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Nitrates and Greens

by Lisa Saffron(more info)

listed in environmental, originally published in issue 23 - November 1997

Nitrate levels are often so high in British lettuce and spinach that our government recently negotiated a relaxation of the EC limits. The EC set limits in order to protect public health. Why are nitrate levels so high in Britain and is our health at risk from British greens?

Vegetables become contaminated with nitrates when crops take up more than they can use for growth. Nitrates are the main form in which the essential plant nutrient, nitrogen, is absorbed by the plant from the soil. Microorganisms in the soil break down organic compounds, converting them to nitrates which are readily taken up by the roots. Fertilisers are added to the soil to increase plant growth. Organic fertilisers are subject to the same processes as organic matter in the soil but mineral fertilisers already contain nitrates and can be used directly by plants.

Nitrates occur in many foods and in drinking water but vegetables are by far the main source. More than three quarters of our average nitrate intake is from vegetables. Certain types of vegetables tend to accumulate nitrate - spinach, lettuce, broccoli, cabbage, celery, radish, and beetroot. Other vegetables do not accumulate as much nitrate - carrots, cauliflower, French beans, parsnips, peas and potatoes. Some plant-foods such as cereal grains contain no nitrates.

The amount of nitrate in vegetables can vary by 40-fold - nitrate in lettuce can be as low as 90 ppm and as high as 3520 ppm (parts per million). The reason for this wide range is not just the amount of fertiliser used, although that is important. Excessive fertiliser use (of either organic or mineral fertilisers) leads to a higher nitrate content.

But other factors are also important - crop variety, type of fertiliser and amount of sunlight being a few. There are large and significant differences between varieties. In a report from Elm Farm Research Centre, the nitrate content of 13 varieties of lettuce was found to range from 474 ppm in the variety "All the year round" to 1351 ppm in the "Diamant" variety.

Organically grown vegetables have significantly lower nitrate levels than those cultivated conventionally but these differences are not particularly large. In one Elm Farm Research Centre study, researchers found that the lettuces fertilised with composted farmyard manure had no more nitrate than unfertilised lettuce and significantly less nitrate than the lettuces fertilised with mineral fertiliser (1184 ppm compared to 1410 ppm).

However, the most important reason for high nitrate levels in British greens is the amount of sunlight received before harvest. Nitrate-accumulating vegetables have a higher nitrate content when grown in winter, the time of year when there is less sunlight. This is true regardless of whether organic or mineral fertilisers are used. Also, crops grown in northern Europe get less sunlight than those grown further south. That is the main reason that Britain and several other northern European countries have had to ask for a relaxation of the EC nitrate limits.

The main public health risk which the EC nitrate limits am meant to protect us from is stomach cancer. The limits are based on the precautionary principle of erring on the side of caution rather than on firm evidence that nitrates are linked to stomach cancer.

The idea that them is a link between nitrates and stomach cancer comes from two observations. One is that, in the body, nitrates are readily converted to cancer-causing chemicals called nitrosamines. The other is that both nitrate intake and stomach cancer death rates am high in certain countries - Japan, Colombia and Chile while both am low in other countries - the UK, Denmark, USA, and Sweden.

However, there is stronger evidence against a link. For one thing, stomach cancer mortality has been declining steadily and dramatically in most developed countries and in many developing countries since 1950 even while nitrate levels have increased. Secondly, an overview of the studies linking nitrates in vegetables with cancers of the digestive tract gives a consistent and dear picture. Eating vegetables protects against digestive tract cancers regardless of the presence of nitrates in the vegetables.

If there is a link between nitrates and stomach cancer, you would expect a higher nitrate intake in areas with a higher stomach cancer rates. In a 1985 UK study, it was thought that people in Wales and northeast England, where stomach cancer rates are relatively high, would have higher nitrate levels than people living in Oxford and southeast England where stomach cancer rates are low. But in this study, the opposite was found. The higher nitrate levels (measured in saliva) were in the people living in the areas with the lower stomach cancer rates.

Similar results come from a 1986 study done in Chile. Nitrate levels in children's bodies (measured in urine) and in locally grown vegetables were determined in two areas with high stomach cancer mortality rates and two areas with low mortality rates. Nitrate levels were highest in the children and vegetables from one of the areas with the lowest stomach cancer mortality.

Since the nitrosamines made from the nitrates are such potent cancer-inducing chemicals and so readily formed in the body, how it is possible that vegetables, our main source of nitrates, do not increase the risk of cancer?

The answer is that vegetables are also an important source of vitamins C and E and of other natural compounds Which are known to protect against cancer. These compounds inhibit the formation in the body of nitrosamines. The effect of vitamin C, in particular, has been demonstrated in a number of studies using a technique of feeding volunteers a diet rich in nitrates and measuring the amount of nitrosamine excreted in the urine. In one study, groups of ten people were given a standard diet over several days with and without addition of nitrate and vitamin C. When vitamin C was taken with the nitrate, the amount of nitrosamine was cut by nearly haft. When the vitamin C was taken 2 hours after the meal, there was no effect on the amount of nitrosamine formed. From this study, it appears that vitamin C must be taken with a meal not afterwards, to prevent nitrosamines being formed in the body from nitrates.

Our health is not at risk from British greens, despite the presence of nitrates. But our health is at risk if we do not eat vegetables. More is known about the protective effect of vegetables than about the risk from nitrates. We protect our health most by eating as much fresh vegetables as possible.

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About Lisa Saffron

Lisa Saffron is a health researcher and writer with a particular interest in the effect of environmental pollution on health. She has a Masters in Environmental Technology and a first degree in microbiology. She is committed to providing accurate and accessible information. Lisa also wrote a regular column in Positive Health magazine.

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