How Does Our Body Get Nitrogen Needed to Grow?

November 18, 2025 •

4 min read

Although nitrogen makes up about 78% of the air we breathe, the human body cannot use it in its gaseous form. The answer to how does our body get nitrogen needed to grow is surprisingly simple: it comes primarily from the food we eat, a process intricately linked to the broader ecosystem through the nitrogen cycle.

Quick Summary

Humans obtain nitrogen for growth and repair from dietary sources like proteins and amino acids, not by breathing it directly from the air. The nitrogen, sourced from plants and animals, is an essential building block for DNA, proteins, and other crucial biological molecules. This process is part of a complex global cycle involving bacteria and the food web.

Key Points

Dietary Intake is Key: The human body obtains its required nitrogen by consuming protein-rich foods from both plant and animal sources.
Amino Acids as Building Blocks: Proteins are broken down into amino acids, which contain nitrogen and are used to build new tissues and other essential molecules.
The Nitrogen Cycle Starts with Bacteria: Special bacteria fix atmospheric nitrogen gas (N₂) into usable forms like ammonia and nitrates, which plants can then absorb from the soil.
Breathing is Not the Answer: Despite the atmosphere being mostly nitrogen, our bodies cannot process it in its gaseous form.
Positive Nitrogen Balance for Growth: A positive nitrogen balance, indicating more nitrogen is consumed than excreted, is essential for growth periods like childhood and pregnancy.
Excretion of Excess Nitrogen: The liver processes excess nitrogen into urea, which the kidneys then filter out and excrete in urine.

The Fundamental Role of Nitrogen

Nitrogen is a cornerstone of life, forming an essential component of proteins, amino acids, and nucleic acids (DNA and RNA). Without a steady supply of this element, our bodies would be unable to synthesize new tissues, repair existing ones, or carry out countless metabolic functions. However, the abundant atmospheric nitrogen gas (N₂) is largely inert and unusable by most living organisms, including humans. The journey of nitrogen from the atmosphere to our bodies is a remarkable ecological relay race, in which we are the final leg.

The Nitrogen Cycle: A Primer

The journey begins with a process called nitrogen fixation. This is where special microorganisms, primarily certain types of bacteria, convert atmospheric nitrogen (N₂) into bioavailable forms like ammonia (NH₃). These nitrogen-fixing bacteria often live in a symbiotic relationship within the root nodules of certain plants, particularly legumes such as peas, beans, and clover.

Nitrogen Fixation: Bacteria and algae convert atmospheric N₂ into ammonia (NH₃) and then ammonium (NH₄+).
Nitrification: Other soil bacteria convert ammonium into nitrites (NO₂⁻) and then nitrates (NO₃⁻).
Assimilation: Plants absorb these nitrates and ammonium ions from the soil through their roots and use them to build their own organic molecules, including amino acids and proteins.
Consumption: Humans and other animals consume these plants (or animals that have eaten plants), ingesting the nitrogen-rich proteins and amino acids.
Decomposition: When plants and animals die, or through animal waste, decomposers return the nitrogen back to the soil as ammonia, and the cycle continues.

Protein Digestion and Amino Acids

When we eat protein-rich foods, our digestive system breaks down these large protein molecules into individual amino acids. These amino acids, which contain the nitrogen our body needs, are then absorbed into the bloodstream. From there, they are transported to cells throughout the body, where they are used as building blocks for a wide range of vital functions. Some amino acids are considered "essential" because our bodies cannot produce them, and we must get them from our diet. The nitrogen from these amino acids is used to synthesize new proteins, enzymes, hormones, and genetic material.

Nitrogen Balance and Utilization

Maintaining a healthy nitrogen balance is crucial for growth and health. Nitrogen balance compares the amount of nitrogen consumed (primarily from protein) with the amount excreted (mostly in urine as urea).

Positive Nitrogen Balance: This occurs when the body takes in more nitrogen than it excretes. It is a sign of growth, as seen in childhood, pregnancy, or during recovery from an injury.
Negative Nitrogen Balance: This indicates that the body is losing more nitrogen than it consumes. This can happen with a low-protein diet, illness, or during periods of muscle wasting.
Equilibrium: In a healthy adult, nitrogen intake is typically balanced with excretion, indicating a stable state of protein synthesis and breakdown.


Feature	Plant-Based Nitrogen Sources	Animal-Based Nitrogen Sources
Primary Source	Proteins and amino acids in legumes, nuts, seeds, grains, and vegetables.	Proteins and amino acids in meat, fish, eggs, and dairy products.
Digestibility	Generally lower due to fiber and antinutrients; higher quantity may be needed to achieve adequate nitrogen balance.	Generally higher due to more concentrated protein and amino acid profiles.
Biological Value	Lower DIAAS (Digestible Indispensable Amino Acid Score) than most animal proteins.	Higher DIAAS, with more complete amino acid profiles.
Nitrogen Contribution	Direct assimilation from plants that fixed nitrogen or absorbed it from the soil.	Ingested from animals that consumed plants, making it an indirect source from the food web.
Sourcing	Directly from crops, often enriched by nitrogen-fixing bacteria in the soil.	Higher up the food chain, where nitrogen has already been assimilated into animal tissues.

The Final Steps: Excretion

The body cannot store excess nitrogen, so any surplus amino acids or those from metabolized proteins are processed and excreted. The liver converts excess nitrogen into urea, which is then transported to the kidneys. The kidneys filter the urea from the blood, and it is eliminated from the body in the urine. This crucial process ensures that toxic nitrogenous waste products, such as ammonia, do not accumulate in the body.

Conclusion

In summary, our bodies acquire the nitrogen necessary for growth not from the air we breathe, but from the food we consume. This dietary nitrogen, bound within proteins and amino acids, is the end product of a vast and complex ecological cycle. Microscopic bacteria perform the essential task of fixing inert atmospheric nitrogen into a usable form for plants. Plants then assimilate this nitrogen, incorporating it into their own structures. When we eat these plants or the animals that feed on them, we tap into this carefully orchestrated system. The intricate process of digestion, synthesis, and excretion allows us to build and repair our bodies, all thanks to the humble beginnings of a tiny bacterium turning inert gas into a building block of life.

Learn more about the intricate biological and industrial processes that make atmospheric nitrogen accessible to living organisms in this detailed article on Nitrogen Fixation from Britannica.

Frequently Asked Questions

The nitrogen in the human body comes primarily from the food we eat, specifically from the proteins found in both plant and animal sources.

No, the human body cannot use atmospheric nitrogen (N₂) directly from the air because it is a very stable, inert gas. It must first be converted into a usable form by bacteria in the environment.

The nitrogen cycle is the process where nitrogen is converted into different chemical forms as it moves through the atmosphere, soil, and living organisms. Humans fit into this cycle as consumers, obtaining our nitrogen by eating plants or other animals that have assimilated nitrogen from the environment.

Nitrogen is a vital component of amino acids, which are the building blocks of proteins, and nucleotides, which make up DNA and RNA. It is essential for cell function, growth, and tissue repair.

The liver converts excess nitrogen into a less toxic compound called urea. The kidneys then filter this urea from the blood and excrete it from the body in urine, maintaining a proper nitrogen balance.

Plants absorb usable nitrogen compounds, like nitrates and ammonium, from the soil through their roots. These compounds are created by nitrogen-fixing bacteria and other microorganisms as part of the nitrogen cycle.

Nitrogen balance is a measure of the difference between the amount of nitrogen consumed and the amount excreted. A positive balance indicates growth, while a negative balance indicates a net loss of body protein.