Nitrates and nitrites are chemical compounds containing nitrogen and oxygen, found in natural environments and human biology. Nitrates (NO3) have one nitrogen atom bonded to three oxygen atoms, while nitrites (NO2) have one nitrogen atom bonded to two oxygen atoms. Nitrates are generally more stable than nitrites. The body can convert nitrates into nitrites.
Sources and Presence
Nitrates and nitrites are found widely in both natural sources and as additives in processed foods. Vegetables, especially leafy greens like spinach, lettuce, celery, and beets, are the primary dietary source of nitrates, contributing around 80% of typical human intake. The concentration of nitrates in vegetables can vary based on agricultural practices, storage conditions, and the levels of nitrates in soil and water used for cultivation.
These compounds also appear in drinking water, primarily due to agricultural runoff from fertilizers, animal waste, and discharge from septic systems. While nitrate concentrations in groundwater are typically low, they can be elevated in areas with concentrated human activities or shallow, privately owned wells. Nitrites are less common in well-oxygenated water due to their rapid conversion to nitrates.
Processed and cured meats like bacon, ham, sausages, and hot dogs contain added nitrates and nitrites. These additives function as preservatives, inhibiting the growth of harmful bacteria such as Clostridium botulinum, which causes botulism. They also contribute to the characteristic pink or red color and salty flavor of cured meats. Dairy products and certain cheeses can also contain low concentrations of these compounds.
How the Body Processes Them
Once consumed, dietary nitrates are rapidly absorbed, primarily in the small intestine. Approximately 60-70% of ingested nitrate is excreted in urine within 24 hours. A portion of the absorbed nitrate, about 25%, is actively transported to the salivary glands and then secreted into saliva.
In the oral cavity, specific bacteria on the tongue possess nitrate reductase enzymes, which reduce nitrates to nitrites. This conversion is a step in the metabolic pathway of dietary nitrates. The nitrites then travel to the acidic environment of the stomach, where they can undergo further non-enzymatic reactions.
Within the stomach, nitrites can be converted into nitric oxide (NO) and other nitrogen compounds. This process is particularly evident under acidic conditions. The formation of nitric oxide from dietary nitrates and nitrites is referred to as the “enterosalivary nitrate-nitrite-nitric oxide pathway.”
Impact on Human Health
The processing of nitrates and nitrites in the body leads to compounds that can exert both beneficial and adverse health effects. Nitric oxide (NO), a byproduct of nitrite conversion, plays a significant role in cardiovascular health. It helps regulate blood pressure by promoting the relaxation and dilation of blood vessels, which improves blood flow. This vasodilating effect supports the cardiovascular system.
Beyond cardiovascular benefits, nitrates and nitrites may offer other positive effects. Nitrates can function as antimicrobials in the digestive system, helping to eliminate harmful bacteria like Salmonella. Dietary nitrates may also enhance physical performance, leading to their consumption by athletes.
Conversely, the conversion of nitrates to nitrites, particularly in excessive amounts, can lead to adverse health outcomes. One concern is methemoglobinemia, a condition where nitrite interferes with the blood’s ability to carry oxygen. Nitrite reacts with hemoglobin, the oxygen-binding protein in red blood cells, oxidizing its iron to form methemoglobin, which cannot bind oxygen. Symptoms can range from bluish skin discoloration (cyanosis) to dizziness, lethargy, and in severe cases, cardiac issues or central nervous system effects. Infants, especially those under four months, are particularly susceptible to methemoglobinemia from nitrate-contaminated water due to their less developed enzyme systems.
Another concern arises from the potential formation of N-nitrosamines, which are compounds considered carcinogenic. Nitrosamines can form when nitrites react with amines (protein components) under specific conditions, such as the low pH environment of the stomach, the presence of certain bacteria, or during high-heat cooking like frying or barbecuing. While nitrates and nitrites themselves are not directly carcinogenic, their conversion to nitrosamines is the basis for their association with an increased risk of certain cancers, particularly in the digestive tract. However, the presence of antioxidants can help reduce this conversion.
Regulation and Safety
Regulatory bodies worldwide establish guidelines for nitrate and nitrite levels in food and drinking water to ensure consumer safety. In the United States, the Environmental Protection Agency (EPA) has set maximum contaminant levels (MCLs) for nitrates in drinking water at 10 milligrams per liter (mg/L) and for nitrites at 1 mg/L. These standards aim to prevent health issues from exposure.
Food safety authorities, such as the U.S. Food and Drug Administration (FDA) and the European Food Safety Authority (EFSA), regulate their use as food additives. Nitrates and nitrites are permitted as curing agents and preservatives in specific meat products, with limits on the amounts that can be added. For instance, sodium nitrate may be limited to 500 parts per million (ppm) and sodium nitrite to 200 ppm in finished meat products for home curing.
Acceptable Daily Intake (ADI) values are set to define the amount of a substance that can be consumed daily over a lifetime without posing an appreciable health risk. The ADI for nitrate is 3.7 milligrams per kilogram of body weight per day (mg/kg bw/day), while for nitrite, it is 0.07 mg/kg bw/day. These values are derived from scientific assessments to protect public health, considering all sources of dietary exposure.