Food Additives and Contaminants: Part B Vol. 4, No. 2, June 2011, 94–98
VIEW DATASET Dietary exposure of Hong Kong adults to nitrate and nitrite from vegetable intake Melva Yung-yung Chena, Stephen Wai Cheung Chungb, Jeff Chuong-hao Tranb, Katherine Siu-kuen Tongb, Yuk-yin Hoa, Constance Hon-yee Chana and Ying Xiaoac* a
Centre for Food Safety, Food and Environmental Hygiene Department, 43/F Queensway Government Offices, 66 Queensway, Hong Kong, China; bFood Research Laboratory, Food and Environmental Hygiene Department, 4/F Public Health Laboratory Centre, 382 Nam Cheong Street, Hong Kong, China; cDepartment of Nutrition and Food Hygiene, School of Public Health, Peking University Health Science Center, 38 Xue Yuan Road, Beijing 100083, China (Received 15 November 2010; final version received 18 March 2011) The aim of this study was to assess the dietary exposure of adults in Hong Kong to nitrate and nitrite from vegetables. If all vegetables consumed were raw, the dietary exposure to nitrate for average consumers was estimated to be 4.4 mg kg1 body weight (bw) day1 and, for high consumers, was estimated to be 13 mg kg1 bw day1, which is about 120 and 350% of acceptable daily intake (ADI), respectively. If all vegetables consumed were cooked, the dietary exposure to nitrate from vegetables for the average adult consumer was estimated to be 3.5 mg kg1 bw day1 and, for high consumer, was estimated to be 10 mg kg1 bw day1, which is about 95 and 270% of ADI, respectively. On the other hand, the dietary exposure to nitrite from vegetables for average and high consumers were well below the ADI. Keywords: vegetables; nitrite; nitrate
Introduction Vegetables are essential to human health as they are good source of vitamins, minerals and biologically active substances. The World Health Organization (WHO) and Food and Agricultural Organization (FAO) recommend an intake of a minimum of 400 g of fruit and vegetables per day for the prevention of chronic diseases (WHO/FAO 2003). However, vegetables also contain nitrate and nitrite, which occur naturally in the environment. They are important plant nutrient and are used in fertilisers. In addition, they are added to some food products as preservatives. Human exposure to nitrate is mainly exogenous through the consumption of vegetables (70–90%), and to a lesser extent water and other foods (USEPA 1991; WHO 2003a; EFSA 2008). On the other hand, oral reduction of nitrate is the most important source of nitrite for human, accounting for 70–80% of human total nitrite exposure (EFSA 2008). Nitrate itself is relatively non-toxic but its metabolites, nitrite, is associated with methaemoglobinaemia. Nitrite may also react with amines to form carcinogenic nitrosamines in the stomach. The association between nitrate intake and cancer risk is debatable. Several studies had found an association between high nitrite intake and gastric and/or oesophageal cancer; however, other studies, particularly prospective cohort *Corresponding author. Email: [email protected] ISSN 1939–3210 print/ISSN 1939–3229 online ß 2011 Taylor & Francis DOI: 10.1080/19393210.2011.574158 http://www.informaworld.com
studies, revealed no such association (WHO 2003b). Overall, the epidemiological studies reviewed by JECFA and EFSA did not provide evidence that nitrate and nitrite are carcinogenic to humans (WHO 2003b; EFSA 2008). In addition, studies revealed a negative correlation between nitrate intake and gastric cancer. Most likely this is due to the known strong protective effect of vegetables and fruits on the risk of gastric cancer (Santamaria 2006). Other than adverse health effects, there are also claimed beneficial effects of nitrate. There is evidence linking nitrate to the prevention of microbial infection, in particular from gastrointestinal pathogens (Dykuizen et al. 1996; McKnight et al. 1999). Dietary nitrate is associated with reduction in hypertension and cardiovascular diseases. Some authors have opined that the physiologic basis for regulating human consumption of plant foods containing nitrate and nitrite should be re-evaluated to include potential health benefits (Hord et al. 2009). The Joint FAO/WHO expert Committee on Food Additives (JECFA) established an ADI of 0–3.7 mg kg1 bw day1 for nitrate and 0–0.07 mg kg1 bw day1 for nitrite (WHO 2003b, 2003c). Exposure to nitrate, via vegetables only, exceeded the ADI among Beijing residents (Feng et al. 2006) and a population in North China (Zhong et al. 2002). While further studies are needed to determine the adverse health effects and
Food Additives and Contaminants: Part B potential benefits of nitrate and nitrite, it is important to obtain dietary exposure data to these chemicals for the population. Although vegetables are not the main source of nitrite, nitrite is more toxic than nitrate, and the adverse health effects of nitrate are associated with its conversion to nitrite. This study was conducted to estimate the exposure of Hong Kong adults to nitrate and nitrite from vegetables.
