How Does Nitrate Enter the Body? Sources, Metabolism, and Health Effects

December 13, 2025 •

4 min read

Approximately 80-90% of the nitrate in a typical human diet comes from vegetables like leafy greens and beets. But beyond dietary sources, how does nitrate enter the body, and what happens to it next? The process involves multiple pathways, from environmental exposure to complex internal physiological cycles.

Quick Summary

The body acquires nitrate from ingested food and water, along with endogenous production. Absorbed nitrate circulates to salivary glands and is converted to nitrite by oral bacteria before being swallowed and absorbed again.

Key Points

Diverse Intake Routes: Nitrate enters the body primarily through consuming vegetables (80-90%), drinking water, and processed meats containing additives.
Endogenous Formation: The body naturally produces nitrate from the oxidation of nitric oxide (NO) as part of a normal physiological cycle.
Entero-Salivary Circulation: A metabolic pathway where ingested nitrate is concentrated in saliva, converted to nitrite by oral bacteria, and re-swallowed.
Oral Bacteria are Key: Specific bacteria in the mouth play a crucial role in reducing nitrate to nitrite, which is essential for the body's use of nitrate.
Health Effects Depend on Dosage: While moderate intake can produce beneficial nitric oxide, high levels of nitrite can cause methemoglobinemia, particularly in infants.
Balancing Risks with Diet: The presence of antioxidants like Vitamin C, found in vegetables, can help mitigate the risk of harmful N-nitroso compounds forming from nitrite.

Primary Sources: How Nitrate Gets into Food and Water

Nitrate is a naturally occurring compound of nitrogen and oxygen, but human activity, particularly in agriculture, has significantly increased its prevalence in the environment. It is highly water-soluble, allowing it to easily move through soil and contaminate groundwater and surface water. This widespread presence means humans are routinely exposed through both natural and man-made sources.

Vegetables: The Main Exogenous Source

The largest source of nitrate for most people is vegetables, especially leafy greens and root vegetables. Plants absorb nitrate from the soil as a vital nutrient for growth. The nitrate content in vegetables can vary widely depending on factors like fertilizer use, light intensity, and time of harvest. Nitrate levels are particularly high in:

Spinach
Lettuce
Arugula
Beetroot
Celery
Radishes

Drinking Water: An Environmental Pathway

Contaminated drinking water is another significant route for nitrate to enter the body, posing a particular risk to rural populations relying on private wells. Contamination typically results from:

Agricultural Runoff: Excessive use of nitrogen-based fertilizers and animal manure from large-scale farming.
Septic Systems: Leaks from poorly maintained or aging septic tanks can release nitrates into groundwater.
Waste Disposal: Landfills and industrial processes can leach nitrogen compounds into water sources.

Processed Meats and Additives

Nitrates and nitrites are also intentionally added to cured and processed meats, such as bacon, hot dogs, and deli meats, acting as preservatives to prevent bacterial growth and maintain color and flavor. While cured meats contribute a smaller percentage to the overall intake compared to vegetables, they represent a direct source of both nitrates and nitrites.

Internal Processes: The Endogenous Pathway

Not all nitrate entering the body comes from external sources. The human body is capable of producing its own nitrate internally (endogenous synthesis). This occurs through the oxidation of nitric oxide (NO), a molecule with important physiological functions, as part of the body's natural nitrogen cycle. The endogenous formation can increase during times of inflammation or infection.

The Entero-Salivary Circulation: A Key Metabolic Cycle

Once ingested, nitrate follows a fascinating metabolic path known as the entero-salivary circulation, which is key to its processing and fate within the body. Approximately 25% of absorbed nitrate is actively concentrated and secreted into saliva by the salivary glands, where its conversion begins. The steps involved are:

Absorption and Recirculation: Dietary nitrate is rapidly absorbed from the small intestine into the bloodstream. A portion of this circulating nitrate is then taken up by the salivary glands.
Bacterial Reduction: Symbiotic bacteria living in the anaerobic crypts of the tongue reduce the nitrate present in saliva to nitrite.
Swallowing and Reaction in the Stomach: The nitrite-rich saliva is swallowed and enters the highly acidic stomach. In this environment, nitrite is converted into nitric oxide (NO) and other reactive nitrogen species.
Further Metabolism: The resulting NO and other compounds are then absorbed systemically or contribute to functions within the gastrointestinal tract.

