Written evidence submission from Professor Gunter Kuhnle, University of Reading (NIT0029)

Supporting Evidence for the House of Lords Environment and Climate Change Committee enquiry into Nitrogen

Professor Gunter G. C. Kuhnle, Professor of Nutrition and Food Science,

Dr Charlotte E. Mills, Hugh Sinclair Lecturer in Nutritional Sciences

Department of Food and Nutritional Sciences, University of Reading

Intake and sources of nitrate (NO3-) and nitrite (NO2-)

1. As part of the re-evaluation of nitrate and nitrite as food additives in 2017, the European Food Safety Authority (EFSA) has conducted an exposure assessment which is based on national nutrition surveys of EU member states[1]. This is currently the most reliable estimate of intake of nitrates and nitrites from different sources.
2. Estimating the intake of nitrate and nitrite from dietary sources is difficult because the nitrate content is very variable and depends on a range of factors, including production methods. The variability in fruits and vegetables is more than 50-fold for most foods (Appendix), making an accurate estimate impossible.
3. Food additives contribute only a small amount to the total intake of nitrate (less than 5%). They contribute approximately 30% of dietary nitrite (Figure 2).
4. According to EFSA’s assessment, the main food categories contributing to nitrate and nitrite exposure include composite foods, fruits and fruit products, poultry, livestock meat, and cheese. For infants, foods specifically formulated for this age group contributed significantly to nitrite exposure.
5. Sausages and preserved meat were important contributors to the total exposure to nitrites from all sources, with sausages contributing approximately 24% in adults.
6. The intake of nitrate and nitrite from food additives did not exceed the respective ADIs in 2017. However, when considering all sources, the amounts of the ADI were exceeded by high consumers (Table 1).

Figure 1:               Contribution of food additives (E249 – E252) to total nitrate and nitrite intake across different age groups based on the 2017 EFSA exposure assessment (EU and UK). The graph represents the percentage contribution of nitrates and nitrites from food additives to overall exposure from all sources, including natural presence in foods, contamination, and additive use. The data account for reduction factors applied to vegetables.

Table 1:              Estimated dietary exposure to nitrites and nitrates from all sources (food additives, natural presence and contamination) based on the 2017 EFSA exposure assessment (EU and UK). Data show the mean and 95th percentile range of the estimated exposure (mg/kg body weight per day for nitrate and nitrite ion).

Drinking water variability

7. Drinking water can be an important source of nitrate and nitrite depending on drinking water composition.
8. Nitrate enters drinking water through multiple pathways, primarily from agricultural sources such as fertiliser runoff and manure application, but also from wastewater discharge and natural geological deposits.
9. The legal limit for nitrate in drinking water in the UK is 50 mg/L, in line with the UK Water Supply (Water Quality) Regulations. This corresponds to an intake of 1.75 mg/kg BW/d[2].
10.         In the UK, drinking water quality is closely monitored, and nitrate concentrations are regularly tested by water companies and regulators such as the Drinking Water Inspectorate (DWI). When nitrate or nitrite levels exceed safe limits, water is subject to treatment measures, such as blending with low-nitrate water sources, ion exchange, or reverse osmosis.
11.         The concentration of nitrate and nitrite in drinking water is highly variable and depends on multiple environmental and anthropogenic factors. These include seasonal changes, rainfall patterns, land use, soil characteristics, and agricultural intensity.

Role of nitrate and nitrite in the association between red and processed meat and cancer

12.         Ingested nitrate is absorbed in the stomach and subsequently excreted via the salivary glands as part of the entero-salivary circulation. In the oral cavity, nitrate is reduced to nitrite by the oral microbiome, increasing nitrite availability in the stomach and intestines, where it can participate in carcinogenic chemical reactions.
13.         Under acidic conditions, nitrite can be converted into nitrosating species (reactive nitrogen species, RNS) that can react with amino groups from proteins to form nitrosamines, a class of carcinogens. The acidic environment of the stomach provides favourable conditions for this reaction, particularly when nitrite is present in high concentrations, such as after the consumption of processed meat.
14.         Our research has shown that haemoglobin present in meat facilitates the formation of nitrosamines und simulated stomach conditions. This effect is particularly relevant in the context of red and processed meat, where haem iron is abundant and can increase formation of nitrosamines in the digestive tract.
15.         In an analysis of colorectal tumours, we could identify mutations commensurate with nitrite-induced nitrosamine formation in patients with high red and processed meat intake, supporting a mechanistic link between nitrosation and cancer development. DNA adducts specific to nitrosamines have also been detected in exfoliated cells from the gut, further supporting the role of nitrosation in DNA damage within the intestinal epithelium.
16.         Dietary fibre and phenolic compounds such as flavonoids can reduce and prevent the formation of nitrosamines. We have shown previously that phenolic compounds can scavenge reactive nitrogen species. As part of the PHYTOME research project, we have shown that adding phenolic compounds to processed meat with nitrite reduces the amount of nitrosamines formed significantly.
17.         The gut microbiome is also involved in the formation of nitrosamines, as nitrosamines are not formed in germ-free rats.

Prevention of the formation of nitrosamines

18.         The formation of nitrosamines during food processing can be controlled by optimising processing conditions. High temperatures, particularly during frying, grilling, and high-temperature curing, promote nitrosation reactions. Lowering cooking temperatures and adjusting processing methods can reduce nitrosamine formation while preserving food safety. In addition, modifying pH conditions in food matrices can help limit the conversion of nitrite into reactive nitrogen species.
19.         Nitrosation inhibitors, such as ascorbic acid (Vitamin C) and alpha-tocopherol (Vitamin E), are often added to processed meats to reduce nitrosamine formation[3].
20.         Polyphenols have also been shown to reduce the formation of nitrosamines under simulated stomach conditions. They can react with reactive nitrogen species (RNS), scavenging them and thereby preventing the formation of nitrosamines. By intercepting RNS, polyphenols disrupt the key chemical pathway that leads to nitrosamine formation, offering a potential protective mechanism against dietary sources of these harmful compounds.
21.         The PHYTOME project[4] investigated whether plant extracts rich in polyphenols could be used as an alternative to nitrite for meat preservation while simultaneously reducing the formation of nitrosamines.
22.         Results from this project demonstrated that the addition of polyphenol-rich plant extracts to processed meat products reduces the formation of nitrosamines even in the presence of nitrite and can potentially replace nitrite as curing agent.
23.         A wide range of different plant extracts with varying polyphenol compositions are available and could be utilised for this purpose. These include extracts from fruits, vegetables, herbs, and spices, many of which contain high levels of flavonoids, tannins, and phenolic acids which can inhibit nitrosation.

Role of nitrate and nitrite in the association between cardiovascular disease risk reduction

24.         Nitric oxide (NO) is a key molecule involved in regulating vascular health produced via the L-arginine-eNOS-NO pathway. The nitrate- nitrite- nitric oxide (NO) pathway is now accepted as an alternative (exogenous/ dietary) means of NO production.
25.         Inorganic (dietary) nitrate is reduced to nitrite (as aforementioned via the entero-salivary circulation) and further to NO in hypoxic conditions or by enzymatic means (e.g. by xanthine oxidase) in circulation.
26.         We, and others have demonstrated that dietary nitrate (typically delivered as beetroot juice) reduces cardiovascular disease risk with the most evidence demonstrating blood pressure reduction[5].

Appendix – nitrate content in vegetables

Nitrate content in vegetables. The purple bar shows the entire measured range and yellow the inter-quartile range[6].

11/03/2025