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