Comparison of Major Nitrate Entry and Metabolism Pathways


Feature	Vegetables	Drinking Water	Processed Meats	Endogenous Production
Source Type	Natural, Dietary	Environmental Contamination	Food Additives	Internal Synthesis
Entry Method	Ingestion	Ingestion	Ingestion	Metabolic Process
% of Total Intake	80-90%	Varies, can be significant in rural areas	~5%	Varies, can be substantial
Associated Factors	Presence of antioxidants (e.g., Vitamin C), cultivation methods	Fertilizers, animal waste, septic systems, well construction	Curing agents, potential for nitrosamine formation during cooking	Inflammation, infection, nitric oxide oxidation
Health Implications	Typically beneficial or neutral due to antioxidants; high intake can impact metabolism	Potential risk for methemoglobinemia (especially in infants), link to cancers at high levels	Possible link to nitrosamine formation; carefully regulated in most countries	Regulates cardiovascular and immune function

Health Effects and the Role of Nitrite

The health effects of nitrate are largely tied to its conversion to nitrite. While nitrate itself is relatively inert, nitrite is more reactive and can have both beneficial and harmful effects. At low concentrations, the nitric oxide produced from nitrite can help regulate blood pressure and improve cardiovascular health. However, excessive nitrite can pose risks.

Methemoglobinemia

Infants are particularly vulnerable to methemoglobinemia, or "blue baby syndrome," caused by excessive nitrite intake. Nitrite can interfere with the hemoglobin in red blood cells, reducing its ability to transport oxygen throughout the body. Infants are more susceptible due to their different gut bacteria, lower stomach acidity, and different type of hemoglobin.

Potential Carcinogenic Nitrosamine Formation

Under certain conditions, such as high nitrate/nitrite intake coupled with high protein and low antioxidant consumption, nitrite can react with amines to form N-nitroso compounds (NOCs). Some NOCs are known carcinogens, but the link to cancer from dietary nitrate is complex and depends heavily on other dietary factors, like the presence of antioxidants from vegetables. The International Agency for Research on Cancer (IARC) classifies ingested nitrate and nitrite under conditions of endogenous nitrosation as probably carcinogenic to humans (Group 2A). For more in-depth information, you can consult the CDC's profile on nitrate and nitrite toxicity.

Conclusion: A Multifaceted Intake

Nitrate enters the human body through several primary routes, including dietary intake from vegetables and cured meats, and from contaminated drinking water. Additionally, the body generates its own nitrate internally. The subsequent metabolism of nitrate via the entero-salivary circulation, involving oral bacteria and gastric acid, produces nitrite and potentially beneficial nitric oxide. The health implications, from cardiovascular benefits to risks like methemoglobinemia in infants and potential nitrosamine formation, are dependent on the concentration of intake and the presence of other dietary compounds like antioxidants. Understanding these complex intake and metabolic pathways is crucial for assessing both the risks and the physiological benefits of nitrate exposure.

Frequently Asked Questions

The largest source of nitrate in the average diet is vegetables, particularly leafy greens like spinach and root vegetables like beets, which can account for 80-90% of dietary intake.

No, boiling water does not remove nitrates and may, in fact, increase the nitrate concentration as the water evaporates.

Infants are more sensitive because their digestive systems contain different bacteria and have lower stomach acidity, which promotes nitrate conversion to nitrite. They also have a type of hemoglobin that is more easily affected by nitrite, which can lead to methemoglobinemia.

While the link is complex and influenced by diet, high nitrite intake under specific conditions can lead to the formation of carcinogenic N-nitroso compounds. Epidemiological studies on nitrate from vegetables have been inconsistent, suggesting other factors like antioxidants play a role.

This is a metabolic loop where absorbed nitrate is concentrated in saliva by the salivary glands. Oral bacteria then reduce the nitrate to nitrite, which is swallowed and can be converted into nitric oxide in the acidic stomach.

Nitrates and nitrites are added to processed meats as preservatives and are regulated by food safety authorities. While there is a potential risk of nitrosamine formation, the levels are controlled, and the process is inhibited by additives like vitamin C.

The only way to know for sure is to have your water tested by a certified laboratory, especially if you have a private well in a rural area with significant agricultural activity nearby.