How to get energy from amino acids?

November 21, 2025 •

4 min read

Over 15% of the body's resting energy expenditure can be supported by the oxidation of amino acids. When primary energy sources like carbohydrates and fats are insufficient, the body turns to protein catabolism to get energy from amino acids. This process involves a complex series of metabolic steps, primarily occurring in the liver and muscles, that break down these protein building blocks for fuel.

Quick Summary

This article details the complex metabolic pathways the body uses to convert amino acids into usable energy sources, such as glucose and ketone bodies. It explains the conditions under which amino acid metabolism occurs and the key biological steps involved.

Key Points

Fuel of last resort: The body primarily gets energy from amino acids when carbohydrates and fats are insufficient, such as during fasting or starvation.
The deamination process: Amino acids must first have their nitrogen-containing amino group removed in a process called deamination, primarily performed by the liver.
Toxic waste conversion: The toxic ammonia produced from deamination is converted into less harmful urea by the liver for safe excretion by the kidneys.
Glucogenic amino acids: Many amino acids can be converted into glucose (via gluconeogenesis) to provide energy for the brain and other tissues.
Ketogenic amino acids: Some amino acids are converted into ketone bodies (via ketogenesis), which act as an alternative fuel source for the brain and muscles.
TCA cycle integration: The carbon skeletons of amino acids are broken down into intermediates that enter the TCA cycle to produce ATP.
Central role of mitochondria: Amino acid metabolism and subsequent energy production are highly dependent on mitochondrial function.
Balanced nutrition is key: A balanced intake of macronutrients is crucial to ensure amino acids are preferentially used for protein synthesis rather than energy.

Understanding the body's fuel hierarchy

While carbohydrates and lipids are the body’s primary and most efficient energy sources, amino acids can also provide significant fuel under specific conditions. The body preferentially uses amino acids for building proteins and other vital molecules, but when needed, it will break them down for energy. This typically happens during periods of low glucose availability, such as fasting, prolonged intense exercise, or starvation. The process, known as catabolism, first involves removing the nitrogen-containing amino group, after which the remaining carbon skeleton is converted into usable energy.

The process of deamination: Removing the nitrogen

The critical first step in generating energy from amino acids is the removal of the alpha-amino group, a process called deamination. This primarily occurs in the liver, with other tissues like the kidneys and muscles also participating. The amino group is transferred to other molecules in a reaction called transamination, often to an alpha-keto acid like alpha-ketoglutarate. This forms a new amino acid and a corresponding alpha-keto acid from the original amino acid. The transferred amino group eventually forms ammonia, a toxic substance. The liver then converts this ammonia into less toxic urea, which is excreted by the kidneys in the urine via the urea cycle. The overall process ensures that the body's nitrogen waste is safely eliminated.

Pathways for converting carbon skeletons into energy

After deamination, the remaining carbon skeletons (also known as alpha-keto acids) are channeled into different metabolic pathways to produce energy. These pathways lead to the production of either glucose, ketone bodies, or intermediates for the tricarboxylic acid (TCA) cycle (also known as the Krebs cycle), which directly generates ATP.

Gluconeogenesis: Making new glucose

Glucogenic amino acids have carbon skeletons that can be converted into intermediates of glycolysis and the TCA cycle, ultimately leading to the synthesis of new glucose. This process is vital for providing energy to the brain and other tissues that depend heavily on glucose for fuel, especially during prolonged fasting. Key examples include alanine and glutamine, which are transported to the liver and kidneys to be converted into glucose.

Ketogenesis: Producing ketone bodies

Ketogenic amino acids have carbon skeletons that are degraded into acetyl-CoA or acetoacetyl-CoA. These cannot be used to produce new glucose but can be converted into ketone bodies (acetoacetate and β-hydroxybutyrate). Ketone bodies serve as an alternative fuel source for the brain, heart, and skeletal muscles during fasting or when carbohydrates are restricted. The exclusively ketogenic amino acids are leucine and lysine.

Glucogenic vs. Ketogenic amino acids: A comparative table

The 20 common amino acids can be categorized based on their catabolic fates, affecting how the body can get energy from amino acids.