Materials and methods Samples A total of 73 varieties of vegetables commonly consumed in Hong Kong were sampled from different local wet markets and other vegetable-seller during winter and summer for a better estimation of exposure. A sample consisted of at least 300 g of vegetable and 10 individual samples (5 for each season) were collected for each type of vegetables to obtain more representative average values. For several types of vegetables, only five individual samples were collected owning to their availability in either winter or summer only.
Analytical determination of nitrate and nitrite All vegetables were analyzed according to a modified BS EN 12014-2:1997. Nitrite in all vegetables was determined according to a modified Part 2 of the above-mentioned standard, followed by flow injection analysis (FIA) (Chung et al. 2011). The limits of detection (LOD) for nitrate and nitrite were 4 and 0.8 mg kg1, respectively; the limits of quantitation (LOQ) for nitrate and nitrite were 20 and 4 mg kg1, respectively. As there is no matrix certified reference material for nitrate and nitrite, two FAPAS quality control samples (T1553 and T1557) were purchased to check the accuracy of nitrate. All recovery results for T1553 and T1557 fell within the specified acceptable limits (n ¼ 7 and 24, respectively).
Food consumption data Food consumption data was extracted from the Hong Kong Population-based Food Consumption Survey 2005–2007 (FEHD 2010). Two non-consecutive days of 24-h dietary intake (24-h recall) questionnaires were used. The survey adopted a two-stage stratified random sample design with quotas for 14 age-gender groups after the required sample size for these categories was determined. A total of 5008 adults participated in the survey. At the end of the survey, a food consumption database was developed, including a Food and Recipe Database with 1429 foods consumed by respondents of this survey, along with 1591 recipes. Since quota sampling was adopted for sample selection, the survey results were weighted, based on
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population distribution, by age and gender to represent a population of approximately 5,394,000 Hong Kong residents aged 20–84 years. The average weight of this population was 62 kg.
Estimation of dietary exposure Daily dietary exposure to nitrate and nitrite from each individual vegetable were obtained by combining the consumption data and the levels of nitrate and nitrite of that vegetable. Total exposure for each adult was obtained by summing exposures from all vegetables. The mean and 95th percentile of daily exposure levels were used to represent the dietary exposure for average and high consumers, respectively.
Results and discussion Vegetable consumption pattern According to the Hong Kong Population-based Food Consumption Survey 2005–2007, Hong Kong adults, on average, consumed 195 g of tested vegetables per day. Leafy vegetables accounted for 56% of total vegetables consumed, while fruiting vegetables accounted for 19%. For other groups of vegetables, each contributed less than 10% to total vegetable consumption. Consumption data for different groups of vegetables are shown in Table 1.
Dietary exposure to nitrate and nitrite from vegetables The concentration of nitrate in each vegetable group is shown in Table 2. Mean concentrations of nitrate in different groups of vegetables, in descending order, were leafy vegetables (2100 mg kg1), root and tuber vegetables (720 mg kg1), fruiting vegetables, cucurbits (370 mg kg1), legume vegetables (140 mg kg1), fruiting vegetables, other than cucurbits and mushrooms (93 mg kg1), and fruiting vegetables/mushrooms (14 mg kg1). This trend was in agreement with those reported in other studies (Feng et al. 2006; Ayaz et al. 2007; EFSA 2008). Nitrite concentrations in vegetables were generally low, with average mean levels of less than 1 mg kg1 and levels in 66% of the vegetable samples were below the LOD. Dietary exposure to nitrate from vegetables for average consumers was estimated to be 4.4 mg kg1 bw day1 and, for high consumers, was estimated to be 13 mg kg1 bw day1, which are about 120 and 350% of the ADI, respectively (Table 3). However, the estimations are based on the nitrate levels in raw vegetables. Most vegetables, especially leafy vegetables consumed in Hong Kong, are cooked. Various studies have shown a reduction in nitrate levels (16–79% loss) when vegetables, such as peas,
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Table 1. Consumption pattern of vegetables for Hong Kong adults.
Type of vegetables
Table 3. Dietary exposure to nitrate for average and high adult consumers in Hong Kong (mg kg1 bw day1).
