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 in the body during digestion.

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

Vegetables

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.

Additives

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 Clostridium botulinum bacterium and results in a red coloration of the meat product.

Drinking water

Nitrate can leach from the soil into groundwater and thus can also be found 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.

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. Most of these compounds have been classified as carcinogenic on the basis of animal studies. 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.

Regulations (EC) No. 1881/2006 and (EU) No. 1258/2011 setting maximum levels for certain contaminants in foodstuffs established maximum levels for nitrates in certain leafy vegetables. Since climatic conditions and cultivation method have a significant influence on nitrate levels, different maximum levels have been introduced depending on the season (winter/summer harvest) or cultivation method (under film/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 2020, 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 79 samples. 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.

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: studies have shown that nitric oxide has a vasodilatory effect and thus reduces blood pressure. 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.