Fig: A sumptuous selection of food. Food is the major pathway of build-up of carcinogenic Cadmium in our bodies (Picture from flickr.com/creativecommons)
Food especially the high fatty type always draws people closer. However, there has been an increased interest in the cultivation and consumption of healthier foods in our age due to the prevalence of lifestyle diseases. A number of people have resorted to avoiding meaty diets in order to have healthier and more vibrant lifestyle. It is however becoming increasingly clear in the scientific community that some instances of lifestyle diseases such as cancer are as a result not of the kind of diet we have but the kind of soils the plants grow on.Plants take up nutrients from the soil for growth. However some other substances present in the soil mimic the behaviour of nutrients but have no biological significance and end up being absorbed by crops. One such kind of a substance is the rarely known Cadmium.
Cadmium is a metallic element found in the earth’s crust. Together with Lead, Mercury, Chromium and Arsenic (also known as the big five), it forms a group of highly notorious, persistent, top-notch and non-biodegradable pollutants generally found in the environment commonly referred to as Toxic Heavy Metals. Cadmium is rare but quite evenly distributed on the surface of the earth mostly associated with zinc or phosphate related rocks. It is therefore present in very small amounts in agricultural soils all over the world. Even though it is not as abundant as Iron or Potassium, it has no biological significance in the body which makes it a toxic element to humans. Some heavy metals like Lead and Mercury are in the same toxic category as Cadmium. However, what sets Cadmium in a class of its own is that it is readily absorbed by some agricultural crops. What is even more stunning is that these crops such as green leafy vegetables, potatoes, some cereals, rice and even tobacco are not affected by the accumulation of Cadmium in their tissues. This makes the element available for storage in plant parts which form sources of food for humans such as the roots and tubers, leaves and even the stem. Because of its accumulation in food crops, cadmium becomes very mobile in the food chain ending up in our diets and accumulating in our bodies since it has no use biologically. This build-up of cadmium can take place over a very long time before its toxic effects are noted. According to the Centre for Diseases Control, the reduction of Cadmium in the body to half its quantity may take place after 10-30 years if no additional Cadmium is taken up (which is rarely the case). Therefore a diet composed of food high in Cadmium levels may lead to its bio-accumulation in the body.
Cadmium has been classified adversely by several Cancer research institutes with the International Agency for Research on Cancer (IARC) which is a department of the WHO and The American Cancer Society categorizing it as a first class carcinogen. What this means is that there is a very high likelihood of developing either lung or prostate cancer as a result of accumulation of even small amounts of Cadmium in the body. Cadmium causes cancer by damaging the DNA or inhibiting its repair. The interference with the self repair process of DNA is known to cause mutations (DNA faults) which may lead to the over- expression of certain genes which are responsible for cell multiplication or the suppression of others responsible for healthy cell destruction. It also has been known to cause the release of very toxic substances also called Reactive Oxygen Species (ROS) as a result of DNA breakdown. Cadmium is also known to be a deadly kidney toxin. It inflicts injury to the kidney tissues which more often than not leads to kidney failure. It is also known to accumulate around the liver.
SOURCES OF CADMIUM IN OUR ENVIRONMENT
Phosphate fertilizers such as DAP and TSP are used in planting of agricultural crops. Phosphate fertilizers are produced from phosphate rocks called hydroxyappatite which usually contains trace levels of Cadmium. The removal of these trace amounts of Cadmium is very hard without compromising the standard of the fertilizers or may be too expensive. Therefore quite some amount Cadmium finds its way into the soils and by extension crops through fertilization.
Fig: Well fertilized corn field. Cadmium enters our food chain through phosphate fertilizers (Picture by flickr.com/creativecommons)
A substance known as Cadmium Stearate is used as a plastic stabilizer in the manufacture of plastics. This is a substance added to the plastic material to make it more resistance to temperature and pressure thereby boosting its durability. Such trace amounts of Cadmium can be released into foods stored in such kinds of plastics or even in the air when these plastics are burnt.
Several kind of industrial processes release Cadmium into the environment. Nickel/Cadmium battery plants may release effluents rich in Cadmium into the nearby sewers; Zinc ores might release traces of Cadmium into the environment during smelting processes; and some electroplating processes may also lead to the release of cadmium into the soil and water around the industries. Some industrial processes involving high temperature may also release traces of Cadmium Oxide into the atmosphere.
The amount of toxic heavy metals contained in waste material is considerably high. This is because these materials are not biodegradable thus end up being released from a source to an accumulator called a sink and on many occasions, the vicious cycle is continuous with no end in sight. Some of the important waste sources are such as incinerators, burning rubbish, sewage and industrial waste.
Fig: Wastewater. Cadmium and other toxic heavy metals are re-introduced into the environment through human and animal waste (picture through flickr.com/creativecommons)
Natural occurrences such as volcanoes can release some traces of heavy metals like Cadmium into the atmosphere and even onto soils. This is however less of an occurrence.
CADMIUM EXPOSURE ROUTES
Food perhaps is the most potent exposure route especially to an unsuspecting population. Crops grown on soils even with trace amounts of Cadmium lead to the accumulation of the element in the crops and later on in the food sources. Continuous consumption of such foods by an unsuspecting population will expose them to a build-up of the toxin leading to medical complications. Also, livestock which are exposed to pasture grown on cadmium contaminated soils may lead to the accumulation of the substance in their kidneys and livers leading to its transfer to the meat consumers up the food chain.
Burning rubbish can be one of the most subtle modes of exposure to Cadmium. Plastics which have been stabilized by Cadmium compounds can end up releasing cadmium in the atmosphere in the form of Cadmium Oxide which is known to be a very potent carcinogen when inhaled. Human and animal wastes are also very key sources of cadmium which is being excreted after being bio-accumulated in the body for a long time.
The tobacco leaves are known to be very conducive hosts for cadmium. This means that tobacco smokers face a double risk of Cadmium toxicity through smoking and even through food.
PRACTICAL STEPS TO REDUCE CADMIUM TOXICITY
Buying Vegetables from reputable sources
Since Cadmium accumulates more readily in leafy vegetables, it is essential to be careful when purchasing and consuming them. Vegetables grown near rubbish dumpsites and sewer lines have a greater risk of Cadmium accumulation. It is important to ensure that the vegetable sources one is consuming are from a hygienic environment in order to mitigate against heavy metal contamination.
Washing Vegetables thoroughly
It is important to wash vegetables thoroughly before cooking them in order to remove traces of Cadmium if possible. A good practice is washing the vegetables three times in clean water. Also ensure to wash one’s own hands during and after washing the vegetables.
Eating a balance diet
The dependence on one form of food might lead to over exposure to considerable Cadmium levels. Leafy vegetables, potatoes, some meats like beef and some cereals like maize have been known to accumulate considerably higher levels of Cadmium compared to other crops. In order to reduce the risk of exposure, it is essential to diversify one’s diet to include fruits, pulses etc. Also foods rich in vitamins E and C found in colored foods like fruits are known anti oxidants which reduce the toxicity of Cadmium in the body.
References and further reading
https://www.ncbi.nlm.nih.gov › NCBI › Literature › PubMed Central (PMC)
http://www.cfs.gov.hk › Programme Areas › Risk Assessment in Food Safety
http://www.cdc.gov › NIOSH Publications & Products › Publication Types