Nutrition Diet: What Happens to Excess Amino Acids in Our Body?

November 10, 2025 •

4 min read

Unlike carbohydrates and fats, the body has no mechanism for storing excess amino acids for later use. This raises the question of what happens to excess amino acids in our body, and the answer involves a complex metabolic process primarily handled by the liver to prevent the accumulation of toxic byproducts.

Quick Summary

The body breaks down excess amino acids in the liver through deamination. The nitrogen is converted to urea and excreted, while the carbon skeletons are used for energy or converted to glucose and fat.

Key Points

No Storage Mechanism: The human body cannot store excess amino acids for later use, unlike fats and carbohydrates.
Deamination in the Liver: Excess amino acids are processed in the liver through deamination, which removes the amino group and produces toxic ammonia.
Urea Cycle for Detoxification: The liver detoxifies ammonia by converting it into less toxic urea via the urea cycle, which is then excreted by the kidneys.
Carbon Skeleton Fates: The remaining carbon skeletons can be converted into glucose (glucogenic), ketone bodies (ketogenic), or used directly for energy.
Potential Health Risks: Long-term high protein intake can strain the kidneys, lead to dehydration, and cause nutrient imbalances due to the metabolic burden.
Amino Acid Imbalances: High doses of specific amino acid supplements can create imbalances that negatively impact the body's functions, such as affecting neurotransmitter synthesis in the brain.

The Body's Approach to Protein Overload

When we consume protein, it is digested into amino acids that are used for building new proteins, repairing tissues, and other vital functions. However, there is no dedicated storage system for amino acids in the body. If the intake of protein exceeds the body's needs, the excess amino acids must be processed and eliminated. This metabolic pathway is efficient but can place a strain on certain organs over time, particularly with consistently high protein consumption. The breakdown and repurposing of excess amino acids are essential to maintain cellular health and prevent the build-up of toxic waste products, most notably ammonia.

The Deamination Process: Removing the Nitrogen

The first and most critical step in processing surplus amino acids is deamination. This process occurs predominantly in the liver and involves removing the amino ($NH_2$) group from the amino acid molecule. This reaction is primarily catalyzed by enzymes, leading to two main outcomes:

The nitrogenous amino group is converted into highly toxic ammonia ($NH_3$).
The remaining part of the amino acid, known as the carbon skeleton or alpha-keto acid, is left to be repurposed for energy or storage.

The immediate conversion of ammonia is essential because even small amounts of this toxin in the bloodstream can be harmful, causing symptoms ranging from lethargy to severe neurological damage.

The Urea Cycle: Detoxifying Ammonia

Following deamination, the liver immediately processes the toxic ammonia through a multi-step pathway known as the urea cycle. This biochemical cycle, discovered by Hans Krebs and Kurt Henseleit, works to convert ammonia into the much less toxic compound, urea.

Key stages of the urea cycle:

Ammonia ($NH_3$) and carbon dioxide (from cellular respiration) are combined to form carbamoyl phosphate in the mitochondria of liver cells.
Carbamoyl phosphate then enters the main urea cycle reactions in the cytoplasm.
Through a series of enzymatic steps, urea is produced.
Once formed, urea is released from the liver into the bloodstream and travels to the kidneys.
The kidneys filter the urea from the blood, and it is excreted from the body in the urine.

This cycle is vital for preventing the accumulation of ammonia. Defects in any of the enzymes involved can cause a dangerous build-up of ammonia, leading to conditions known as urea cycle disorders.

The Fate of the Carbon Skeleton: Energy and Storage

After the amino group is removed, the remaining carbon skeleton has several potential fates, depending on the body's needs and the specific type of amino acid. Amino acids are broadly classified into two groups based on what their carbon skeletons can be converted into.

Glucogenic Pathways

Glucogenic amino acids have carbon skeletons that can be converted into glucose through a process called gluconeogenesis. This is particularly important during periods of fasting or low carbohydrate intake, as it helps maintain stable blood glucose levels, which are crucial for the brain and red blood cells. The carbon skeletons of these amino acids are converted into intermediates of the Krebs cycle or pyruvate, which can then be used to synthesize new glucose.

