Intake assessment

First draft prepared by

T. Hambridge
Australia New Zealand Food Authority, Canberra, Australia

Nitrate and nitrite are found naturally in the environment, and form part of the nitrogen cycle. Nitrate is formed from oxidation of organic wastes by the action of nitrogen-fixing bacteria. The nitrite ion is relatively unstable and is readily oxidized to nitrate. Nitrate and nitrite are also found in soil and water due to use of fertilizers (WHO, 1998a). Nitrate has been shown to be produced endogenously (McKnight et al., undated). It is actively secreted into saliva, where some is reduced to nitrite by oral microflora (Dich et al., 1996; WHO, 1998a). Salivary nitrate is swallowed and recirculated. Nitrate is also found in air, at concentrations of 0.1–0.4 µg/m³, although concentrations up to 40 µg/m³ have been reported (WHO, 1998a).

Nitrate is found naturally in foods and in high concentrations in certain vegetables. The concentrations in vegetables depend on a number of factors including season, light, temperature, method of growth, fertilizer use and storage (Dich et al., 1996). In Europe, the concentrations of nitrate are generally higher in winter (European Commission, 1998) owing to the low light intensity and fewer daylight hours.

Nitrate and nitrite are also used as food additives in processed foods, as preservatives, antimicrobial agents and colour fixatives (Tompkin, 1993; Institute of Medicine, 1996). The main processed foods in which it is used include cured meat, poultry and fish, other processed meats and cheese (Coultate, 1989; Tompkin, 1993; Maga & Tu, 1995).

Nitrate is permitted for use in the Codex draft General Standard for Food Additives (GSFA) in numerous food categories, including cheese and cheese products, raw and processed meats, edible casings, processed fish and fish products and spirits and liqueurs. The GSFA also permits use of nitrite in numerous food categories, including cheese and cheese products, meat and processed meats, edible casings, cooked and preserved fish and roe.

Ideally, all sources of nitrate and nitrite should be considered in order to determine total estimated intake and the risks to health; however, only the intake from food (non-additive sources and as a food additive) and water are considered below.

The ADI for nitrate of 0–3.7 mg/kg bw, expressed as nitrate ion, was retained by the Committee at its present meeting, and the ADI for nitrite was set at 0–0.07 mg/kg bw, expressed as nitrite ion (Annex 1, reference 116). All values for nitrate and nitrite in the intake assessments are expressed as the ions to allow direct comparison with the respective ADIs.

Data on nitrate and nitrite for review by the Committee, including national intake assessments, were submitted by Australia, the Netherlands, New Zealand and the United Kingdom (Table 1). Both Australia and New Zealand submitted estimated dietary intakes of nitrate and nitrite based on individual dietary records. The Netherlands submitted data from monitoring and the results of a duplicate diet study (Vaessen & Schothorst, 1999). The United Kingdom submitted the results of monitoring that also included intake assessments, a survey of nitrate and nitrite in bacon and cured meats (Ministry of Agriculture, Fisheries and Food, 1998a), reports of two total diet studies (Ministry of Agriculture, Fisheries and Food, 1997a, 1998b), the results of a duplicate diet study of nitrate intake by vegetarians (Ministry of Agriculture, Fisheries and Food, 1998b) and a study of the metabolism of dietary nitrate (McKnight et al., undated).

Table 1. Summary of submissions on intake of nitrate

A regulation of the European Commission (194/97) requires Member States to monitor the concentrations of nitrate in lettuce and spinach to ensure that they remain acceptable. The Netherlands submitted the results of monitoring of nitrate concentrations in lettuce and spinach (van der Schee, 1997; van der Schee & Speek, 2000a,b), a survey of nitrate in other vegetables (van der Schee, 1998) and the results of monitoring for adherence to good agricultural practice in spinach and lettuce (de Kreij, 1998; de Kreij & van Winkel, 2000; de Kreij, 2001). The nitrate concentrations in lettuce collected over all seasons for a number of years ranged between 40 and 5500 mg/kg, while those in spinach were between 30 and 6000 mg/kg. The mean concentrations in other vegetables were 1900 mg/kg (maximum, 3400 mg/kg) in endive and 1800 mg/kg (maximum, 4700 mg/kg) in beetroot.