Mean consumption Percentage (%) (g day1)
Leafy vegetables Brassica vegetables Stalk and stem vegetables Bulb vegetables Legume vegetables Root and tuber vegetables Fruiting vegetables Others (not specified)
110 9 5 7 14 15 36 1
56 4 2 3 7 8 19 51
Note: Data are rounded to whole figure.
Table 2. Concentration of nitrate (mg kg1) in vegetables (uncooked) marketed in Hong Kong.
Vegetables Leafy vegetables (including brassica leafy vegetables)a Brassica (cole or cabbage) vegetables, head cabbages, flowerhead cabbagesb Stalk and stem vegetablesc Bulb vegetablesd Legumes vegetablese Root and tuber vegetablesf Fruiting vegetables, cucurbitsg Fruiting vegetables, mushroomsh Fruiting vegetables, other than cucurbits and mushroomsi
No. of samples Mean
Range
175
2100
79–6300
30
620
16–2800
55 70 63 125 70 60 60
830 8–4600 520 5–2300 140 ND–830 720 ND–4100 370 11–1400 14 ND–140 93 ND–470
Notes: ND denotes not detected. LOD for nitrate ion is 4 mg kg1. a Including Chinese cabbage, Chinese flowering cabbage, Chinese kale, Chinese spinach, European lettuce, spinach, watercress, etc. b Including broccoli, cauliflower, and European variety cabbage. c Including bamboo shoot, celery, mungbean sprouts, etc. d Including common bean, green string bean, snow pea, etc. e Including bulb vegetables, Chinese chives, garlic spears, spring onion, etc. f Including carrot, Chinese white radish, lotus root, etc. g Including bitter gourd, cucumber, hairy gourd, wax gourd, etc. h Including oyster mushroom, shiitake mushroom, straw mushroom, etc. i Including bell pepper, eggplant, tomato, sweet corn etc.
cabbage, beans, carrots, potatoes spinach, endives and celery leaves, are cooked in water (EFSA 2008). A study showed that nitrate in vegetables (i.e. Chinese flowering cabbage, Chinese spinach and celery) could be reduced significantly (12–31%) after blanching for 1–3 min (Chung et al. 2011). Assuming a conservative reduction of 20% in nitrate levels in cooked
Scenario 1a Scenario 2b
Average consumer
% ADI
High consumer
% ADI
4.4 3.5
120 95
13 10
350 270
Notes: aAssuming all vegetables are consumed raw. b Assuming all vegetables are consumed cook with 20% reduction in nitrate.
vegetables, dietary exposure to nitrate from vegetables for the average adult consumer was estimated to be 3.5 mg kg1 bw day1 and, for the high consumer, was estimated to be 10 mg kg1 bw day1, which are about 95 and 270% of the ADI, respectively. Since nitrite were not detected (ND) in 66% of samples, estimated exposure was presented in range, upper bound (ND ¼ LOD) and lower bound (ND ¼ 0). The dietary exposure to nitrite from vegetables for average consumers was estimated to be 0.0038 mg kg1 bw day1 (lower bound) to 0.0051 mg kg1 bw day1 (upper bound) and, for high consumers, was estimated to be 0.012 mg kg1 bw day1 (lower bound) to 0.015 mg kg1 bw day1 (upper bound). The estimated exposures to nitrite were well below the ADI of 0.07 mg kg1 bw day1. When assessing risk from nitrate in vegetable, both the potential risk of nitrate and the benefits of eating vegetables have to be considered. In view of the well-known benefits of vegetables and the lack of data on the possible effects of vegetable matrices on the bio-availability of nitrate, the JECFA considered it to be inappropriate to compare exposure to nitrate from vegetables directly with the ADI and, hence, to derive limits for nitrate in vegetables directly from it (WHO 1996). EFSA considered that benefits of vegetable and fruit consumption outweigh any perceived risk of developing cancer from the consumption of nitrate and nitrite in these foods (EFSA 2008). Nevertheless, it is prudent to reduce dietary nitrate intake by maintaining a balanced diet and avoiding over-indulgence in vegetables containing high nitrate levels.
International comparison of nitrate exposure from foods The estimated exposure to nitrate for the Hong Kong population was higher than Western countries (Ysart et al. 1999; Thomson 2004; Menard et al. 2009) and Korea (Chung et al. 2003) but lower than Beijing (Feng et al. 2006) and North China (Zhong et al. 2002) (Table 4). This may due to higher consumption of leafy vegetables by Hong Kong adults and the relatively high nitrate concentrations in these vegetables.