Ketogenic Pathways

Ketogenic amino acids have carbon skeletons that are converted into acetyl-CoA or acetoacetate. These products can then be used to produce ketone bodies, an alternative fuel source for the body and brain during prolonged fasting or carbohydrate restriction. Only two amino acids, leucine and lysine, are exclusively ketogenic.

Amphibolic Pathways

Some amino acids are both glucogenic and ketogenic, meaning their carbon skeletons can be funneled into both pathways. Examples include isoleucine, phenylalanine, tryptophan, and tyrosine.

Potential Health Consequences of Excess Amino Acids

While the body is adept at handling occasional excess protein, persistently high intake can lead to potential health concerns:

Increased Kidney Workload: The kidneys are responsible for filtering urea and other waste products. Long-term high protein intake increases the volume of nitrogenous waste, potentially straining kidney function over time.
Dehydration: The excretion of urea requires water, so a higher protein intake necessitates increased fluid intake to avoid dehydration.
Digestive Issues: High protein diets often displace fiber-rich foods, leading to digestive problems like constipation and bloating.
Nutrient Imbalance: Focusing excessively on protein can lead to the neglect of other vital macronutrients like carbohydrates and healthy fats, causing imbalances and potential deficiencies.
Specific Amino Acid Toxicity: Taking high doses of individual amino acid supplements can cause imbalances and negative side effects. For example, excess branched-chain amino acids (BCAAs) can compete with tryptophan for transport to the brain, potentially reducing serotonin levels and affecting mood and appetite.

Comparison of Glucogenic and Ketogenic Amino Acid Fates


Feature	Glucogenic Amino Acids	Ketogenic Amino Acids
Metabolic Fate	Converted into glucose via gluconeogenesis.	Converted into acetyl-CoA or acetoacetate to produce ketone bodies.
Energy Use	Primarily provides glucose for energy, especially for the brain and red blood cells.	Provides an alternative energy source for the brain and muscles during fasting.
Key Product	Pyruvate and Krebs cycle intermediates.	Acetyl-CoA and acetoacetate.
Exclusively	Most amino acids are at least partially glucogenic.	Leucine and Lysine are the only exclusively ketogenic amino acids.

Conclusion: The Body's Efficient Recycling System

In summary, the body has no warehouse for surplus amino acids. Instead, it utilizes a sophisticated and multi-stage process to break them down. This ensures that excess nitrogen, a byproduct of protein metabolism, is safely converted into urea and eliminated, preventing toxicity. The remaining carbon skeletons are efficiently repurposed as an energy source or converted into glucose or fat for storage, depending on the body's immediate metabolic needs. While this system is highly effective, maintaining a balanced diet is key to avoiding overworking these processes and potential health complications. The process of protein metabolism is a testament to the body's remarkable ability to adapt to nutritional intake, even though it is not designed to permanently store protein beyond what is needed for structural and functional purposes.

Frequently Asked Questions

No, the body does not have a dedicated storage system for excess amino acids. Unlike fat or glycogen stores, amino acids that are not used immediately for protein synthesis or other functions are metabolized and eliminated.

Deamination is the process of removing the amino group (containing nitrogen) from an amino acid. It occurs primarily in the liver. This process is necessary to break down excess amino acids, but it produces the highly toxic byproduct, ammonia.

The liver immediately converts the toxic ammonia into less harmful urea through a metabolic pathway called the urea cycle. The urea is then transported to the kidneys, where it is filtered out of the blood and excreted in the urine.

The remaining carbon skeleton, or alpha-keto acid, can be utilized in several ways. It can be used directly for energy production, converted into glucose through gluconeogenesis, or stored as fat.

Consistently high protein intake can increase the workload on the kidneys, potentially leading to issues in individuals with pre-existing kidney conditions. It can also cause dehydration if fluid intake is insufficient and may lead to digestive problems or nutrient imbalances.

Yes, excessive consumption of amino acid supplements can be harmful. For example, high intake of branched-chain amino acids (BCAAs) can interfere with the transport of other essential amino acids to the brain, affecting neurotransmitter levels and function.

Glucogenic amino acids can be converted into glucose, while ketogenic amino acids are converted into ketone bodies or acetyl-CoA. Most amino acids are glucogenic, but leucine and lysine are exclusively ketogenic, and some can serve both purposes.