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Nitrate in Food

Introduction

1.
Recently, the local media reported cases of methaemoglobinaemia (a rare condition under which haemoglobin is unable to carry oxygen to body tissues) in a 9-month-old baby and a woman of about 50 years of age in Hong Kong. Nitrate naturally present in Chinese spinach/amaranth has been reported to be the incriminated agent.
   
2.
Nitrate occurs in the environment, in air, food (particularly in vegetables and fruits) and water, and is produced inside living organisms. It can also be used as a food additive, mainly as preservative in cheese products and cured meat (e.g. Chinese preserved sausage and ham).
   
3.
The level of nitrates in vegetables varies greatly among species and can also be affected by growing conditions (seasons, light, temperature, growth method, fertilizer use etc.), storage (time and temperature etc., as enzymatic reaction and bacterial activity that lead to conversion of nitrate to nitrite which is a more toxic metabolite, are inactivated under cold storage) as well as processing (washing, peeling, blanching, boiling etc.). Levels reported ranged from a low of 1 mg/kg (e.g. peas) to a high of over 4,000 mg/kg (e.g. rucola) (see Appendix). Higher levels of nitrate have been consistently reported in certain vegetables such as leafy vegetables. This may be due to the fact that higher levels of nitrate tend to be found in leaves rather than in seeds or tubers.
   
4.
Human exposure to nitrate is mainly through the consumption of vegetables, and to a lesser extent water and other foods (such as cured meat).

Safety and Public Health Significance

1.
Nitrate itself is relatively non-toxic, but its metabolites such as nitrite have raised concern because of implications for adverse health effects such as methaemoglobinaemia and cancers.
   
2.
Nitrate in the body can be converted to nitrite, which can oxidize haemoglobin in blood and make it unable to carry oxygen to the body tissues. Having insufficient oxygen, the person develops cyanosis as a result, and the condition is called methaemoglobinaemia. Population subgroups such as young infants (i.e. less than 4-6 months of age) and people with glucose-6-phosphate dehydrogenase (G6PD) deficiency are more susceptible to the above condition. However, the Joint Food and Agriculture Organization / World Health Organization Expert Committee on Food Additives (JECFA) in 2002 considered the evidence on the potential of a high nitrate intake to cause methaemoglobinaemia from human studies equivocal.
   
3.

The International Agency for Research on Cancer (IARC) of the World Health Organization has evaluated the carcinogenicity of ingested nitrate and nitrite and considered that –

there is limited evidence in humans for the carcinogenicity of nitrite in food;
there is limited evidence in experimental animals for the carcinogenicity of nitrite itself; and
there is inadequate evidence in humans for the carcinogenicity of nitrate in food.

IARC concluded that ingested nitrate or nitrite under conditions that result in endogenous nitrosation (i.e. conversion into nitroso compounds such as nitrosamines) is probably carcinogenic to humans (i.e. Group 2A). However, the European Food Safety Authority (EFSA) concluded from the available evidence that nitrate intake from diet or drinking water is not associated with increased cancer risk in humans.

   
4.
JECFA has evaluated the safety of nitrate and allocated an acceptable daily intake (ADI) of 0-5 mg per kg body weight (bw), expressed as sodium nitrate, or 0-3.7 mg/kg bw, expressed as nitrate ion.
   
5.
EFSA compared the risks and benefits of exposure to nitrate from vegetables in 2008 and concluded that “overall, the estimated exposures to nitrate from vegetables are unlikely to result in appreciable health risks, therefore the recognised beneficial effects of consumption of vegetables prevail”.

Regulatory Control

1.
In Hong Kong, the use of nitrates (in forms of sodium nitrate or potassium nitrate) in specified food and their respective maximum permitted levels are governed under the Preservatives in Food Regulations (Cap. 132). Any person who contravenes the above Regulations is liable to a maximum fine of HK$50,000 and imprisonment for 6 months.
   
