Background of the study
Vegetables play an important role in all human nutrition. They belong to about thirteen plant families and have originated from widely differing part of the earth (Chandy K. T. 2000).
Vegetables may be defined as those annual plants of which immature succulent parts are commonly used for culinary purposes and also these perennial non woody plant of which the stem, `root, leaves, stock or leaves are eaten (Karlowski K. 2002).
Vegetables are generally low in fat, high in fibre and high in vitamins and minerals. Dark green leaves of leafy vegetables such as spinach and broccoli are good sources of vitamin A in the form of carotone. Vegetables also contain small but significant quantities of iron, calcium and phosphorus (Philips 2005). Additionally vegetables are packed with soluble as well as insoluble dietary fiber known as non-starch polysaccharides (NSP) such as cellulose-mucilage, hemi-cellulose, gums, pectin (Lyons D. J. et al, 2000).
Vegetables are a very important part of human diet because they provide humans with nutrient and anti oxidants. However, boiling vegetables destroys these essential nutrients and not all the health benefits will be received (Hill, M. 2001). Some nutrients in Vegetables known as water soluble vitamins dissolve in water. The longer vegetables are submerged in water, the more vitamins seep out.
The loss of vitamins is increased when these vegetables are also exposed to heat such as during boiling and sun drying (Voronina, 2002)
Vegetable production must be increased to meet the demand of vegetables in our country. Vegetables that may accumulate nitrate in their tissues are leafy vegetables such as spinach, lettuce and cabbage. The amount of nitrate in vegetables like lettuce is usually between 90 and 3520 ppm (Cieslik, 2001).
The term heavy refers to any metallic chemical element that has a relatively high density and is toxic or poisonous at low concentrations. Living organisms require trace amount of some heavy metals, including sodium (Na), magnesium (Mg), potassium (K), calcium (Ca), chromium (Cr), manganese (Mn), iron (Fe), Cobalt (Co), Copper (Cu), Zinc (Zn), Selenium (Sc) and molybdenum (Mo), but excessive levels can be detrimental to the organism of the non-essential metals, mercury (Hg), Lead (Pb), Cadmium (Cd), and Arsenic (As) are recognized as health hazardous and all have caused major health problems as a result of environmental pollution (Berglund et al, 2004).
Natural and anthropogenic sources of soil contamination are widespread and variable. Heavy metals occur naturally in rocks. Mineral rock weathering and anthropogenic sources provide two of the main types of metal inputs to soils. In localized mineral zones, or around volcanoes on elevated level of the element arsenic, lead, cadmium, copper and zinc can also be found (Adriano, 2000). Cadmium is associated with sptalirite, and head is found in many ores (ATSDR, 2002).
Anthropogenic activities, such as mining, industrial effluents and sewage sludge application, are the main source of heavy metals, contamination in the environment (Lee Stuebing, 1999). This sources increases heavy metal concentrations in the soil and considered to pose possibly serious hazards in the soil-plant-animal system (Lester, 2002).
The prevalence of contamination from both natural and anthropogenic sources has increased concern about the health effects of chronic low level exposures. Certain plants can accumulate heavy metals in their tissues. Uptake is generally increased in plants that are grown in areas with increased soil concentrations (Khan et al, 2003).
Many people could be at risk of adverse health effect from consuming vegetables cultivated in contaminated soil. Some vegetables are capable of accumulating high levels of metals from the soil (Cobb et al., 2000).
Heavy metals such as Cd and Pb, have long been known as a major contamination problem, not only for working conditions but also for the environment (Merian, 2004). Although zinc and copper are essential trace elements which may also serve as plant nutrients, their undue presence in the environment can also be hazardous to man (WHO, 1999).
Pb and Cd have no essential biological function and are highly toxic to plants, animals and humans (Pendias, 2002). Cd is being relatively more toxic, in lower concentrations than Pb to plants (Pahlsson, 1999). Cadmium is recognized as an extremely significant pollutant due to its high toxicity and large solubility of its compounds in water.