Amino Acid Type	Catabolic End Product	Can it form glucose?	Can it form ketones?	Examples of Amino Acids
Glucogenic	Pyruvate or TCA intermediates	Yes	No	Alanine, Glycine, Serine, Cysteine, Aspartate, Asparagine, Glutamate, Glutamine, Proline, Arginine, Histidine, Valine, Methionine
Ketogenic	Acetyl-CoA or Acetoacetate	No	Yes	Leucine, Lysine
Both (Glucogenic & Ketogenic)	Both	Yes	Yes	Tryptophan, Tyrosine, Phenylalanine, Isoleucine, Threonine

The tricarboxylic acid (TCA) cycle

Once the carbon skeletons are converted into intermediates of the TCA cycle (like pyruvate, acetyl-CoA, α-ketoglutarate, succinyl-CoA, fumarate, and oxaloacetate), they are oxidized to generate adenosine triphosphate (ATP), the body's primary energy currency. This process is crucial for generating a significant portion of the energy derived from amino acids. The branched-chain amino acids (BCAAs)—leucine, isoleucine, and valine—are particularly important in this regard, especially during exercise, as they are primarily oxidized in the skeletal muscles.

The role of mitochondria

Amino acid metabolism is deeply interconnected with mitochondria, the powerhouse of the cell. Most of the enzymes involved in the degradation of amino acids are located within the mitochondria. These processes—including parts of the TCA cycle and oxidative phosphorylation—are vital for converting the energy within amino acid carbon skeletons into usable ATP. This mitochondrial function is critical for energy production, protein synthesis, and cellular homeostasis.

Conclusion

Generating energy from amino acids is a multi-step metabolic process that the body uses primarily when its preferred fuel sources, carbohydrates and fats, are in short supply. It begins with the removal of the amino group, followed by the conversion of the remaining carbon skeleton into either glucose (via gluconeogenesis) or ketone bodies (via ketogenesis). These resulting molecules, along with other TCA cycle intermediates, are then used to produce ATP for immediate energy needs. While amino acids are not the body's first-choice fuel, their metabolic versatility ensures that the body can maintain energy homeostasis under demanding conditions like fasting or intense exercise, underscoring the critical importance of protein in overall nutrition.

Maximizing energy from amino acids

Prioritize Carbohydrates: To spare amino acids for building proteins, ensure a sufficient intake of carbohydrates, especially for fuel-intensive activities.
Adequate Protein Intake: Consume enough protein to support protein synthesis and avoid the body breaking down its own muscle tissue for fuel.
Balanced Diet: A balanced diet with adequate carbohydrates, proteins, and fats helps the body use each nutrient appropriately without resorting to breaking down lean muscle mass.
Targeted BCAA Intake: Athletes may use branched-chain amino acid supplements to support muscle energy and recovery, particularly during prolonged or intense exercise.
Stay Hydrated: Proper hydration is essential to support the kidney's role in the urea cycle, which clears the nitrogenous waste produced during amino acid catabolism.

Frequently Asked Questions

The primary role of amino acids is to serve as the building blocks for proteins, enzymes, and hormones. Using them for energy is a secondary function that occurs only when carbohydrates and lipids are scarce.

The nitrogen-containing amino group is removed through deamination and converted into ammonia. The liver then processes this toxic ammonia into urea, which is safely eliminated from the body via the kidneys.

Amino acids whose carbon skeletons can be converted into glucose are called glucogenic. Examples include alanine, glycine, serine, and many others that form intermediates of the TCA cycle.

Ketogenic amino acids are those whose carbon skeletons are converted into acetyl-CoA or acetoacetyl-CoA, which can then be used to form ketone bodies. The only exclusively ketogenic amino acids are leucine and lysine.

While the brain relies heavily on glucose, it cannot directly use amino acids for fuel. Instead, glucogenic amino acids are converted into glucose, or ketogenic amino acids are converted into ketone bodies, which the brain can use for energy during specific conditions like fasting.

The body primarily uses amino acids for energy when preferred fuels (carbohydrates and lipids) are in short supply. This is common during prolonged fasting, intense exercise, or starvation.

Amino acids provide energy at a rate of 4 kcal per gram, the same as carbohydrates, but are a less efficient energy source than fats. The process of breaking them down is complex and is not the body's primary energy pathway.

Mitochondria are crucial for amino acid catabolism, as most enzymes involved are located there. They house the TCA cycle and are responsible for converting the final carbon skeletons into ATP.