What Happens to Proteins in Our Body? A Deep Dive

January 13, 2026 •

4 min read

Every cell in the human body contains protein, making up about 42% of the body's dry weight. This vital macronutrient is essential for numerous biological functions, but what happens to proteins in our body after we eat them? The process involves intricate digestion, absorption, and a metabolic journey that ultimately supports life itself.

Quick Summary

Proteins are broken down into amino acids during digestion. These amino acids are then absorbed and used by cells for vital functions like repairing tissues, creating enzymes, and providing energy. The body cannot store excess protein, so it must be consumed consistently.

Key Points

Digestion and Absorption: Dietary proteins are broken down by stomach acid and enzymes in the stomach and small intestine into individual amino acids, dipeptides, and tripeptides.
Amino Acid Pool: Absorbed amino acids enter a shared pool, which is used for protein synthesis and other functions throughout the body.
Protein Synthesis: Cells use amino acids from the pool to build thousands of new proteins with diverse functions, from forming structural tissues to acting as enzymes and hormones.
No Dedicated Storage: The body does not have a storage mechanism for excess protein, so a consistent daily intake is necessary.
Excess Protein Use: Surplus amino acids are deaminated, with the nitrogen excreted as urea and the carbon skeleton converted to energy, glucose, or fat.
Health Impacts: Adequate protein is essential for growth, repair, and immune function, while excessive intake can put a strain on the kidneys.

The Journey of Protein: From Plate to Cellular Workhorse

When you consume protein-rich foods, you are providing your body with the building blocks it needs to thrive. This complex process begins the moment food enters your mouth and continues on a cellular level, ensuring every system has the resources it needs to function optimally.

Digestion and Absorption: Breaking Down the Building Blocks

The digestion of protein is a multi-step process that dismantles the long chains of amino acids. Here is a step-by-step breakdown:

In the Mouth and Esophagus: The journey begins with mechanical digestion. Chewing breaks food into smaller pieces, but salivary enzymes do not significantly contribute to protein digestion.
In the Stomach: As the protein-containing food enters the stomach, it's mixed with gastric juices. Hydrochloric acid (HCl) in the gastric juice denatures proteins, unfolding their complex 3D structures and making them more accessible to enzymes. Pepsin, an enzyme activated by HCl, begins to break the long polypeptide chains into smaller segments.
In the Small Intestine: The partially digested food (chyme) moves into the small intestine. The pancreas releases bicarbonate to neutralize the stomach acid, protecting the intestinal lining. The pancreas also secretes several inactive proteases, like trypsinogen and chymotrypsinogen, which are activated within the small intestine. These powerful enzymes, along with brush border enzymes on the intestinal wall, break the protein fragments down into individual amino acids, dipeptides, and tripeptides.
Absorption and Transport: These smaller units are then absorbed through the intestinal wall and into the bloodstream. The liver acts as a checkpoint, regulating the distribution of amino acids to the rest of the body.

The Amino Acid Pool and Protein Synthesis

Once amino acids enter the bloodstream, they join what is known as the body's "amino acid pool." This pool is a mix of amino acids from both dietary protein and the breakdown of existing body proteins. The body is in a constant state of protein turnover, continuously breaking down old proteins and synthesizing new ones.

Unlike fat and carbohydrates, the body has no dedicated storage organ for protein. This is why a consistent daily intake is critical for providing the necessary amino acids. Cells all over the body draw from this pool to synthesize new proteins, following instructions encoded in our DNA through the processes of transcription and translation.

Diverse Functions of New Proteins

The new proteins synthesized by the body serve a vast array of functions:

Enzymes: These proteins catalyze nearly all metabolic reactions, from digestion to energy production.
Structural Components: Proteins like collagen and keratin provide structure and rigidity to tissues such as skin, hair, and bones.
Hormones: Many hormones, such as insulin and growth hormone, are proteins that act as messengers to coordinate bodily functions.
Antibodies: The immune system produces protein antibodies to identify and neutralize foreign invaders like bacteria and viruses.
Transport and Storage: Proteins like hemoglobin transport oxygen, while ferritin stores iron.
Fluid Balance: Albumin and globulin proteins in the blood help regulate fluid balance.

What Happens to Excess Protein?

Because the body cannot store protein, any amino acids consumed beyond what is needed for protein synthesis are broken down. This process, called deamination, removes the nitrogen-containing amino group. This nitrogen is then converted into urea in the liver and excreted by the kidneys in urine. The remaining carbon skeleton can be used for several purposes:

Energy Production: The carbon can be used as fuel to produce energy.
Glucose Conversion: It can be converted into glucose to fuel the brain and other tissues.
Fat Storage: In cases of excessive protein intake, the carbon skeletons can be converted into fat and stored in adipose tissue.

Comparison of Protein Fate: Normal vs. Excess Intake


Feature	Normal Protein Intake	Excess Protein Intake
Protein Synthesis	Primarily used for synthesizing new proteins and repairing tissues.	Used for protein synthesis up to the body's requirement.
Energy Use	Used as a supplementary energy source when carbohydrate and fat stores are insufficient.	Excess amino acids are deaminated and used for energy.
Nitrogen Processing	Nitrogen from normal protein turnover is converted to urea and excreted via the kidneys.	High volume of excess nitrogen requires the liver to work harder to convert it to urea, and the kidneys to work harder to excrete it.
Body Weight	Helps maintain or build muscle mass alongside resistance training.	Can lead to weight gain as converted carbon skeletons are stored as fat.
Kidney Health	No significant risk to healthy kidneys.	Poses an additional risk to individuals predisposed to kidney disease due to increased workload.

Conclusion: The Dynamic Role of Proteins

The journey of proteins in our body is a highly dynamic and efficient process. From the moment we consume them, they are broken down, distributed, and repurposed to serve countless life-sustaining roles. The body's inability to store protein highlights the importance of consistent dietary intake to provide the amino acids required for growth, repair, and overall bodily function. While excess protein can be used for energy, it is not the body's preferred source and can be taxing on organs like the kidneys over time. A balanced diet ensures your body has the high-quality protein it needs to function as the intricate and complex machine it is.

Learn more about the intricate mechanics of protein metabolism and their effects by consulting reputable resources like the NCBI Bookshelf on Physiology, Proteins.

Frequently Asked Questions

Protein turnover is the continuous process of breaking down and rebuilding proteins that occurs in all cells of the body. This recycling mechanism allows the body to adapt to changing needs and repair damaged tissues.

Essential amino acids are the nine amino acids that the human body cannot produce on its own. They must be obtained through the diet from sources like meat, eggs, and certain plant-based foods.

No, simply eating more protein does not build muscle. While adequate protein is necessary for muscle repair and growth, it must be paired with resistance training or exercise to stimulate muscle synthesis. Excess protein that isn't needed is used for energy or stored as fat.

The nitrogen-containing amino group is removed from excess amino acids in a process called deamination. The liver converts this nitrogen into urea, which is then transported to the kidneys for excretion in the urine.

The digestion and absorption of protein typically take several hours. The exact timeline depends on the type of protein, the portion size, and individual metabolism. Simpler proteins may digest faster than those combined with high fat.

Although severe protein deficiency is rare in developed countries, signs can include edema (swelling), loss of muscle mass, skin and hair problems, weakened bones, and a compromised immune system.

For healthy individuals, consuming the recommended daily amount of protein is not harmful to the kidneys. However, a persistently very high protein diet can increase the workload on the kidneys, posing a risk to people with pre-existing kidney conditions.