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Leafy Vegetables and Nitrates

by David Kennedy May 9, 1995

We are being encouraged to eat far more vegetables, especially dark green leafy vegetables, by virtually every agency and institution that addresses health and nutrition concerns, and there are several compelling reasons to do so.

1. At least 35% of all cancers are directly attributable to diet. Dark green leafy vegetables are very rich in the vitamins, ascorbic acid and betacarotene, and contain a fair amount of vitamin E. These three antioxidant vitamins plus other compounds found in plant leaves, such as indoles, flavonoids, and phenols are being shown to greatly reduce the likelihood of cancer developing.

2. The antioxidants and soluble fiber in greens lower the risk of heart disease.

3. Iron deficiency anemia is the most common nutritional problem in the world. Anemia makes people always feel exhausted and can lower their resistance to infectious disease. Most greens are rich in iron. Vitamin C in greens helps the iron to be better absorbed. Greens also supply folate, which is very important in preventing anemia.

4. In most developed nations and among many urban populations in developing countries the diet has become dangerously low in fiber. For instance, the average American only consumes 11 grams of fiber per day. 20 35 grains is recommended to reduce risks of heart disease, colon cancer, and other intestinal diseases. Dark green leafy vegetables are an excellent source of fiber.

While the daily consumption of dark green leafy vegetables is still far below the recommended levels everywhere, there are some signs that they are becoming more popular. Grocery stores are beginning to carry more varieties and seed companies are offering a greater choice in growing greens at home. This is an exciting and encouraging trend in nutrition, but it raises a question that needs to be considered. What about the increased intake of nitrates?

Nitrates are fairly stable nitrogen compounds that can be degraded into nitrites. Nitrites are unstable and can combine readily with other compounds in the digestive tract to form carcinogenic nitrosamines. Currently about 65 of the average 73 milligrams of the nitrates we consume daily come from vegetables. If people increase their vegetable consumption sharply, as they are advised to do, is there a danger to their health from the associated increase in nitrates? And if there is, what steps can be taken to minimize this increased risk?

The amount of nitrate in plants is determined mainly from its genetically based metabolism, the age of the plant, and the amount of available nitrate in the soil. Leafy green vegetables and some root crops contain the highest concentrations of nitrates. Among commonly eaten vegetables, beetroot, celery, lettuce, spinach, and radishes have the most nitrate. There is often a tenfold variation in nitrate levels of the same variety of vegetables sampled from supermarkets. This is largely a function of the age of the vegetable when picked and the amount of nitrate fertilizer used to grow the crop. Nitrate levels of vegetables have gone up significantly in recent years because of increased use of nitrate fertilizers. Nitrate levels in carrots, lettuce and spinach, for example have roughly doubled since the 1970s in the US.

There are two basic strategies to reduce the risks of nitrosamine exposure while greatly increasing your consumption of vegetables, especially leafy vegetables. The first is to reduce the amount of nitrate in your diet; and the second is to prevent the nitrate from being converted to nitrites in the body. Some plants such as lettuce and spinach are very high in nitrates relative to the nutritional contribution they make, and it may be reasonable to use other vegetables in their place when possible. Varietal differences in nitrate content can be very significant, and it is prudent to choose a low nitrate variety, such as the smooth leaved spinach "cv. Tuftegard" over a high nitrate variety, such as the common home garden spinach "cv Bloomsdale", which has over 3 1/2 times as much nitrate. Increased consumer demand for low nitrate vegetable varieties could quickly lead to selection and breeding programs focused toward this end.

Vegetable plants grown without excessive nitrogen fertilizer have far less nitrate. Nitrate fertilizer applied shortly before harvest causes the greatest increase in nitrate levels and should be avoided. Slower releasing nitrogen sources such as animal and green manures can produce vegetables with significantly lower nitrates, and this is an area where the organic foods movement has led the way. If nonorganic fertilizers are being used, ammonium nitrogen and nitropyrin will grow lower nitrate vegetables than those fertilized with nitrate nitrogen.

Greens harvested in the afternoon on a sunny day will contain less nitrate than those picked on a cloudy day, or early in the morning. Generally, low light intensity, such as that found in cloudy climates, high latitudes, and winter greenhouses contribute to higher nitrate levels. Molybdenum deficiency in the soil can also lead to excessive nitrate accumulation in vegetables.

Vitamin C, or ascorbic acid is very efficient at preventing the conversion of nitrate to nitrite in plant tissue and within the human body. Greens that are very rich in Vitamin C, such as kale, may have enough vitamin C to protect us completely against the nitrates they contain. There are many other good reasons to increase vitamin C intake, besides its role in protecting against nitrite formation, and it is an inexpensive nutritional insurance.

Leaf For Life offers two techniques that can reduce the danger of nitrite and nitrosamine formation from increased consumption of greens. A leaf concentrate that is essentially nitrate free, can be made by grinding the leaf crop, pressing out the leaf juice and heating the juice to coagulate a curd. This curd when pressed will have about 27 % protein, but an estimated 98% of the nitrates are washed out in the residual liquid.

The second technique involves drying and grinding of leaf crops. Because we grind them to a flour like consistency, we can use more mature plants which are lower in nitrates. These plants are normally considered too tough and stringy to be marketable. By selecting species and varieties that are low in nitrate and high in vitamin C, and growing them without excessive nitrate fertilizer, we can dramatically reduce the danger from nitrates in vegetables. By making leaf concentrate or by drying and fine grinding more mature leaf crops, we can reduce the danger to an absolute minimum.

See: Nitrates and Nitrites in Food and Water, ed. Michael Hill, 1991, Ellis Horwood Publishers, London, 195 pp.


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