Unpacking the Connection: What is the relationship between protein and energy?

November 11, 2025 •

4 min read

While protein, like carbohydrates, contains 4 calories per gram, its primary role is not to be the body's main energy source. The nuanced relationship between protein and energy is crucial for understanding how the body prioritizes fuel sources, builds and repairs tissue, and maintains overall health and vitality.

Quick Summary

Protein is primarily used for building and repairing tissues, but can be converted to energy through a process called gluconeogenesis. The body prefers to use carbohydrates and fats for fuel, turning to protein only when other energy stores are depleted. This is a less efficient process that can lead to muscle mass loss.

Key Points

Protein's Primary Role: Protein is fundamentally a structural nutrient, used for building and repairing tissues, producing enzymes, and supporting immune function, not for primary energy supply.
The Energy Hierarchy: The body preferentially uses carbohydrates for quick energy and fats for sustained energy, turning to protein only when other fuel sources are depleted, such as during starvation or intense exercise.
Gluconeogenesis: The process of converting protein into glucose for energy is called gluconeogenesis and occurs primarily in the liver, putting extra stress on the kidneys and potentially leading to muscle tissue loss.
Indirect Energy Benefits: Protein supports energy levels indirectly by stabilizing blood sugar when consumed with carbohydrates and increasing satiety, which helps prevent energy crashes.
Macronutrient Balance: For optimal health, energy, and muscle maintenance, it is crucial to consume a balanced diet with adequate carbohydrates and fats to spare protein for its primary structural and functional roles.

Protein's Primary Function: More Than Just Fuel

At its core, protein's main function is a structural one. Composed of amino acids, protein serves as the fundamental building block for a vast array of tissues and functions within the body. This includes:

Building and repairing tissues: Protein is essential for the growth and maintenance of muscles, bones, skin, and connective tissues. During periods of growth or recovery from injury, the demand for protein increases significantly.
Enzyme and hormone production: Most of the body's enzymes, which catalyze metabolic reactions, and many hormones, which act as chemical messengers, are proteins. Hormones like insulin and glucagon, for instance, are critical in regulating energy metabolism.
Transport and storage: Specialized proteins act as carriers for nutrients and other substances throughout the bloodstream. Hemoglobin, a protein in red blood cells, transports oxygen, while ferritin stores iron.
Immune function: Antibodies, which are vital for fighting off infections and illnesses, are also made of protein.

Because of these critical functions, the body prioritizes using protein for building and repair rather than burning it for immediate energy. Using protein for fuel is generally reserved for survival scenarios when other energy sources are scarce.

The Energy Hierarchy: Where Protein Fits In

Your body has a preferred order for metabolizing macronutrients for energy. This hierarchy prioritizes efficiency and preservation of vital structures.

Carbohydrates (Fastest Fuel): Carbohydrates are the body's most immediate and preferred energy source, particularly for the brain and during high-intensity exercise. They are quickly broken down into glucose, which is used for cellular respiration to produce adenosine triphosphate (ATP), the body's energy currency. Excess glucose is stored as glycogen in the liver and muscles for later use.
Fats (Long-Term Storage): As the most concentrated source of energy (9 calories per gram), fats serve as a slow-burning, long-term energy reserve. When carbohydrate stores are low, the body taps into fat reserves for fuel, a process that takes longer than metabolizing carbohydrates but yields more energy.
Protein (The Last Resort): Under normal dietary conditions, protein contributes a small portion (around 5%) of the body's total energy needs. Its use as a primary fuel source increases only when carbohydrate and fat stores are insufficient, such as during prolonged fasting, intense endurance exercise, or undernourishment.

The Process of Protein-to-Energy Conversion

When the body is forced to use protein for energy, it undergoes a metabolic process called gluconeogenesis, which literally means "the creation of new glucose".

Deamination: Amino acids, the building blocks of protein, are first stripped of their nitrogen-containing amino group. This process, called deamination, happens primarily in the liver and results in the formation of ammonia, a toxic compound.
Urea Cycle: To safely remove the ammonia from the body, the liver converts it into urea, which is then excreted by the kidneys.
Conversion to Glucose: The remaining carbon skeleton of the amino acid is then converted into glucose, which can be used for energy.

