In winter, the nitrate content in plants is just one and a half times higher than in summer. Beets, spinach, endive, radishes, celery and purslane, among others, are among the vegetables that contain more than one gram (1000 mg) of nitrate per kilogram. The stem, petiole and leaf veins of these vegetables contain the most nitrate; leaves, flowers and fruits least.
In addition to vegetables, we also get some nitrate through drinking water and in the form of preservatives. The acceptable daily intake of nitrate has been set at 3.7 mg per kg of body weight. That equates to 235 mg of nitrate for a person weighing 10 stone (63.5kg/140lb).
Nitrate in itself is not very harmful. However, this does not apply to the nitrite formed from nitrate. Bacteria are responsible for this conversion, a process that is quite slow under normal circumstances. Under the influence of heat, this process is much faster and toxic nitrite is formed. Above 60°C the bacteria die and the formation of nitrite stops. Nitrosamines can be formed from nitrite and the proteins found in food. These have been classified as ‘probably carcinogenic to humans’ by the International Agency for Research on Cancer (IARC). However, the European Food Safety Authority (EFSA) does not consider it likely that nitrates from vegetables pose a health risk. Where previously, due to the possible formation of nitrosamines, a combination of protein-rich fish with a nitrate-rich vegetable was not recommended, the Dutch Institute for Public Health and the Environment (RIVM) found that the risk of cancer with said combination is so small that combinations of fish and nitrate-rich vegetables can simply be eaten.
There are exceptions: for babies younger than six months, the intake of nitrate-rich vegetables should be limited. Babies still make little stomach acid, which means that nitrite is formed from nitrate more quickly. In addition, this nitrite binds more easily to the iron in the blood in babies, making oxygen absorption very difficult; normally oxygen binds to the iron. The result is a serious oxygen deficiency. Another source for nitrate can be nitrate-rich water that is used to prepare bottled food, for example spring water.
Daily consumption of nitrate-rich, concentrated beet juice by athletes is also not recommended.
Nitrate and nitrite, in combination with salt, are used as preservatives in the production of cheese and in the preservation of meat products. Maximum permitted amounts of 100 to 500 mg per kg apply to the permitted preservatives potassium nitrite (E249), sodium nitrite (E250), sodium nitrate (E251) and potassium nitrate (E252). Incidentally, that is far below the nitrate content that vegetables may contain, which is 4500 mg per kg. In England, Northern Ireland and Wales vegetables from greenhouse cultivation are checked for nitrate content.