2.
For foods containing natural nitrate, there is no international consensus on their reference levels.

Advice to the Public

1.
Vegetables are an essential component of a healthy diet. Members of the public are advised to take a balanced diet and eat a variety of fruits and vegetables so as to avoid excessive exposure to chemicals from a small range of food items.
   
2.

To reduce the health risk of nitrate due to consumption of vegetables, members of the public can –

store the vegetables under refrigeration;
wash, peel, remove the stem, blanch the vegetables before cooking, when appropriate; and
cook vegetables soon after cutting or mashing.

The above measures can be adopted together for further risk reduction.

   
3.
Babies below 6 months of age are recommended to avoid vegetables of high nitrate content (e.g. spinach, beets) and processed food with nitrates added as food additives (e.g. cheese, cured meats). It is generally recognized that babies about six months of age are physiologically and developmentally ready for solid food.

Advice to the Trade

1.
Farmers are advised to observe good agricultural practice (GAP) (e.g. proper use of fertilizer) with an aim to minimize nitrate concentrations in vegetables.
   
2.
Be cautious about the origins from which food products are sourced, and should only do so from reliable sources.
   
3.
The trade should also take note of the regulatory requirement under the Public Health and Municipal Services Ordinance, Cap. 132, regarding the use and labelling of food additives.

Risk Assessment Section
Centre for Food Safety
August 2008

 

 

  Appendix

Levels of naturally occurring nitrate in vegetables

Food

Reported Level (mg/kg)

Europea

Beijing , Chinab,c

Bulb vegetable

Chives

1 ~ 2 949

643 ~ 2 257

Garlic

8 ~ 161

 

Leek

5 ~ 975

 

Onion

1 ~ 638

 

Brassica vegetable, Head cabbages, Flowerhead cabbages

Broccoli

16 ~ 758

 

Brussels sprouts

1 ~ 100

 

Cabbage

47 ~ 833

780 ~ 4 489

Cauliflower

7 ~ 390

164 ~ 854

Leafy vegetable

 

Amaranth

439 ~ 3 483

 

Chinese cabbage

77 ~ 1 928

1 419 ~ 3 832

Chinese kale

 

3 072 ~ 5 473

Chrysanthemum greens

 

2 892 ~ 5 480

Indian lettuce

 

444 ~ 2 901

Lettuce

56 ~ 3 660

929 ~ 5 742

Mixed lettuce

281 ~ 5 242

 

Pak-choi

 

1 503 ~ 6 534

Rucola

1 528 ~ 7 340

 

Spinach

64 ~ 3 048

1 388 ~ 5 214

Fruiting vegetables, Cucurbits

Balsam pear, bitter melon

 

192 ~ 979

Cucumber

22 ~ 409

56 ~ 570

Pumpkin

8 ~ 4617

 

Zucchini

 

784 ~ 1 675

Fruiting vegetables, other than Cucurbits

Eggplant

29 ~ 572

169 ~ 643

Capsicum

1 ~ 476

93 ~ 353

Chili pepper

4 ~ 120

70 ~ 507

Tomato

1 ~ 144

12 ~ 72

Mushroom

31 ~ 100

 

Legume vegetables

French beans

4 ~ 3 970

142 ~ 814

Peas

1 ~ 100

 

Root and tuber vegetables

 

Beetroot

110 ~ 3 670

 

Carrot

21 ~ 1 574

99 ~ 582

Potato

10 ~ 340

74 ~ 541

White radish

135 ~ 3 488

 

Stalk and stem vegetables

Asparagus

1 ~ 1 459

 

Celery

18 ~ 3 319

1 485 ~ 6 761


a
EFSA. 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. The EFSA Journal (2008) 69, 1-79. Available from: URL: http://www.efsa.europa.eu/EFSA/Scientific_Opinion/contam_ej_689_nitrate_en.pdf
   
b
FENG J. et. al. Assessment of nitrate exposure in Beijings residues via consumption of vegetables. Chinese Journal of Food Hygiene (2006) 18(6): 514-516. [Article in Chinese]
   
c

The figures have been rounded to the nearest whole number.

 

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Last Revision Date : 20-08-2008