Food Additives and Contaminants: Part B Table 4. International comparison of average daily exposure to nitrate (as nitrate ion) (mg kg1 bw day1).
Beijing, Chinaa Franceb Hong Kong, Chinac Koread New Zealande North Chinaf United Kingdomg
Average consumer
High consumer
5.5 1.5 4.4 1.7 0.53 7.0 1.6
49 3.3 13 – 3.7 – 2.3
Notes: Daily exposure data of Beijing China, Korea, North China and UK were converted to per body kilogram weight assuming an average weight of 60 kg. a Feng et al. (2006). Data based on consumption of vegetables only. High consumer refers to 95th percentile. b Menard et al. (2008). Data based on consumption of a variety of foods. High consumer refers to 97.5 percentile. c Present study. Data based on consumption of vegetables only. High consumer refers to 95th percentile. d Chung et al. (2003). Data based on consumption of vegetables only. e Thomson (2004). Data based on consumption of a variety of foods. High consumer refers to 99th percentile. f Zhong et al. (2002). Data based on consumption of vegetables only. g Ysart et al. (1999). Data based on consumption of a variety of foods. High consumer refers to 97.5th percentile.
However, one should be cautious when comparing these results. As there are large variations in nitrate concentrations among vegetables, even of the same type, the representativeness of the samples is dependent on the sampling methods (e.g. sample size and types). Thus, due to differences in study design, calculation methods and analytical techniques, exposure data may not be directly comparable.
Conclusions This study provides the first exposure estimation of nitrate and nitrite from vegetables for Hong Kong adults. Estimated dietary exposure to nitrate from vegetables for average and high consumers approached or exceeded the ADI, while for nitrite it was well below the ADI. Current knowledge and evidence indicates there are strong beneficial effects in the consumption of vegetables. To maximise the health benefits from eating vegetables, measures must be taken to reduce nitrate exposure. Farmers should observe good agriculture practice (GAP) to minimize nitrate concentrations in vegetables and the public is recommended to eat a wide variety of vegetables, including fruiting and legume vegetables, instead of predominantly leafy vegetables.
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References Ayaz A, Topcu A, Yurttagul M. 2007. Survey of nitrate and nitrite levels of fresh vegetables in Turkey. J Food Tech. 5(2):177–179. BS EN 12014-2:1997. Foodstuffs: Determination of nitrate and/or nitrite content. Part 2: HPLC/IC method for the determination of nitrate content of vegetables and vegetable products. Chung SY, Kim JS, Kim M, Hong MK, Lee JO, Kim CM, Song IS. 2003. Survey of nitrate and nitrite contents of vegetables grown in Korea. Food Addit Contam A. 20(7):621–628. Chung SWC, Tran JCH, Tong KSK, Chen MYY, Xiao Y, Ho YY, Chan CHY. 2011. Nitrate and nitrite levels in commonly consumed vegetables in Hong Kong. Food Addit Contam B. 4(1):34–41. Dykhuizen RS, Frazer R, Duncan C, Smith CC, Golden M, Benjamin N, Leifert C. 1996. Antimicrobial effect of acidified nitrite on gut pathogens: importance of dietary nitrate in host defense. Antimicrob Agents Chemother. 40:1422–1425. Available from: http://www.ncbi.nlm.nih. gov/pmc/articles/PMC163343/pdf/401422.pdf EFSA. 2008. Opinion of the scientific panel on contaminants in food chain on a request from the European Commission to perform a scientific risk assessment on nitrate in vegetables. EFSA J. 69:1–79. Available from: http:// www.efsa.europa.eu/EFSA/Scientific_Opinion/contam_ej _689_nitrate_en.pdf FEHD. 2010. Hong Kong population-based food consumption survey 2005–2007. Final Report. Available from: http://www.cfs.gov.hk/english/programme/programme_firm/ files/FCS_final_report.pdf Feng J, Shi ZX, Wu YN, Zhao YF. 2006. Assessment of nitrate exposure in Beijing residunts via consumption of vegetables. Chin J Food Hyg. 18(6):514–516. [Article in Chinese] Hord NG, Tang YP, Byran BS. 2009. Food sources of nitrates and nitrite: the physiologic context for potential health benefits. Am J Clin Nutr. 90:1–10. Available from: http://www.ajcn.org/ McKnight G, Duncan CW, Leifert C, Golden MH. 1999. Dietary nitrate in man: friend or foe? Br J Nutr. 81: 349–358. Menard C, Volatier HJL, Leblanc JC. 2008. Assessment of dietary exposure of nitrate and nitrite in France. Food Addit Contam A. 25(8):971–988. Santamaria P. 2006. Review – Nitrate in vegetables: toxicity content, intake and EC regulation. J Sci Food Agric. 86:10–17. Thomson B. 2004. Nitrates and nitrites dietary exposure and risk assessment: prepared as part of a New Zealand Food Safety Authority contract for scientific services. Christchurch Science Centre: Institute of Environmental Science & Research Limited. Available from: http://www.nzfsa.govt.nz/consumers/food-safetytopics/chemicals-in-food/residues-in-food/consumerresearch/nitrite-nitrate-report.pdf
