Nitrogen, an abundant element constituting approximately 78% of Earth’s atmosphere, is fundamental to life. While humans are surrounded by atmospheric nitrogen, they cannot directly utilize it in its gaseous form. Instead, it must be acquired through external sources, primarily the diet.
Nitrogen’s Essential Role in the Body
Nitrogen is a building block for many biological molecules. It is an integral part of amino acids, which form proteins. Proteins are diverse and perform numerous functions, including forming structural components like muscles and tissues, acting as enzymes that facilitate metabolic reactions, and functioning as antibodies for the immune system.
Beyond proteins, nitrogen is also a component of nucleic acids, specifically DNA and RNA, which carry genetic information and direct protein synthesis within cells. Additionally, it is found in adenosine triphosphate (ATP), the primary energy currency of the cell, and in other compounds such as neurotransmitters and hormones that regulate various bodily processes. The body contains about 3.3% nitrogen by weight, underscoring its widespread presence and importance for overall physiological function.
Dietary Sources of Nitrogen
Humans obtain nitrogen through protein-rich foods. When dietary proteins are ingested, the body breaks them down into individual amino acids, which then become available for synthesizing new nitrogen-containing compounds. This process allows the body to build its own necessary structures and molecules.
Animal-based proteins, which contain all essential amino acids, include meats such as beef, pork, and poultry, as well as fish, eggs, and dairy products like milk, cheese, and yogurt. Plant-based sources also contribute significantly to nitrogen intake and include legumes (such as beans, lentils, and chickpeas), nuts, seeds, and whole grains like quinoa. Certain vegetables, including spinach, broccoli, and leafy greens, also contain nitrogen.
How Nitrogen Enters the Food Chain
The journey of nitrogen into the human diet begins with the nitrogen cycle, which converts atmospheric nitrogen into usable forms. Atmospheric nitrogen gas (N2) is converted into ammonia or nitrates through a process called nitrogen fixation, mainly by bacteria in the soil or within the root nodules of plants like legumes. Lightning also contributes to nitrogen fixation.
Once converted into these usable forms, plants absorb the nitrogen compounds from the soil through their roots. Plants then use this nitrogen to synthesize their own proteins and other nitrogen-containing molecules. When animals consume these plants, or when other animals consume plant-eating animals, nitrogen is transferred up the food chain.
Maintaining Nitrogen Balance
Nitrogen balance describes the relationship between the amount of nitrogen consumed and excreted by the body. Positive nitrogen balance occurs when nitrogen intake exceeds its output, meaning the body retains more nitrogen than it loses. This is typical during growth, such as childhood, pregnancy, or muscle building and tissue repair.
Conversely, negative nitrogen balance means the body loses more nitrogen than it consumes. This can occur during starvation, severe illness, or muscle wasting, as the body breaks down its own proteins to meet energy or functional demands.
Chronic nitrogen deficiency can lead to adverse effects like muscle loss, impaired immune function, and stunted growth. While excessive nitrogen intake is rare with a normal diet, extreme amounts could strain the kidneys.