This is a far less efficient process than using carbohydrates or fats, and it can put additional stress on the kidneys. Furthermore, because the body does not have a dedicated protein storage depot like it does for glycogen (carbohydrates) and fat (adipose tissue), using protein for energy often means breaking down functional muscle tissue.

Comparison of Macronutrients for Energy


Feature	Carbohydrates	Protein	Fats
Primary Role	Primary and quick energy source	Building, repairing, and maintenance	Long-term energy storage
Energy Content (per gram)	4 calories	4 calories	9 calories
Conversion to Energy	Quickest and most efficient	Slow and least efficient	Slow-burning and efficient for low-intensity activity
Thermic Effect of Food (TEF)	~5-10% of calories burned in digestion	~20-30% of calories burned in digestion	~0-3% of calories burned in digestion
Storage in Body	Limited storage as glycogen	Not stored for energy; constant turnover	Large storage in adipose tissue
Primary Use during Fasting	Initial stages (glycogen)	Later stages (muscle breakdown)	Extended periods of fasting

Indirect Ways Protein Impacts Your Energy

While protein isn't a direct source of instant energy, it profoundly affects your energy levels in several indirect ways:

Steady Blood Sugar: Consuming protein alongside carbohydrates helps stabilize blood sugar levels. Protein slows down the digestion and absorption of carbs, preventing rapid spikes and crashes in blood sugar that can cause fatigue.
Increased Satiety: Protein is the most satiating of the macronutrients, meaning it helps you feel full for longer. This can help regulate overall calorie intake and prevent the energy dips often associated with hunger.
Muscle Maintenance: Maintaining lean muscle mass is metabolically active, meaning it requires energy. Adequate protein intake ensures muscle mass is preserved, which supports a higher resting metabolic rate and overall energy expenditure.
Iron Storage: Protein is required for storing iron, and low iron levels are a common cause of fatigue.

Conclusion: Balancing Your Macros for Optimal Energy

The relationship between protein and energy is a dynamic and interdependent one. Protein is not the body's go-to fuel source; rather, it is a crucial building and repair nutrient whose energy is tapped only when other reserves are low. This metabolic hierarchy highlights the importance of a balanced diet that includes all macronutrients.

Ensure adequate carbohydrate intake to provide readily available fuel and spare protein for its vital functions.
Include healthy fats to provide a concentrated, slow-release energy source.
Consume sufficient protein to support tissue repair, muscle growth, and maintain steady blood sugar levels.

Prioritizing a balanced intake of all three macronutrients ensures your body functions optimally, providing you with consistent energy while protecting your precious muscle tissue from being cannibalized for fuel. For more detailed information on nutrition, consult the U.S. Department of Agriculture's Dietary Guidelines for Americans.

Frequently Asked Questions

Yes, protein can provide energy, delivering 4 calories per gram, the same as carbohydrates. However, your body prefers carbohydrates and fats for fuel and only uses protein for energy when those other sources are in short supply.

When the body is in a calorie deficit or during intense, prolonged exercise, it can break down proteins into amino acids. These amino acids are then converted into glucose through a process called gluconeogenesis, which is used as fuel.

Using protein for energy is not ideal because it is a less efficient process and can involve breaking down functional muscle tissue. A balanced diet with sufficient carbs and fats is recommended to ensure protein is reserved for its critical roles in building and repair.

When consumed with carbohydrates, protein helps to slow down the absorption of sugar into the bloodstream. This prevents the rapid spike and subsequent crash in blood sugar that can leave you feeling fatigued.

Protein is not the body's preferred energy source because its main purpose is to build, repair, and maintain tissues. Sacrificing this function for fuel is an inefficient use of a vital nutrient.

The thermic effect of food (TEF) is the energy required to digest and process food. Protein has a higher TEF than carbohydrates or fats, meaning your body burns more calories breaking it down.

While sufficient protein is necessary for overall vitality, simply eating more won't provide a quick energy boost. Excess protein that isn't used for its primary functions is either converted to fat or broken down and excreted, which can strain the kidneys.