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US EPA. 1991. Intergrated risk information system – Nitrate (CASRN 14797-55-8). Available from: http://www.epa. gov/iris/subst/0076.htm WHO. 1996. Nitrate. In: Safety evaluation of certain food additives. Food additives Series 35. Geneva: WHO. Available from: http://www.inchem.org/documents/jecfa/ jecmono/v35je14.htm WHO. 2003a. Nitrate and nitrite – intake assessment. In: Safety evaluation of certain food additives. Food additives Series 50. Geneva: WHO. Available from: http:// www.inchem.org/documents/jecfa/jecmono/v50je07.htm WHO. 2003b. Nitrate (and potential endogenous formation of N-nitroso compounds). In: Safety evaluation of certain food additives. Food additives Series 50. Geneva: WHO. Available from: http://www.inchem.org/documents/jecfa/ jecmono/v50je06.htm
WHO. 2003c. Nitrite (and potential endogenous formation of N-nitroso compounds). In: Safety evaluation of certain food additives. Food additives Series 50. Geneva: WHO. Available from: http://www.inchem.org/documents/jecfa/ jecmono/v50je05.htm WHO/FAO. 2003. Diet, nutrition and the prevention of chronic diseases. Report of the Joint WHO/FAO expert consultation. Technical Report Series 916. Available from: http://www.fao.org/docrep/005/AC911E/ AC911E00.HTM Ysart G, Miller P, Barrett G, Farrington D, Lawrance P, Harrison N. 1999. Dietary exposures to nitrate in the UK. Food Addit Contam A. 16(12):521–532. Zhong W, Hu C, Wang M. 2002. Nitrate and nitrite in vegetables from north China: content and intake. Food Addit Contam A. 19(12):1125–1129.
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VIEW DATASET Dietary exposure of Hong Kong adults to nitrate and nitrite from vegetable intake Melva Yung-yung Chena, Stephen Wai Cheung Chungb, Jeff Chuong-hao Tranb, Katherine Siu-kuen Tongb, Yuk-yin Hoa, Constance Hon-yee Chana and Ying Xiaoac* a
Centre for Food Safety, Food and Environmental Hygiene Department, 43/F Queensway Government Offices, 66 Queensway, Hong Kong, China; bFood Research Laboratory, Food and Environmental Hygiene Department, 4/F Public Health Laboratory Centre, 382 Nam Cheong Street, Hong Kong, China; cDepartment of Nutrition and Food Hygiene, School of Public Health, Peking University Health Science Center, 38 Xue Yuan Road, Beijing 100083, China (Received 15 November 2010; final version received 18 March 2011) The aim of this study was to assess the dietary exposure of adults in Hong Kong to nitrate and nitrite from vegetables. If all vegetables consumed were raw, the dietary exposure to nitrate for average consumers was estimated to be 4.4 mg kg1 body weight (bw) day1 and, for high consumers, was estimated to be 13 mg kg1 bw day1, which is about 120 and 350% of acceptable daily intake (ADI), respectively. If all vegetables consumed were cooked, the dietary exposure to nitrate from vegetables for the average adult consumer was estimated to be 3.5 mg kg1 bw day1 and, for high consumer, was estimated to be 10 mg kg1 bw day1, which is about 95 and 270% of ADI, respectively. On the other hand, the dietary exposure to nitrite from vegetables for average and high consumers were well below the ADI. Keywords: vegetables; nitrite; nitrate
Introduction Vegetables are essential to human health as they are good source of vitamins, minerals and biologically active substances. The World Health Organization (WHO) and Food and Agricultural Organization (FAO) recommend an intake of a minimum of 400 g of fruit and vegetables per day for the prevention of chronic diseases (WHO/FAO 2003). However, vegetables also contain nitrate and nitrite, which occur naturally in the environment. They are important plant nutrient and are used in fertilisers. In addition, they are added to some food products as preservatives. Human exposure to nitrate is mainly exogenous through the consumption of vegetables (70–90%), and to a lesser extent water and other foods (USEPA 1991; WHO 2003a; EFSA 2008). On the other hand, oral reduction of nitrate is the most important source of nitrite for human, accounting for 70–80% of human total nitrite exposure (EFSA 2008). Nitrate itself is relatively non-toxic but its metabolites, nitrite, is associated with methaemoglobinaemia. Nitrite may also react with amines to form carcinogenic nitrosamines in the stomach. The association between nitrate intake and cancer risk is debatable. Several studies had found an association between high nitrite intake and gastric and/or oesophageal cancer; however, other studies, particularly prospective cohort *Corresponding author. Email: [email protected] ISSN 1939–3210 print/ISSN 1939–3229 online ß 2011 Taylor & Francis DOI: 10.1080/19393210.2011.574158 http://www.informaworld.com
