A nitrate ion is a chemical compound composed of nitrogen and oxygen that carries a negative electrical charge. This molecule is a natural part of the environment, playing a role in the global nitrogen cycle that sustains plant life. Humans regularly ingest nitrate through diet, where it serves as a precursor for other biologically active molecules. Understanding its transformation within the body is key to appreciating its dual influence on human health.
The Chemistry of the Nitrate Ion
The nitrate ion is a polyatomic molecule with a central nitrogen atom covalently bonded to three oxygen atoms. This arrangement results in a trigonal planar structural shape. The entire structure carries a single negative charge, classifying it as an anion, with the chemical formula NO3-. The nitrate ion is stable and, when paired with a positively charged ion, forms a salt. Almost all inorganic nitrate salts exhibit high solubility, meaning they dissolve readily in water, which explains the ease with which nitrate moves through soil and water systems.
Natural Occurrence and Dietary Sources
Nitrate is a ubiquitous component of the natural world, originating primarily from the nitrogen cycle. Soil bacteria convert organic nitrogen compounds into nitrate through nitrification, making it the primary form of nitrogen absorbed by plant roots. Leafy green vegetables are the largest source of dietary nitrate, contributing an estimated 80 to 90 percent of human intake. Vegetables like spinach, arugula, chard, and beetroot accumulate high concentrations of the compound. Nitrate and its close relative, nitrite, are also added to processed meats like bacon, ham, and cured sausage, functioning as preservatives to inhibit the growth of harmful bacteria, such as Clostridium botulinum.
Transformation within the Human Body
Once ingested, nitrate enters a metabolic pathway that alters its structure and biological activity. After absorption, a significant portion is actively transported and concentrated by the salivary glands, resulting in concentrations 10 to 20 times higher than in the blood. When nitrate-rich saliva is swallowed, commensal bacteria on the tongue use the enzyme nitrate reductase to reduce nitrate (NO3-) to nitrite (NO2-). The resulting nitrite enters the stomach, where the highly acidic environment converts a portion of it into nitric oxide (NO), a potent gaseous signaling molecule. This sequence, known as the entero-salivary pathway, serves as an alternative to the body’s primary, oxygen-dependent nitric oxide production system, and the remaining nitrite is absorbed into the bloodstream for further conversion.
Dual Impact on Human Health
The transformation of nitrate into nitrite and nitric oxide creates a dual impact on human health, involving both positive physiological effects and toxicological risks.
Beneficial Effects
The beneficial effects are attributed to the resulting nitric oxide (NO) molecule. Nitric oxide is a potent vasodilator, relaxing and widening blood vessels. This effect reduces blood pressure and improves blood flow throughout the body. Dietary nitrate supplementation, often through beetroot juice, enhances athletic performance by lowering the oxygen cost of submaximal exercise.
Toxicological Risks
The adverse health effects stem from the intermediate nitrite molecule, particularly when present in excess. One severe but rare risk is methemoglobinemia, often called blue baby syndrome, which primarily affects infants under six months. Nitrite oxidizes the iron in hemoglobin, creating methemoglobin, which is incapable of effectively carrying oxygen. Infants are susceptible because they lack sufficient enzyme levels and their higher stomach pH allows more nitrate-reducing bacteria to thrive. Another concern is the formation of N-nitrosamines, a class of probable human carcinogens, which occurs when nitrite reacts with amines in the stomach or during the cooking of processed meats; due to these risks, the U.S. Environmental Protection Agency (EPA) established a maximum contaminant level for nitrate in drinking water at 10 milligrams per liter.