What is the third source of energy for the body?

November 17, 2025 •

4 min read

The human body is an incredibly efficient machine, primarily fueled by carbohydrates and fats from our diet. However, in extreme circumstances like prolonged fasting or intense endurance exercise, the body turns to what is the third source of energy for the body: protein, as a survival mechanism.

Quick Summary

Protein serves as the body's third energy source, tapped into when carbohydrate and fat stores are exhausted. The body breaks down protein into amino acids, which are then used for glucose synthesis, especially during fasting or prolonged exercise.

Key Points

Third Energy Source: Protein is the body's third and least preferred source of energy, utilized primarily during starvation or prolonged exercise when other fuel stores are low.
Fuel Hierarchy: The body prioritizes carbohydrates for quick energy, followed by fats for long-term endurance, before resorting to protein.
Protein Catabolism: To use protein for energy, the body breaks it down into amino acids, which can then be converted into glucose via gluconeogenesis in the liver.
Consequence of Use: Relying on protein for fuel leads to the breakdown of muscle tissue (muscle wasting), compromising the body's structural and functional integrity.
Ketone Bodies: During carb restriction, the liver can produce ketone bodies from fats to fuel the brain and other tissues, offering an alternative energy pathway that is not the same as using protein.
Nutrient Intake is Key: For optimal health and performance, it is crucial to consume sufficient carbohydrates and fats to spare protein for its primary functions.

The Body's Energy Hierarchy

Our bodies rely on a predictable sequence of fuel sources to meet their energy demands, with the primary fuel determined by the availability of nutrients and the intensity of activity. The ultimate goal of these metabolic pathways is the production of adenosine triphosphate (ATP), the universal energy currency for cellular functions. The body's energy substrates—carbohydrates, fats, and proteins—are digested and broken down into smaller components: glucose, fatty acids, and amino acids, respectively.

The Preferred Energy Sources

Carbohydrates: Our most readily available and preferred source of energy. Carbs are broken down into glucose, which can be used immediately or stored as glycogen in the liver and muscles for quick access during exercise or between meals. A gram of carbohydrate provides about 4 calories.
Fats: This is the body's primary long-term energy storage, providing more than twice the energy per gram than carbohydrates or proteins. Stored as triglycerides in adipose tissue, fat is a highly efficient fuel for rest and low-to-moderate intensity, long-duration activities. The process of converting fat into ATP is slower than using carbs, but its reserves are nearly limitless.

Understanding the Third Source: Protein Catabolism

Protein is generally not considered a primary energy source, as its main role is to build and repair tissues, synthesize hormones, and support various cellular functions. However, when both carbohydrate and fat reserves are significantly depleted, the body's survival mechanisms prioritize fueling essential organs like the brain and heart, turning to protein as a last resort.

The Process of Protein as a Fuel

Protein Breakdown: The process begins with protein catabolism, where proteins are broken down into their amino acid components. This primarily involves breaking down muscle tissue, which leads to muscle wasting if sustained.
Amino Acid Conversion: The amino acids are then modified to enter energy-generating pathways. Glucogenic amino acids can be converted into glucose through a process called gluconeogenesis, primarily occurring in the liver.
Krebs Cycle Entry: Ketogenic amino acids can be converted to intermediates that enter the Krebs cycle (citric acid cycle) to produce ATP.
Nitrogen Excretion: A byproduct of amino acid metabolism is the nitrogen component, which is converted to urea and excreted via the kidneys. This places an added strain on the kidneys.

The Role of Ketone Bodies

Another metabolic response to low carbohydrate availability is the production of ketone bodies by the liver, derived from fatty acids. During fasting or a ketogenic diet, the brain and other tissues can adapt to using ketones for fuel, sparing limited glucose for cells that depend on it. This process is different from using protein for energy, although both occur during periods of prolonged energy deficit.

Protein vs. Other Macronutrients for Energy


Feature	Carbohydrates	Fats	Protein
Primary Role	Quick Energy, Glycogen Storage	Long-Term Energy Storage, Cell Membranes, Hormones	Building and Repairing Tissues, Enzymes, Hormones
Energy Yield	4 kcal/gram	9 kcal/gram	4 kcal/gram
Primary Use	High-Intensity Exercise, Brain Function	Rest, Low-Intensity Endurance Activity	Not a Primary Source; Used as Last Resort
Energy Pathway	Glycolysis, Oxidative System	Beta-oxidation, Oxidative System	Catabolism, Gluconeogenesis, Oxidative System
Availability	Readily available	Most abundant energy reserve	Used only when other sources are depleted

Implications for Health and Athletic Performance

Relying on protein for energy is not an optimal or sustainable strategy for performance or health. For athletes, adequate carbohydrate intake is crucial to fuel high-intensity exercise and spare muscle tissue. Inadequate energy from carbs and fats forces the body into a catabolic state, breaking down precious muscle mass. For general health, a balanced diet is key to ensuring protein can perform its essential functions rather than being diverted for emergency fuel.

How to Fuel Your Body Correctly

Prioritize a Balanced Diet: Ensure your daily intake includes sufficient carbohydrates, fats, and protein to support your energy needs and activity levels.
Consider Activity Intensity: The mix of fuel your body uses depends on your activity. Higher intensity relies more on carbs, while lower intensity can burn a higher ratio of fat.
Manage Fasting Wisely: While intermittent fasting is popular, prolonged or severe energy deficits can trigger protein breakdown. Understand your body’s limits.

Conclusion

In summary, while carbohydrates and fats serve as the body's primary and secondary fuel sources, protein is what is the third source of energy for the body, utilized only under duress. This process of protein catabolism and gluconeogenesis is a survival mechanism that allows the body to continue functioning when other fuel reserves are exhausted, but it comes at the cost of muscle tissue. Maintaining a balanced diet with adequate carbohydrate and fat intake is the most effective way to ensure protein is reserved for its critical roles in building and repair, supporting long-term health and performance.

National Institutes of Health (NIH) | (.gov)

Frequently Asked Questions

The third source of energy for the body is protein. It is used as a last resort when the more readily available carbohydrate and stored fat reserves are exhausted, such as during periods of starvation or prolonged, intense exercise.

The body breaks down proteins into their amino acid building blocks through a process called protein catabolism. Some of these amino acids are then converted into glucose by the liver through a pathway called gluconeogenesis, which can then be used for energy.

Protein's primary role is building and repairing tissues, creating enzymes, and supporting other vital bodily functions. Using it for energy sacrifices these essential functions, often leading to muscle loss. This is why the body prefers to use carbohydrates and fats for fuel.

The body primarily uses protein for energy under extreme conditions, including prolonged fasting, starvation, or during long-duration, high-intensity exercise when glycogen (stored carbohydrates) and fat reserves are depleted.

No, if you eat more protein than your body needs, it is generally stored as fat, not immediately used for energy. Your body will still prioritize carbs and fat for fuel, only using protein when necessary.

When the body uses protein for energy, it often breaks down muscle tissue to access the necessary amino acids, a process known as muscle wasting. This is detrimental to health and athletic performance.

No. Ketone bodies are an alternative fuel produced from fatty acids during periods of low carbohydrate availability. While both processes are survival mechanisms used during energy deficits, ketones are derived from fat, not protein.