Humans need nitrogen for bodily functions. Though air is 78% nitrogen, humans cannot directly use this atmospheric form. Instead, nitrogen must be acquired through diet in specific chemical forms. This acquired nitrogen is necessary for creating essential biological molecules, supporting metabolism and overall bodily function.
Essential Roles of Nitrogen in the Body
Nitrogen is a building block for proteins, complex molecules essential for most bodily functions. Proteins are composed of amino acids, and nitrogen is a primary component of every amino acid. These proteins form structural components of muscles, skin, hair, and blood, and they also function as enzymes that facilitate metabolic processes.
Beyond proteins, nitrogen is a component of nucleic acids, DNA and RNA. DNA carries genetic information, and RNA is involved in protein synthesis; both rely on nitrogen-containing bases. Nitrogen is also found in adenosine triphosphate (ATP), the primary energy currency of cells, allowing them to store and use energy from metabolism. Nitrogen also contributes to neurotransmitter formation, transmitting signals between nerve cells and impacting brain function.
Dietary Nitrogen and Its Acquisition
Humans obtain nitrogen primarily through protein-rich foods. Animals, including humans, cannot directly absorb nitrogen gas from the air or soil. Instead, plants play an important role by absorbing nitrogen compounds from the soil, often converted by bacteria, and incorporating it into their proteins.
When humans consume plants or animals that have eaten plants, they take in this nitrogen. The body then breaks down these dietary proteins into individual amino acids during digestion. These amino acids are absorbed and reassembled by the body to create human proteins and other nitrogen-containing molecules. Common dietary sources of nitrogen include meat, fish, eggs, dairy products, legumes, nuts, and grains.
Nitrogen Balance and Health Implications
Nitrogen balance describes the relationship between the amount of nitrogen consumed and the amount excreted by the body. This balance indicates protein metabolism and nutritional status. Nitrogen is primarily lost through urine in the form of urea, but also through feces, sweat, hair, and skin.
A positive nitrogen balance occurs when nitrogen intake exceeds nitrogen loss, indicating that the body is retaining protein. This state is typical during growth, pregnancy, and recovery from illness or injury, as the body builds and repairs tissues. Conversely, a negative nitrogen balance means more nitrogen is lost than gained, signifying protein breakdown. This can result from insufficient protein intake, severe injuries, burns, fasting, or medical conditions, potentially leading to muscle wasting and impaired immune function.
An equilibrium in nitrogen balance, where intake equals loss, is generally observed in healthy adults maintaining body tissues. While a balanced diet typically provides sufficient nitrogen, extreme deficiencies can lead to muscle weakness, delayed wound healing, and hair loss. Excessive protein intake can strain the kidneys, which filter nitrogenous waste products like urea from the blood. Though healthy kidneys manage a higher protein load, prolonged excessive intake, especially of animal proteins, may concern individuals with pre-existing kidney conditions.