Heavy metal toxicity can result in damaged or reduced mental and central nervous function, lower energy levels, and damage to blood composition, lungs, kidneys, liver and other vital organs (Maverick, L. 2000).
Consuming foods contaminated by Pb and Cd can seriously deplete body stores of Fe, Vitamin C and other essential nutrients, leading to impaired psycho-social faculties and disabilities associated with malnutrition (Nair, 2000).
Vegetables act as neutralizing agents for acidic substances formed during digestion. As human activities increases, especially with the application of modern technologies; pollution and contamination of the human food chain has become inevitable. (Williams S. 2000).
Heavy metal contamination in vegetables cannot be under estimated as these food stuffs are important components of human diets (Abraham, V., 1999).
High concentration of nitrate has adverse effect on environment, animals and humans (Cobb et al, 2000):
Environment: The high concentration of nitrate in water causes a phenomenon known as “Eutrophication” which means an excessive growth of the algae in water which consumes the oxygen gas dissolved in water causing the death of fishes in that water (Hambridge, 2003).
Animals: Especially in ruminant animals such as cows, sheep and goats. When ruminants consumes food with high nitrate level, the nitrate can be converted to nitrite, which causes both nitrate and nitrite accumulation in the rumen (Voronina, 1999).
As a result of the accumulation of both nitrate and nitrite in animal rumen, it causes acute and chronic symptoms which run as reduction in weight gain, reduction in milk production, low appetite, aborted breathing, blue colouring of mucus membrane, abortions and premature death of calves (Blom et al, 2005).
Humans: Nitrate itself is not toxic, however the conversion of nitrate to nitrite in human and animal bodies is very dangerous if it accumulates in high concentrations (Chandy K. T. 2000).
Nitrates naturally occur in plant based food. Plants typically uptake nitrogen from the environment and assimilate it is produce amines and amides. The nitrate contents in plants increases with increased doses of mineral nitrogen fertilization (Hullsek et al, 1999).
However, nitrate content in consumable plant organs is small and should not raise concern provided that the recommended fertilization and harvest terms of the original plants are observed (Galler, 2000).
The nitrates content are largely diversified and according to various sources (Chung et al, 2003). In the alimentary tract, nitrates can be reduced to nitrites and further take part in forming carcinogenic compounds.
A possibility exist that nitrate can react with amines or amides in the body to form nitrosamine which is known to cause cancer. Nitrate must be converted to nitrite before nitrosamine can be formed (MC Knight et al, 2002).
Vegetables are major sources of nitrate in human diet (Hadidi, 2001). It is estimated that they contribute about 85 percent and 14 – 43 percent of the daily dietary intakes of nitrate respectively in a number of societies (Cassens, 2002).
A number of factors influence nitrate accumulation in vegetable such as nitrate supply from the soil, genotype of plants and environment under which they grow such as light, intensity and photo period (Von der Boon et al, 2002).
Agriculture is considered the major source of nitrate in the environment. Fertilizers use is rapidly increased, causing excessive nitrogen loading to the environment (Cash et al, 2002).
Globally the use of fertilizers was increased from less than 14 million tones in 1950 to 265 million tones in 2006 (Kolbe, 2006).
On the other hand, the following could be occurring as a result of the accumulation of large close of nitrite for human health:
Methemoglobenemia: Nitrate is reduced to nitrite which combines with hemoglobin to form methemoglobenemia. Methemoglobin is a compound that cannot combine with oxygen from lungs to body tissues causing a condition known as “Blue Baby syndromes” or “methomoglobenemia” (Rudkowsko et al, 2001).
Carcinogenicity: Nitrate reacts in stomach with nitro sable compounds to form N – nitraso compounds (Bremier, 2002).
High levels of nitrate intake were also linked with the non. Hodgkin’s Lymphoma, bolder cancer, pancreatic cancerand stomach cancer (Anulso, 2000).
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