Can Proteins Be Broken Down to Make ATP? Yes, and Here's How

December 29, 2025 •

4 min read

While carbohydrates and fats are the body's preferred and most efficient fuel sources, it's a fundamental metabolic fact that proteins can be broken down to make ATP. This process typically occurs when other energy reserves are depleted, such as during periods of starvation or prolonged intense exercise.

Quick Summary

The body can catabolize proteins into amino acids, which are then deaminated. The remaining carbon skeletons enter the cellular respiration pathways to ultimately produce ATP for energy, especially during starvation.

Key Points

ATP from Protein: Yes, proteins can be broken down to make ATP, primarily during energy-depleted states like starvation.
Amino Acid Conversion: The process starts with protein catabolism into individual amino acids, which are then deaminated to remove nitrogen.
Pathway Entry: The remaining carbon skeletons enter cellular respiration pathways, such as the Krebs cycle, to generate energy.
Inefficient Fuel: Using protein for ATP is inefficient and is considered a last-resort measure due to its primary role in building tissues.
Role of Gluconeogenesis: The liver can convert glucogenic amino acids into new glucose via gluconeogenesis, vital for brain function during fasting.
Fuel Hierarchy: The body preferentially uses carbohydrates first, then fats, and finally proteins for energy production.

The Body's Energy Hierarchy

Our bodies are designed with a clear hierarchy of fuel usage. The primary source is glucose from carbohydrates, which is readily converted into ATP through glycolysis and the Krebs cycle. When glucose is scarce, the body turns to fats, breaking them down into fatty acids and glycerol to power the cells. Only after these stores are significantly depleted does the body rely on protein for energy. Using protein for fuel is generally inefficient and is considered a last-resort measure because proteins have a far more critical role as the building blocks for tissues, enzymes, and hormones.

The Journey from Protein to ATP

The conversion of protein into usable energy is a multi-step metabolic process known as protein catabolism. It's not a straightforward path and involves several complex biochemical reactions.

Step 1: Digestion and Amino Acid Liberation

Before proteins can be used for energy, they must be broken down into their individual amino acid components. This begins in the stomach and small intestine, where digestive enzymes like proteases and peptidases cleave the long protein chains into smaller peptides and, finally, into free amino acids. These amino acids are then absorbed and transported to the liver and other cells throughout the body.

Step 2: Deamination – Removing the Nitrogen

Unlike carbohydrates and fats, amino acids contain nitrogen. This nitrogen group (an amine group) must be removed before the molecule can enter the energy-producing pathways. This process, called deamination, primarily occurs in the liver. The amine group is converted into ammonia and subsequently into urea, which is then excreted from the body via urine. The remaining carbon skeleton of the amino acid is what is used for energy production.

Step 3: Entry into the Cellular Respiration Pathways

The fate of the deaminated amino acid carbon skeleton depends on its specific structure. There are two main types of amino acids based on their metabolic fate:

Glucogenic Amino Acids: These amino acids are converted into pyruvate or other intermediates of the Krebs cycle. These can then be used in gluconeogenesis (the creation of new glucose) or continue through cellular respiration to make ATP.
Ketogenic Amino Acids: These amino acids are converted into acetyl-CoA, which can be used to synthesize ketone bodies or enter the Krebs cycle. Leucine and lysine are the only two exclusively ketogenic amino acids.

The carbon skeletons, now in various forms, enter the standard cellular respiration pathway, including the Krebs cycle and the electron transport chain, to generate ATP.

Gluconeogenesis: Making Glucose from Amino Acids

A particularly important process that utilizes glucogenic amino acids is gluconeogenesis. When blood glucose levels are low and the body needs a quick energy source for the brain and red blood cells, which rely almost exclusively on glucose, the liver initiates gluconeogenesis. Here, specific amino acids are converted into glucose, providing a vital energy supply under fasting conditions.

When Does the Body Use Protein for Energy?

Protein is used for energy in specific scenarios:

Prolonged Starvation or Fasting: When carbohydrate and fat stores are exhausted, the body will begin to break down muscle tissue to access the amino acids for gluconeogenesis.
Intense, Prolonged Exercise: In endurance sports, if athletes do not consume enough carbohydrates, their bodies may begin to use protein for energy during the later stages of a race.
Low-Carbohydrate Diets: Individuals on very low-carb diets may use amino acids more frequently for energy, as their glucose supply from food is limited.

Comparing Fuel Sources for ATP Production


Feature	Carbohydrates	Fats	Proteins
Primary Role	Primary energy source	Long-term energy storage	Structural and functional building blocks
Energy Yield	Moderate (~4 kcal/g)	High (~9 kcal/g)	Moderate (~4 kcal/g)
Speed of Use	Fast (readily available)	Slow (more complex process)	Slow (last resort)
Metabolic Byproducts	Water, CO2	Water, CO2, Ketones	Water, CO2, Urea
Nitrogen Excretion	None	None	Yes (as urea)

The Efficiency of Protein as a Fuel

Using protein for energy is an inefficient process for the body. The deamination step is energy-intensive, and the byproducts, like urea, require energy to be excreted. Furthermore, breaking down structural proteins, such as those in muscle tissue, leads to a loss of lean body mass. This is why a balanced diet with adequate carbohydrates and fats is crucial to spare protein for its more essential roles. For more detailed information on metabolic pathways, a resource like the National Institutes of Health can be beneficial.

Conclusion

In summary, while not the body's preferred fuel, proteins can and are broken down to make ATP. This metabolic pathway is an essential survival mechanism that allows the body to continue functioning during periods of energy deficit. Understanding this process highlights the importance of consuming a balanced diet rich in carbohydrates and fats to ensure proteins are reserved for their vital roles in building and repairing the body, rather than being repurposed for energy.

Note: The content provided is for informational purposes and should not be considered medical advice. Always consult with a healthcare professional regarding your specific dietary needs.

Learn more about cellular respiration from NIH

Frequently Asked Questions

Using protein for energy is not the body's normal or preferred state. It happens mainly when carbohydrates and fats are not sufficiently available, such as during prolonged fasting, starvation, or intense, long-duration exercise without adequate fuel intake.

The first step is the breakdown of dietary or tissue proteins into individual amino acids through digestion and proteolysis, respectively.

The nitrogen-containing amine group is removed from amino acids in a process called deamination, primarily in the liver. It is then converted into urea and excreted from the body in urine.

No. Amino acids are categorized as either glucogenic or ketogenic. Glucogenic amino acids can be converted into glucose, while ketogenic amino acids are converted into acetyl-CoA and ketone bodies.

Using protein for energy is less efficient because the body has to expend energy to remove the nitrogen group (deamination) and convert the carbon skeleton into usable fuel. It also leads to the loss of lean body mass.

Consuming more protein won't necessarily boost your energy levels more than carbohydrates or fats, as it is primarily used for building and repairing tissues. Excess protein can be used for energy, but it's not the most efficient source.

During long-duration, high-intensity exercise, if glycogen stores are depleted, the body may increase its use of amino acids for energy. This is often seen in endurance athletes who don't properly fuel during an event.