How much energy is in 1 gram of protein?

November 26, 2025 •

3 min read

According to the Atwater system, the standard nutritional model used for food labeling, 1 gram of protein provides approximately 4 kilocalories (kcal) of metabolizable energy. This figure, however, represents the net energy your body can utilize, not the total energy that is released when the substance is burned.

Quick Summary

One gram of protein contains roughly 4 calories of usable energy, a value established by the Atwater system for nutrition labeling. This is less than the total energy released when burned in a lab due to metabolic costs.

Key Points

Standardized Value: 1 gram of protein is officially counted as 4 kilocalories (kcal) for nutritional labeling purposes.
Metabolizable Energy: The 4 kcal/g figure represents the net energy your body can use, factoring in the energy costs of digestion and metabolism.
Not the Primary Energy Source: Protein's main function is to build and repair tissues, not to be used as a primary fuel source.
Inefficient Conversion: The body expends energy to digest protein, giving it a high thermic effect of food (TEF), which reduces the net energy yield.
Factors Affecting Digestion: The true digestibility and energy absorption can vary based on the protein's source, how it's processed, and individual health factors.
Laboratory vs. Body: The energy released when protein is burned in a lab is higher than the energy your body can extract because the body cannot completely oxidize it.

The Atwater System: The '4-9-4' Standard

For decades, the standard for determining the energy content of food has been the Atwater system, developed by chemist Wilbur Olin Atwater in the late 19th and early 20th centuries. This system assigns general energy conversion factors to the three major macronutrients: protein, carbohydrates, and fat.

Protein: 4 kilocalories per gram (kcal/g)
Carbohydrates: 4 kilocalories per gram (kcal/g)
Fat: 9 kilocalories per gram (kcal/g) These values are estimates of the metabolizable energy—the energy the body can actually extract from the food after accounting for digestion, absorption, and metabolic processing. This is why the value is lower than the raw energy content measured in a lab using a bomb calorimeter. The Atwater system is the basis for the nutrition facts labels found on packaged foods worldwide.

Why the Body Uses Protein Inefficiently for Energy

While protein provides 4 kcal/g, it's not the body's preferred source of energy. Its primary roles are building and repairing tissues, creating hormones, and producing enzymes. The body turns to protein for energy only when it doesn't have enough carbohydrates or fats available. This process, called gluconeogenesis, requires more metabolic effort than converting carbohydrates into energy. Some of the energy is lost as heat during digestion, which is known as the thermic effect of food (TEF). Protein has the highest TEF of all macronutrients, meaning a significant portion of its caloric value is used simply for its own digestion.

The Difference Between Laboratory and Body Caloric Value

The discrepancy between the energy a substance contains and what a body can use is a key concept. A bomb calorimeter measures the total heat of combustion—the maximum potential energy if the substance were completely burned. However, the human body is a biological system, not a furnace. It cannot utilize 100% of the energy from food. For protein, this difference is substantial because:

Incomplete Oxidation: The body cannot fully oxidize protein. The nitrogen component of amino acids is not used for energy and is instead excreted as urea.
High Thermic Effect: As mentioned, digesting protein is energy-intensive. This metabolic cost subtracts from the net energy available for other functions.
Amino Acid Variation: The specific energy yield can vary slightly depending on the amino acid composition of the protein source. The Atwater value is an average used for convenience.

Macronutrient Energy Comparison


Macronutrient	Metabolizable Energy (kcal/g)	Primary Role in Body	Energy Density
Protein	4	Building & Repairing Tissues	Moderate
Carbohydrates	4	Primary Energy Source	Moderate
Fats	9	Long-Term Energy Storage	High
Alcohol	7	Secondary Energy Source	High

Factors Influencing the Digestion of Protein

Not all protein is digested and absorbed equally. Several factors affect how much usable energy your body can extract from a protein source:

Source of Protein: Animal proteins like eggs and milk typically have higher digestibility than plant-based proteins, such as those from legumes and cereals, which contain fiber and other anti-nutritional factors that can impede absorption.
Processing: How food is processed can alter protein structure and digestibility. High-heat processing can sometimes make proteins less digestible.
Individual Variation: Factors such as age and gut health can affect an individual's ability to digest and absorb nutrients efficiently.

Conclusion: The Final Word on Protein's Energy

The simple answer is that 1 gram of protein contains approximately 4 kilocalories of energy. This figure is the standardized Atwater conversion factor used on nutrition labels and represents the net metabolizable energy your body can derive. It is a highly practical and widely accepted number for dietary planning. However, this is not the full story. The biological and metabolic processes involved mean that protein is a less efficient energy source compared to carbohydrates or fats, and its primary function is not to provide fuel but to build and maintain the body's structure and function. So, while you can technically use protein for energy, your body prefers to use it for its more critical roles. The 4 kcal/g figure is a reliable shorthand, but understanding the metabolic context provides a much more complete picture of protein's nutritional value.

Additional Resources

For more information on nutrition and food science, explore the resources from authoritative sources like the USDA National Agricultural Library.

Frequently Asked Questions

To calculate the calories from protein, you simply multiply the number of grams of protein by the Atwater factor of 4. For example, a food with 20 grams of protein would provide 80 calories from protein (20g * 4 kcal/g).

Protein is not the body's preferred source of energy. While it can be used for fuel, its primary roles are structural and functional. The body uses carbohydrates and fats for energy first, turning to protein only when these are insufficient.

Fat contains more than twice the amount of metabolizable energy per gram compared to protein. Fat molecules are more energy-dense and do not contain nitrogen, which the body must excrete when processing protein, leading to a loss of energy.

The Atwater system is the method used by food scientists to calculate the approximate metabolizable energy of food. It assigns a standardized value of 4 kcal/g for protein and carbohydrates and 9 kcal/g for fat, which is the basis for modern nutrition labels.

No, your body does not absorb all the energy. The digestion of protein is energy-intensive, and some energy is lost as heat (the thermic effect of food). The 4 kcal/g figure accounts for these metabolic losses.

No, protein digestibility varies by source. Animal proteins are generally more digestible than plant-based proteins, which contain fiber and other compounds that can reduce absorption.

The energy value listed on a food label is the metabolizable energy, which is the net energy your body can use. It is not the total potential energy of the food as measured by a bomb calorimeter.