studies, revealed no such association (WHO 2003b). Overall, the epidemiological studies reviewed by JECFA and EFSA did not provide evidence that nitrate and nitrite are carcinogenic to humans (WHO 2003b; EFSA 2008). In addition, studies revealed a negative correlation between nitrate intake and gastric cancer. Most likely this is due to the known strong protective effect of vegetables and fruits on the risk of gastric cancer (Santamaria 2006). Other than adverse health effects, there are also claimed beneficial effects of nitrate. There is evidence linking nitrate to the prevention of microbial infection, in particular from gastrointestinal pathogens (Dykuizen et al. 1996; McKnight et al. 1999). Dietary nitrate is associated with reduction in hypertension and cardiovascular diseases. Some authors have opined that the physiologic basis for regulating human consumption of plant foods containing nitrate and nitrite should be re-evaluated to include potential health benefits (Hord et al. 2009). The Joint FAO/WHO expert Committee on Food Additives (JECFA) established an ADI of 0–3.7 mg kg1 bw day1 for nitrate and 0–0.07 mg kg1 bw day1 for nitrite (WHO 2003b, 2003c). Exposure to nitrate, via vegetables only, exceeded the ADI among Beijing residents (Feng et al. 2006) and a population in North China (Zhong et al. 2002). While further studies are needed to determine the adverse health effects and
Food Additives and Contaminants: Part B potential benefits of nitrate and nitrite, it is important to obtain dietary exposure data to these chemicals for the population. Although vegetables are not the main source of nitrite, nitrite is more toxic than nitrate, and the adverse health effects of nitrate are associated with its conversion to nitrite. This study was conducted to estimate the exposure of Hong Kong adults to nitrate and nitrite from vegetables.
Materials and methods Samples A total of 73 varieties of vegetables commonly consumed in Hong Kong were sampled from different local wet markets and other vegetable-seller during winter and summer for a better estimation of exposure. A sample consisted of at least 300 g of vegetable and 10 individual samples (5 for each season) were collected for each type of vegetables to obtain more representative average values. For several types of vegetables, only five individual samples were collected owning to their availability in either winter or summer only.
Analytical determination of nitrate and nitrite All vegetables were analyzed according to a modified BS EN 12014-2:1997. Nitrite in all vegetables was determined according to a modified Part 2 of the above-mentioned standard, followed by flow injection analysis (FIA) (Chung et al. 2011). The limits of detection (LOD) for nitrate and nitrite were 4 and 0.8 mg kg1, respectively; the limits of quantitation (LOQ) for nitrate and nitrite were 20 and 4 mg kg1, respectively. As there is no matrix certified reference material for nitrate and nitrite, two FAPAS quality control samples (T1553 and T1557) were purchased to check the accuracy of nitrate. All recovery results for T1553 and T1557 fell within the specified acceptable limits (n ¼ 7 and 24, respectively).
Food consumption data Food consumption data was extracted from the Hong Kong Population-based Food Consumption Survey 2005–2007 (FEHD 2010). Two non-consecutive days of 24-h dietary intake (24-h recall) questionnaires were used. The survey adopted a two-stage stratified random sample design with quotas for 14 age-gender groups after the required sample size for these categories was determined. A total of 5008 adults participated in the survey. At the end of the survey, a food consumption database was developed, including a Food and Recipe Database with 1429 foods consumed by respondents of this survey, along with 1591 recipes. Since quota sampling was adopted for sample selection, the survey results were weighted, based on
95
population distribution, by age and gender to represent a population of approximately 5,394,000 Hong Kong residents aged 20–84 years. The average weight of this population was 62 kg.
