Nitrate, nitrite and nitrosamines
Nitrates (NO3-) are nitrogen compounds that occur naturally in the soil. However, they are also a component of fertilizers and thus enter the soil in increased quantities. Nitrate serves plants as a nutrient and is an important growth factor. It is absorbed from the soil through the roots and helps build organic compounds such as proteins and nucleic acids. Nitrite (NO2-) is an intermediate product in the conversion of nitrate and is formed with the help of microorganisms and enzymes. It can be formed in food or produced in the body during digestion. Nitrosamines are reaction products of nitrites or nitrogen oxides with amines or substances with amino groups. They can be formed in the presence of these substances both in food and in the body. A variety of foods, for example sausages, fish products, beer, cheese, soy sauce, oils and vegetable products, can contain nitrosamines.
Excess nitrate is stored by plants mainly in stems, leaf panicles and the outer leaves (water-conducting parts). The uptake and storage capacity differs according to plant species and is also influenced by the intensity of solar radiation.
Factors affecting nitrate content in food:
- Sunlight and heat favor the decomposition of nitrate in the plant. Drought, however, increases nitrate accumulation.
- Different plant species have different nitrate contents
- Fertilization increases the nitrate content in plants
- Vegetables from glasshouses and foil cultures have higher nitrate contents than field vegetables due to the lower solar radiation
- Room temperature increases the conversion of nitrate to nitrite: storage and transport of nitrate-rich foods should therefore be refrigerated
Plants that store nitrates particularly well include arugula, lettuce, lamb's lettuce, chard, spinach, radishes, radishes, red beets, kohlrabi and collard greens. Naturally occurring nitrite levels in plant foods are very low. The occurrence of nitrosamines in vegetables, especially leafy greens and pickled/salted vegetables, is due to high nitrate content, low pH, the presence of nitrite and glucosinolates (which are mustard oil glycosides containing sulfur and nitrogen). Furthermore, some species of fungi, namely Fusarium monoliforme, can reduce nitrate to nitrite and increase the amount of secondary amines in vegetables.
Meat and meat products
Cured meat products in particular often contain detectable levels of nitrosamines, mainly due to the use of nitrite as a preservative. Several additional factors influence their occurrence, such as temperature, pH, processing conditions (i.e., raw material and storage), and the presence of free amines, especially biogenic amines.
Nitrites (E249-250) and nitrates (E251-252) are also used to preserve foods such as meat, cured meats such as ham, bacon and smoked meats, fish, and hard and semi-hard cheeses. The use of nitrate and nitrite curing salt is intended to prevent the growth of the bacterium Clostridium botulinum and results in a red coloration of the meat product.
Nitrate can leach from the soil into groundwater and thus be present in drinking water. A parameter value of 50 mg/l applies to nitrate in drinking water. This means that water with higher nitrate concentrations is not suitable as drinking water and therefore not suitable for the preparation of baby food. For nitrite, a parameter value of 0.1 mg/l drinking water applies. Traces of nitrosamines may also be present in drinking water. They represent by-products of disinfection that occur when water is treated with certain disinfectants, such as chloramine or sodium hypochlorite. Other possible sources of nitrosamines in drinking water may be pollutants from the environment, particularly from diesel vehicle exhaust, the application of sewage sludge to nitrate- or nitrite-rich soils, and the use of certain contaminated pesticides.
Nitrate in itself is relatively harmless to health. However, one should also not consume too much nitrate in the diet, as nitrate can be converted to nitrite in food or through digestion by bacteria or enzymatic conversion: In the body, nitrite converts the red blood pigment hemoglobin into methemoglobin. Unlike hemoglobin, methemoglobin cannot bind oxygen and transport it to the tissues; this leads to a lack of oxygen in the tissues and, in the worst case, can lead to internal asphyxiation.
For infants in particular, a high nitrate or nitrite intake is dangerous in the first months of life, as it can lead to so-called methemoglobinemia ("cyanosis," an undersupply of oxygen to the blood). One possible cause of this is a high nitrate load in the water used to produce infant formula.
