and used as a backup fuel. This ability to switch energy sources is vital, and understanding what is the secondary source of energy in our body reveals a key aspect of metabolic flexibility.

Quick Summary

The body's primary energy source is glucose from carbohydrates, but when this is insufficient, stored fat (triglycerides) becomes the crucial secondary fuel source. This switch involves breaking down fats into fatty acids and, eventually, ketone bodies.

Key Points

Primary Fuel: The body's first choice for energy is glucose, a simple sugar derived from carbohydrates.
Secondary Source: Stored fat, in the form of triglycerides, is the body's backup energy supply, mobilized when glucose is scarce.
Fat to Energy: The process of converting fat to usable energy is called lipolysis, which breaks down triglycerides into fatty acids.
Ketone Bodies: During prolonged fasting or low-carb states, the liver produces ketone bodies from fats to fuel the brain.
Protein as Last Resort: Protein from muscle tissue is only used as an energy source in extreme, long-term starvation.
Exercise and Fuel: The body uses different proportions of carbs and fats depending on the intensity and duration of the exercise.

The Body’s Preferred Energy: Glucose

Before diving into the secondary fuel, it is important to understand the body's primary energy source. The human body's first choice for fuel is glucose, a simple sugar derived from the carbohydrates we consume. Glucose is readily available and can be quickly converted into adenosine triphosphate (ATP), the chemical energy currency of our cells. Excess glucose is stored in the liver and muscles as glycogen, a rapidly mobilizable reserve. For most daily activities and short bursts of high-intensity exercise, glucose is the dominant fuel.

The Role of Glycogen

Glycogen acts as the body's short-term energy battery. When blood glucose levels begin to drop, such as between meals, the pancreas releases the hormone glucagon. Glucagon signals the liver to break down its stored glycogen (a process called glycogenolysis) and release glucose back into the bloodstream to maintain stable blood sugar levels. Muscle glycogen is used directly by muscle cells during exercise and is not released into the general circulation. The body’s glycogen stores, however, are limited and can be depleted in as little as 24 hours of fasting or after intense, prolonged exercise.

The Secondary Energy Source: Stored Fat

Once the body's glycogen reserves are significantly diminished, a metabolic switch occurs, and the body turns to its long-term, and much larger, energy reserves: stored fat. This is the secondary source of energy in our body. This fat is stored in adipose tissue primarily in the form of triglycerides.

How Fat Becomes Fuel

The process of breaking down stored fat is known as lipolysis.

Hormonal Signal: Low insulin and high glucagon levels in the blood, triggered by fasting or exercise, activate hormone-sensitive lipases.
Lipolysis: These enzymes break down triglycerides in fat cells into their component parts: glycerol and three fatty acid molecules.
Transport: The liberated fatty acids are released into the bloodstream and carried to active tissues, such as the heart and skeletal muscles.
Beta-Oxidation: In the mitochondria of these cells, the fatty acids undergo a series of reactions called beta-oxidation. This process breaks the fatty acid chains into two-carbon units of acetyl-CoA.
ATP Generation: The acetyl-CoA enters the citric acid (Krebs) cycle to produce large quantities of ATP. Fats are incredibly energy-dense, yielding more than twice the calories per gram compared to carbohydrates (9 kcal/g vs. 4 kcal/g).

The Role of Ketone Bodies

During periods of prolonged starvation or very low carbohydrate intake, the body's metabolism enters a deeper state of fat utilization known as ketosis. In this state, the liver converts excess acetyl-CoA (from fatty acid breakdown) into water-soluble molecules called ketone bodies. The two primary ketone bodies used for fuel are acetoacetate and beta-hydroxybutyrate. Ketones are crucial because, unlike fatty acids, they can cross the blood-brain barrier to provide a vital energy source for the brain, which typically relies on glucose.

The Last Resort: Protein

In extreme situations of prolonged fasting or starvation, when both glucose and fat stores are insufficient, the body will begin to break down protein. Amino acids from muscle and other tissues can be converted into glucose through a process called gluconeogenesis. While this can provide a much-needed energy supply, it comes at the cost of muscle mass and is considered a last-resort survival mechanism.

Comparison: Carbohydrates vs. Stored Fat


Feature	Carbohydrates (Primary)	Stored Fat (Secondary)
Energy Yield	4 calories per gram	9 calories per gram
Availability	Fast and readily available	Slow-to-mobilize, long-term reserve
Metabolism Speed	Rapid, efficient, requires less oxygen	Slower, requires more oxygen
Storage Capacity	Limited (glycogen)	Large (triglycerides)
Key Process	Glycolysis	Lipolysis and Beta-Oxidation
Primary Function	Immediate fuel for the brain and muscles	Sustained fuel for low-intensity activity and fasting

Conclusion

While glucose from carbohydrates is the body's primary and most immediate source of fuel, stored fat serves as the essential secondary source of energy when carbohydrate reserves are low. This metabolic flexibility ensures a constant supply of energy, supporting both daily functions and survival during fasting. The switch to fat metabolism involves breaking down triglycerides through lipolysis, providing a high-energy, long-lasting fuel source for the body's cells. In dire circumstances, protein provides a tertiary fuel source, but at a significant biological cost. For more detailed information on human metabolism, consider reviewing the comprehensive resources available from the National Institutes of Health.(https://www.ncbi.nlm.nih.gov/books/NBK546690/)

Frequently Asked Questions

The primary energy source for the human body is glucose, which is a simple sugar derived from the carbohydrates we consume. This glucose is used for immediate energy needs and is stored as glycogen for quick access.

When the body's glucose and glycogen stores are low, it initiates lipolysis, a process that breaks down stored triglycerides (fat) into fatty acids. These fatty acids are then transported to cells and broken down through beta-oxidation to produce ATP (energy).

Ketone bodies are alternative fuel molecules produced by the liver from fatty acids when glucose is limited. They are particularly important for providing energy to the brain during prolonged fasting or carbohydrate restriction, as the brain cannot use fatty acids directly.

The body begins to rely more heavily on fat for energy when its glycogen reserves are depleted. This typically occurs after several hours of fasting or during prolonged, low-to-moderate intensity exercise.

Yes, protein can be used for energy, but it is considered a last resort. The body breaks down protein from tissues, such as muscles, in a process called gluconeogenesis, primarily during severe, long-term starvation when both carbohydrate and fat stores are insufficient.

Fats provide significantly more energy per gram than carbohydrates. Fat contains about 9 calories per gram, while carbohydrates contain 4 calories per gram. However, carbohydrates are metabolized faster for quicker energy.

Exercise intensity has a major effect on fuel usage. High-intensity exercise primarily uses carbohydrates for quick energy. Conversely, low-to-moderate intensity and longer duration exercise, like a long walk or light jog, relies more on fat as its fuel source.