Estimation of dietary exposure Daily dietary exposure to nitrate and nitrite from each individual vegetable were obtained by combining the consumption data and the levels of nitrate and nitrite of that vegetable. Total exposure for each adult was obtained by summing exposures from all vegetables. The mean and 95th percentile of daily exposure levels were used to represent the dietary exposure for average and high consumers, respectively.
Results and discussion Vegetable consumption pattern According to the Hong Kong Population-based Food Consumption Survey 2005–2007, Hong Kong adults, on average, consumed 195 g of tested vegetables per day. Leafy vegetables accounted for 56% of total vegetables consumed, while fruiting vegetables accounted for 19%. For other groups of vegetables, each contributed less than 10% to total vegetable consumption. Consumption data for different groups of vegetables are shown in Table 1.
Dietary exposure to nitrate and nitrite from vegetables The concentration of nitrate in each vegetable group is shown in Table 2. Mean concentrations of nitrate in different groups of vegetables, in descending order, were leafy vegetables (2100 mg kg1), root and tuber vegetables (720 mg kg1), fruiting vegetables, cucurbits (370 mg kg1), legume vegetables (140 mg kg1), fruiting vegetables, other than cucurbits and mushrooms (93 mg kg1), and fruiting vegetables/mushrooms (14 mg kg1). This trend was in agreement with those reported in other studies (Feng et al. 2006; Ayaz et al. 2007; EFSA 2008). Nitrite concentrations in vegetables were generally low, with average mean levels of less than 1 mg kg1 and levels in 66% of the vegetable samples were below the LOD. Dietary exposure to nitrate from vegetables for average consumers was estimated to be 4.4 mg kg1 bw day1 and, for high consumers, was estimated to be 13 mg kg1 bw day1, which are about 120 and 350% of the ADI, respectively (Table 3). However, the estimations are based on the nitrate levels in raw vegetables. Most vegetables, especially leafy vegetables consumed in Hong Kong, are cooked. Various studies have shown a reduction in nitrate levels (16–79% loss) when vegetables, such as peas,
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Table 1. Consumption pattern of vegetables for Hong Kong adults.
Type of vegetables
Table 3. Dietary exposure to nitrate for average and high adult consumers in Hong Kong (mg kg1 bw day1).
Mean consumption Percentage (%) (g day1)
Leafy vegetables Brassica vegetables Stalk and stem vegetables Bulb vegetables Legume vegetables Root and tuber vegetables Fruiting vegetables Others (not specified)
110 9 5 7 14 15 36 1
56 4 2 3 7 8 19 51
Note: Data are rounded to whole figure.
Table 2. Concentration of nitrate (mg kg1) in vegetables (uncooked) marketed in Hong Kong.
Vegetables Leafy vegetables (including brassica leafy vegetables)a Brassica (cole or cabbage) vegetables, head cabbages, flowerhead cabbagesb Stalk and stem vegetablesc Bulb vegetablesd Legumes vegetablese Root and tuber vegetablesf Fruiting vegetables, cucurbitsg Fruiting vegetables, mushroomsh Fruiting vegetables, other than cucurbits and mushroomsi
No. of samples Mean
Range
175
2100
79–6300
30
620
16–2800
55 70 63 125 70 60 60
830 8–4600 520 5–2300 140 ND–830 720 ND–4100 370 11–1400 14 ND–140 93 ND–470
Notes: ND denotes not detected. LOD for nitrate ion is 4 mg kg1. a Including Chinese cabbage, Chinese flowering cabbage, Chinese kale, Chinese spinach, European lettuce, spinach, watercress, etc. b Including broccoli, cauliflower, and European variety cabbage. c Including bamboo shoot, celery, mungbean sprouts, etc. d Including common bean, green string bean, snow pea, etc. e Including bulb vegetables, Chinese chives, garlic spears, spring onion, etc. f Including carrot, Chinese white radish, lotus root, etc. g Including bitter gourd, cucumber, hairy gourd, wax gourd, etc. h Including oyster mushroom, shiitake mushroom, straw mushroom, etc. i Including bell pepper, eggplant, tomato, sweet corn etc.