Bacterial infections of the gastrointestinal tract also pose a risk of increased conversion of nitrate to nitrite in the intestine. Infants and children suffering from bacterial gastrointestinal infections should therefore not eat spinach.
In addition, so-called N-nitroso compounds(e.g. nitrosamines) can be formed from nitrite in the digestive tract. In rodents, tumors caused by nitrosamines occur mainly in the liver, but also in the upper gastrointestinal tract and in the respiratory tract. Whether this also applies to humans, or to what extent the intake of nitrite or nitrate via food leads to the formation of these carcinogenic compounds, has not yet been clearly clarified.
Situation in Austria
Regulation(EC) No 2023/915 setting maximum levels for certain contaminants in foodstuffs established maximum levels for nitrates in certain leafy vegetables. Since climatic conditions and the cultivation method have a significant influence on the nitrate content, different maximum levels have been introduced depending on the season (harvest in winter/summer) or cultivation method (under foil/glass, open field).
As part of the official food control, the regulated product groups are regularly checked for compliance with the legal maximum levels. In 2021, as part of the National Nitrate Control Program for monitoring the nitrate content of spinach, lettuce and arugula, the legal maximum levels were exceeded in two samples of spinach out of 105. In a focus action in 2020, the nitrate content was checked in 71 raw sausages and raw cured products from direct marketing. In two samples, the maximum levels for nitrate were exceeded.
Reduce nitrate and nitrite intake:
- Consume more low-nitrate vegetables such as tomatoes and cucumbers
- Remove leaf stalks, leaf panicles and outer green leaves from salads and cabbage vegetables
- Store vegetables in a short, light-protected and cool place
- Limit consumption of cured meats and do not grill them, as high temperatures form nitrosamines together with protein components from food.
Nitrate is mainly absorbed via vegetables and water. However, it is also formed endogenously to a limited extent. Nitrate is rapidly absorbed into the plasma via the stomach and small intestine. Up to 25% of the ingested nitrate is transported via the salivary glands into the oral cavity, where it is released with saliva. Some of the nitrate in saliva is reduced by bacteria in the oral cavity to nitrite, which is swallowed with unconverted nitrate. In healthy adults, about 5-7% of ingested nitrate is normally converted to nitrite in saliva. Infants and patients with gastrointestinal disease, who have a higher gastric pH, may have a significantly greater conversion rate. Nitrate is excreted primarily in the urine. The acceptable daily intake(ADI) for the nitrate ion is 3.7 mg/kg body weight from 4 months of age (World Health Organization, WHO). The ADI for the nitrite ion is 0.07 mg/kg bw/d. This is the amount that a person can consume daily throughout his or her life without any health risks being expected. If this acceptable daily intake is exceeded for a short time, there is no health risk for consumers.
In Austria, the average dietary nitrate intake for children is 0.75 mg/kg bw/d, for adolescents 0.34 mg/kg bw/d, and for adults 0.39 mg/kg bw/d. Leafy vegetables contribute most to nitrate intake. Intake of nitrate via drinking water is estimated to be 1.43 -1.67 mg/kg bw/d. The ADI is not exceeded for an average intake. Highly consuming children exceed the ADI by 1.5 times.
The average daily intake of nitrite for children is 0.009-0.01 mg/kg bw/d, for adolescents 0.005-0.006 mg/kg bw/d, and for adults 0.003 mg/kg bw/d. However, only meat products were considered for nitrite intake. The main sources of intake were sausages. Foods such as fish and cheese were not considered. The ADI is not exceeded here.
Possible positive effects
Nitric oxide(NO) is formed from nitrite in the stomach: Nitric oxide has been shown in studies to have a vasodilatory and thus blood pressure-lowering effect. It is also thought to have a protective effect on the stomach, as it stimulates blood circulation in the gastric mucosa and has a positive effect on the thickness of the mucus layer on the stomach wall. However, nitric oxide is also associated with increasing oxidative stress and DNA damage.
The antibacterial effect of nitrite was described some time ago. In experiments with the bacterium Helicobacter pylori, which is considered to trigger tumors in the gastrointestinal tract, nitrite was shown to be an effective bactericide.
Last updated: 17.05.2023