The United Kingdom submitted the results of numerous monitoring surveys for nitrate in lettuce and spinach (Ministry of Agriculture, Fisheries and Food, 1997b, 1998c, 1999, 2001) and in other vegetables (Ministry of Agriculture, Fisheries and Food, 1996, 1998d). The nitrate concentrations in lettuce collected over all seasons for a number of years ranged between 50 and 5300 mg/kg, while those in spinach were between 25 and 4600 mg/kg. The concentrations in other vegetables were < 3 mg/kg (below the limit of detection) for leeks and 4200 mg/kg for radishes. The mean residue of nitrate in all cured meat products was 180 mg/kg. The mean residue of nitrite was 120 mg/kg in bacon and 65 mg/kg in other cured meats (Ministry of Agriculture, Fisheries and Food, 1998a).

Nitrate and nitrite occur naturally in water, but farming and effluents can increase the levels of these chemicals in some waters. Nitrite is rapidly oxidized to nitrate in water and is rarely found in well-oxygenated or chlorinated water (Cooperative Research Centre, 1996). The WHO guideline for nitrate in drinking-water is 50 mg/l, and the provisional guideline for nitrite is 0.2 mg/l (WHO, 1998b). The guideline levels in drinking-water in Australia and New Zealand are 50 mg/l for nitrate and 3 mg/l for nitrite (National Health and Medical Research Council and Agriculture and Resource Management Council of Australia and New Zealand, 1996; Ministry of Health, 2000).

The concentration of nitrate in surface water is usually low, between 0 and 18 mg/l, while the natural concentration in groundwater is only a few milligrans per litre. The concentration in drinking-water in most countries is < 10 mg/l (WHO, 1998a), although a level of 1500 mg/l was reported for nitrate in India. The nitrate concentration in water in Australia is usually < 0.15 mg/L but can range up to 18 mg/l The nitrite concentration in water in the USA does not exceed 0.3 mg/l, and that in drinking-water in the Netherlands is usually < 0.1 mg/l (Cooperative Research Centre, 1996).

The intakes of nitrate and nitrite from food were calculated at a global level on the basis of mean food consumption in the GEMS/Food regional diets (WHO, 1998c), and the mean concentrations in foods in Europe from the submitted data. Intake from drinking-water was added, assuming a water consumption of 2 l/day in all diets except the European diet, in which it was considered to be 1 l/day. The mean concentration in water that was used in the intake calculations was 4 mg/l for nitrate and 0.3 mg/l for nitrite, which are representative of the usual concentrations found in water (WHO, 1998a). An average body weight of 60 kg was used for the global intake assessments. The calculations of intake (from consumption figures and mean concentrations) are shown in Table 2, and the concentrations and sources are summarized in Table 3. The estimated intakes are summarized in Table 4.

Table 2. Calculation of nitrate and nitrite intakes from sources other than food additives at the global level

Table 3. Concentrations of nitrate and nitrite (mg/kg) used to calculate mean concentrations for use in global intake calculations

Table 4. Estimated intakes of nitrate and nitrite from sources other than food additives at the global level

^a Based on 60 kg body weight

2.2.1 Screening by the budget method

The budget method is used to estimate the theoretical maximum level of a food additive in the proportion of the food and/or beverage supply likely to contain it that would not result in exceedance of the ADI by the population (Hansen, 1979). The factors taken into consideration are physiological energy requirements and the energy density of food. The budget method is used to assess whether use of the additive should be restricted and whether an intake assessment is required.

It is not entirely appropriate to use the budget method for nitrate and nitrite, because not all the intake of these chemicals, and therefore the contribution to the ADI, is derived from use as food additives, as is generally the case when the budget method is used. However, as the proportion of the ADI contributed by sources other than additives is not known, the full ADI was used for the budget method calculations.