cabbage, beans, carrots, potatoes spinach, endives and celery leaves, are cooked in water (EFSA 2008). A study showed that nitrate in vegetables (i.e. Chinese flowering cabbage, Chinese spinach and celery) could be reduced significantly (12–31%) after blanching for 1–3 min (Chung et al. 2011). Assuming a conservative reduction of 20% in nitrate levels in cooked
Scenario 1a Scenario 2b
Average consumer
% ADI
High consumer
% ADI
4.4 3.5
120 95
13 10
350 270
Notes: aAssuming all vegetables are consumed raw. b Assuming all vegetables are consumed cook with 20% reduction in nitrate.
vegetables, dietary exposure to nitrate from vegetables for the average adult consumer was estimated to be 3.5 mg kg1 bw day1 and, for the high consumer, was estimated to be 10 mg kg1 bw day1, which are about 95 and 270% of the ADI, respectively. Since nitrite were not detected (ND) in 66% of samples, estimated exposure was presented in range, upper bound (ND ¼ LOD) and lower bound (ND ¼ 0). The dietary exposure to nitrite from vegetables for average consumers was estimated to be 0.0038 mg kg1 bw day1 (lower bound) to 0.0051 mg kg1 bw day1 (upper bound) and, for high consumers, was estimated to be 0.012 mg kg1 bw day1 (lower bound) to 0.015 mg kg1 bw day1 (upper bound). The estimated exposures to nitrite were well below the ADI of 0.07 mg kg1 bw day1. When assessing risk from nitrate in vegetable, both the potential risk of nitrate and the benefits of eating vegetables have to be considered. In view of the well-known benefits of vegetables and the lack of data on the possible effects of vegetable matrices on the bio-availability of nitrate, the JECFA considered it to be inappropriate to compare exposure to nitrate from vegetables directly with the ADI and, hence, to derive limits for nitrate in vegetables directly from it (WHO 1996). EFSA considered that benefits of vegetable and fruit consumption outweigh any perceived risk of developing cancer from the consumption of nitrate and nitrite in these foods (EFSA 2008). Nevertheless, it is prudent to reduce dietary nitrate intake by maintaining a balanced diet and avoiding over-indulgence in vegetables containing high nitrate levels.
International comparison of nitrate exposure from foods The estimated exposure to nitrate for the Hong Kong population was higher than Western countries (Ysart et al. 1999; Thomson 2004; Menard et al. 2009) and Korea (Chung et al. 2003) but lower than Beijing (Feng et al. 2006) and North China (Zhong et al. 2002) (Table 4). This may due to higher consumption of leafy vegetables by Hong Kong adults and the relatively high nitrate concentrations in these vegetables.
Food Additives and Contaminants: Part B Table 4. International comparison of average daily exposure to nitrate (as nitrate ion) (mg kg1 bw day1).
Beijing, Chinaa Franceb Hong Kong, Chinac Koread New Zealande North Chinaf United Kingdomg
Average consumer
High consumer
5.5 1.5 4.4 1.7 0.53 7.0 1.6
49 3.3 13 – 3.7 – 2.3
Notes: Daily exposure data of Beijing China, Korea, North China and UK were converted to per body kilogram weight assuming an average weight of 60 kg. a Feng et al. (2006). Data based on consumption of vegetables only. High consumer refers to 95th percentile. b Menard et al. (2008). Data based on consumption of a variety of foods. High consumer refers to 97.5 percentile. c Present study. Data based on consumption of vegetables only. High consumer refers to 95th percentile. d Chung et al. (2003). Data based on consumption of vegetables only. e Thomson (2004). Data based on consumption of a variety of foods. High consumer refers to 99th percentile. f Zhong et al. (2002). Data based on consumption of vegetables only. g Ysart et al. (1999). Data based on consumption of a variety of foods. High consumer refers to 97.5th percentile.
However, one should be cautious when comparing these results. As there are large variations in nitrate concentrations among vegetables, even of the same type, the representativeness of the samples is dependent on the sampling methods (e.g. sample size and types). Thus, due to differences in study design, calculation methods and analytical techniques, exposure data may not be directly comparable.
Conclusions This study provides the first exposure estimation of nitrate and nitrite from vegetables for Hong Kong adults. Estimated dietary exposure to nitrate from vegetables for average and high consumers approached or exceeded the ADI, while for nitrite it was well below the ADI. Current knowledge and evidence indicates there are strong beneficial effects in the consumption of vegetables. To maximise the health benefits from eating vegetables, measures must be taken to reduce nitrate exposure. Farmers should observe good agriculture practice (GAP) to minimize nitrate concentrations in vegetables and the public is recommended to eat a wide variety of vegetables, including fruiting and legume vegetables, instead of predominantly leafy vegetables.
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