What is the energy contained in protein?

December 30, 2025 •

4 min read

According to the Food and Agriculture Organization, one gram of protein contains approximately 4 kilocalories of energy. This article explores what is the energy contained in protein, detailing its caloric value and how the body utilizes it differently from other macronutrients like carbohydrates and fats.

Quick Summary

Protein provides four calories per gram, but primarily serves as the body's building block for tissues and enzymes, not as a primary energy source. It is only metabolized for fuel during times of calorie restriction or depleted glycogen stores.

Key Points

Caloric Value: Protein provides 4 calories per gram, the same amount as carbohydrates.
Secondary Fuel Source: The body prefers carbohydrates and fats for energy, using protein primarily for building and repair.
Gluconeogenesis: During calorie restriction or intense exercise with low glycogen, protein can be converted to glucose for energy.
High Thermic Effect: Protein requires more energy to digest, absorb, and metabolize (20-30% of its calories), which can aid metabolism.
Essential Functions: Protein's core role is to build and repair tissues, produce enzymes, and support countless bodily functions.
Amino Acid Building Blocks: Energy from protein comes from breaking down its amino acid components, which is less efficient than using other macros.

Protein as a Macronutrient

Protein, alongside carbohydrates and fats, is one of the three essential macronutrients that supply the body with energy. These macronutrients, composed of amino acids, provide the raw material for building and repairing body tissues, producing hormones and enzymes, and maintaining overall bodily function. While protein does contain stored energy, its role as a fuel source is secondary to that of carbohydrates and fats under normal dietary conditions.

The Atwater System: Calculating Protein's Energy

The standard caloric value of protein is based on the Atwater system, a method developed to calculate the energy provided by food. In this system, one gram of protein is assigned an energy value of 4 kilocalories (kcal), which is equivalent to 17 kilojoules (kJ). This value represents the average usable energy the body can extract after digestion and metabolism. It's the same energy density assigned to carbohydrates, though fat provides a more concentrated 9 kcal per gram.

Protein Metabolism for Energy

Under ordinary circumstances, the body prioritizes carbohydrates as its primary energy source, followed by fats. Protein is the last resort. When the body's glycogen (stored carbohydrate) reserves are depleted, and fat stores are being utilized, amino acids from protein can be converted into glucose through a process called gluconeogenesis. This process is inefficient compared to using carbs or fats, and it involves the removal of the amino group from the amino acid in a process called deamination. The nitrogenous waste is then processed into urea by the liver and excreted, a process that places stress on the kidneys.

The Thermic Effect of Food (TEF) for Protein

The Thermic Effect of Food (TEF) is the energy expenditure required for digestion, absorption, and metabolism of nutrients. Protein has a significantly higher TEF than carbohydrates and fats. Digesting protein can burn approximately 20-30% of the calories it contains, meaning that for every 100 calories consumed from protein, the body uses 20-30 calories just to process it. This is a major reason high-protein diets are often linked with improved metabolism and weight management. For comparison, the TEF for carbohydrates is about 5-15%, and for fat, it's a mere 0-5%.

Comparison of Macronutrient Energy

To better understand protein's place in the energy hierarchy, it's helpful to compare it directly with the other macronutrients. The following table outlines the key differences in how the body uses these fuels.


Macronutrient	Calories per Gram	Primary Role in the Body	Role as Energy Source	Thermic Effect of Food (TEF)
Protein	4 kcal	Building and repairing tissues, hormones, enzymes	Backup source, used when carbs/fats are low	High (20-30%)
Carbohydrates	4 kcal	Primary, fast-acting energy source for body and brain	Primary, readily available fuel	Moderate (5-15%)
Fat	9 kcal	Long-term energy storage, insulation, vitamin absorption	Secondary, long-term fuel source	Low (0-5%)

Protein's Dual Functionality

While our body can and does extract energy from protein, this is not its primary function. Protein's main job is to provide the amino acids necessary for growth, maintenance, and repair of all body cells and tissues. This is why eating a well-balanced diet with sufficient carbohydrates and fats is crucial. It spares protein from being used for energy, allowing it to perform its more critical functions. Athletes, in particular, need adequate carbs and fats to fuel their training, so protein can focus on muscle repair and recovery.

Sources of Protein

High-quality protein sources are essential for a healthy diet. Different sources offer varying profiles of amino acids, particularly the nine essential amino acids the body cannot produce itself.

Animal-Based Sources: Lean meats (chicken, beef), fish (salmon, tuna), eggs, and dairy products (milk, yogurt, cheese) are considered 'complete proteins' because they contain all essential amino acids.
Plant-Based Sources: Lentils, chickpeas, tofu, quinoa, nuts, seeds, and beans are excellent plant-based options. While most plant proteins are not 'complete' on their own, a varied plant-based diet can provide all essential amino acids throughout the day.
Supplements: Protein powders like whey, casein, or plant-based blends can offer a convenient way to meet daily protein goals, especially for athletes or those with higher needs.

Conclusion

In summary, the energy contained in protein is approximately 4 calories per gram, a figure that is widely accepted in nutritional science. However, understanding this number alone is insufficient without context. The body is a highly efficient machine, preferring carbohydrates for immediate energy and fat for longer-term fuel. Protein's energy is reserved for situations when other fuel sources are scarce. Its primary and more vital functions involve the building, repairing, and maintenance of the body's structure. Therefore, while protein can provide energy, its role as a structural and functional component is far more important for overall health and performance. A balanced diet ensures protein can fulfill these essential duties rather than being converted into an inefficient fuel source.

For more information on nutrition, the Food and Agriculture Organization of the United Nations is an excellent resource, with extensive details on dietary guidelines.

Frequently Asked Questions

There are approximately 4 kilocalories in a single gram of protein. This is the same caloric value as carbohydrates but less than fat, which provides 9 calories per gram.

No, protein is not the body's primary energy source. The body preferentially uses carbohydrates for quick energy and fats for sustained, long-term energy. Protein is typically reserved for its primary role in building and repairing tissues.

The body turns to protein for energy when other fuel sources, particularly carbohydrate (glycogen) stores, are depleted. This can occur during extended periods of fasting, intense endurance exercise, or severe calorie restriction.

The thermic effect of food (TEF) refers to the energy required to digest and process nutrients. Protein has a high TEF, meaning the body burns 20-30% of its calories just to metabolize it, a much higher percentage than for carbohydrates or fat.

If you consume more protein than your body needs for its building and repair functions, it can be broken down. The carbon skeleton may be converted into glucose or fat and stored as an energy reserve.

Using protein for energy is inefficient because the body must first remove the nitrogenous waste from the amino acids, a process that requires energy and puts a strain on the kidneys. It also means protein cannot perform its more important structural and enzymatic functions.

Eating adequate protein supports healthy energy levels indirectly by helping to stabilize blood sugar when consumed with carbohydrates, which prevents energy crashes. It also helps build and maintain muscle mass, boosting overall metabolism.