The budget method was used to calculate nitrate in food and beverages, as the GSFA has determined maximum permitted levels for both. As the proportion of use in solid foods and in beverages is unknown, it was assumed to be 50% for each. It was also assumed that 50% of the food supply and 25% of beverages contain the additive. The theoretical maximum level of 150 mg/kg for solid food (see Table 5) is below the maximum permitted level of the GSFA (360 mg/kg), and the theoretical maximum level of 72 mg/kg for beverages is similar to the maximum permitted level of the GSFA (73 mg/kg). An intake assessment is therefore required, and restricted use of the food additive should be considered.

Table 5. Estimated theoretical maximum levels of nitrate and nitrite by the budget method

^a Highest level in food and beverages according to the GSFA

^b Highest levels in food and beverages in national submissions

The budget method was used to calculate nitrite only in food, as the GSFA provides a maximum permitted level only for solid foods. It was assumed that 50% of the solid food supply contains the additive. The theoretical maximum level for solid food of 5.6 mg/kg (see Table 5) is well below the maximum permitted level in the GSFA of 420 mg/kg. An intake assessment is therefore required, and restricted use of the additive should be considered.

Comparison of the theoretical maximum levels with the national maximum permitted levels resulted in conclusions similar to those based on the GSFA.

2.2.2 Assessments based on individual dietary records

Intakes of nitrate and nitrite as food additives estimated from individual dietary records were submitted by Australia and New Zealand (Table 6). In these assess-ments, intake was calculated for each individual on the basis of body weight, before population statistics were derived. The intake assessments of both countries included estimates of intake based on the maximum permitted level of each additive in foods in each country and on those of the GSFA. The Australian assessments also included dietary intake estimates based on levels of use of nitrite in industry. The national and GSFA maximum permitted levels and the data on food consumption aggregated from data on individual food consumption used in the intake assessments are shown in Tables 7 and 8, respectively.

Table 6. Summary of estimates of intake of nitrate and nitrite as food additives based on individual dietary records

^a ADI, 0–3.7 mg/kg bw for nitrate and 0–0.07 mg/kg bw for nitrite

Table 7. National and draft GSFA maximum permitted levels of use used in the intake assessments for Australia and New Zealand

^a Australia New Zealand Food Classification System

Table 8. Aggregated food consumption from individual data used in intake assessments with Australian, New Zealand and GSFA maximum permitted levels

^a All respondents: 13 858 in Australia and 4636 in New Zealand

2.3.1 Assessments based on total diet studies

Model diets are constructed from information on food consumption. They are designed to represent the typical diet of a general population or a specified population subgroup and include foods containing the food additive.

The intakes estimated from the total diet studies submitted by the United Kingdom are presented in Table 9. The results are presented for the whole population; the lower-bound estimates are for analytical concentrations below the limit of detection, assumed to be zero, and the upper-bound estimates are for analytical concentrations at the limit of detection. The results for adult consumers include estimated intake of nitrate from water and beer and of nitrite from water only, in order to estimate the ‘total diet’ intake. The assessments are based on measured concentrations in foods. The intakes in the submitted reports were compared with the ADIs of the Scientific Commission for Food of 3.6 mg/kg bw for nitrate and 0.06 mg/kg bw for nitrite. For the present assessment, the estimated intakes were compared with the ADIs established by the Committee.

The nitrate and nitrite concentrations found in the total diet studies are shown in Table 10. Table 11 shows consumption data for each food group used to estimate population intake. Data on food consumption by consumers only were not provided.

Table 9. Estimates of intake of nitrate and nitrite from total diet studies

^a ADI, 0–3.7 mg/kg bw for nitrate, 0–0.07 mg/kg bw for nitrite

Table 10. Nitrate and nitrite concentrations used in total diet studies in the United Kingdom in 1994 and 1997