Health effects of Cadmium – An Overview



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Piyasiri Amilasith Yapa


Cadmium is one of the elements implicated with chronic kidney disease or CKD by scientists’ world over for some years. Cadmium in its purest form is a soft, silver white metal that is found naturally in the earth’s crust. The most common forms of cadmium found in the environment exist in combinations with other elements such as oxygen (cadmium oxide), chlorine (cadmium chloride), and sulfur (cadmium sulfide). Cadmium doesn’t have a taste or smell.


Cadmium is usually obtained as a byproduct of smelting zinc, copper and lead ores. It is used in electroplating, in plastic stabilizers, in pigments, in solder and nickel-cadmium batteries.


Cadmium is also present in phosphate fertilizer as an impurity. It can enter the food chain and then humans through plants. Phosphate fertilizer is known to contain 5-100 mg Cd kg -1 and this would increase the soil and plant cadmium levels when the fertilizers are used. In Sri Lanka high concentrations of cadmium in rice has been attributed to excessive usage of low quality fertilizer distributed through the subsidy scheme during the last several years. High cadmium levels have been described as one of the reasons for spread of CKD in NCP of Sri Lanka.


In has been estimated in the UK, the average addition of cadmium to crop lands is 0.4mg m-2 a-1 ( air deposition, 41%; phosphate fertilizers, 54%; sewage sludge, 5% ). When sewage sludge is used, over 90% of the added cadmium can come from the sludge (O’Niell, 1998).


There are two ways by which cadmium can enter the body i. e. ingestion and inhalation. One can get exposed to cadmium by breathing cadmium contaminated air. Exposure can also occur by eating foods containing low levels of cadmium. For most of us, the most common source of exposure to cadmium is mainly through eating food containing cadmium since it can enter the food chain. Plants absorb or take up cadmium from soil and fish we eat absorb cadmium from the water they live in.


Cigarette smoke is another source of exposure. Since cadmium is not biodegradable, any cadmium that is naturally present in soil, or is added as contaminated water or added to soil as fertilizer accumulates and is absorbed by plants. Tobacco is a very efficient in absorbing cadmium from soil and the cadmium levels in tobacco plants can be quite high. Each cigarette smoked exposes smokers to1.7 micrograms of cadmium. Most people who smoke are known to have about twice as much cadmium in their bodies as nonsmokers.


Some of the food that we consume also may contain a range of cadmium levels that can be harmful to humans if the foods are eaten in large proportions. One example of this is the high levels of cadmium in bivalve mollusks and crustaceans. These animals absorb high levels of cadmium from the water they live in. In one study carried out among people associated with the oyster industry it was observed that people who consumed more than six oysters per week, were above the recommended WHO level of 70 micrograms of cadmium per day. In another study analyzing a mixed diet and a high shellfish diet in women, it was found an increase of 63% in blood cadmium and an increase of 24% in urinary cadmium in women with a high shellfish diet (Satarug et al, 2010).


Oil seeds, which include sunflower seeds, peanuts, flaxseed and linseed are also known to collect cadmium from soil. Sunflower seeds contain cadmium levels of 0.2 to 2.5 mg/kg of seeds consumed. It was found in a study that the average sunflower seed consumer who consumes more than 28 grams of sunflower seeds consumes 36 micrograms of cadmium per day; and showed negative effects on the kidney, which means that ‘cadmium in sunflower kernels possess a high nephrotoxic potential" (Satarug et al, 2010).


It is reported that 80% of dietary intake of cadmium comes from cereals and vegetables. Among vegetables beet root is reported to accumulate cadmium. Ingestion of cadmium through consumption of rice, staple food of many nations is common, where we Sri Lankans have to be concerned.


Japanese Itai – Itai disease


The toxic effect of cadmium received widespread attention as a result of some Japanese developing a disease condition known as Itai-Itai, during world wars 1 and 2. The name came from the severe pain developed by the sufferers as lumbago-type pains progressed to become severe bone damage with multiple fractures of the softened bones. Death was attributed to kidney failure. The victims were mainly post-menopausal women suffering from malnutrition, low vitamin D intake and calcium deficiency.


Itai-Itai was a result of sudden high intake of cadmium. They were exposed to nearly 600 micrograms of cadmium per day which is much higher than the WHO limit of 70 micrograms per day. This was due to cadmium being leaked from some zinc-lead mining and smelting factories into air and also into streams used for fishing and cultivating rice. Rice grown in paddy fields contained up to 3.4 mg Cd kg-1 .


Renal failure


It is now well known that chronic exposure to cadmium can cause both renal proximal tubular damage and decline in glomerular filtration rate (GFR) in humans. This has been confirmed in experimental models (Ferraro et al, 2010). After pulmonary and/or gastrointestinal absorption, cadmium binds to serum albumin and accumulates in the liver, where it is complexed to a metal binding protein with a high affinity for cadmium, metallothionein-1.


The Cd-metallothionein-1 complex then reaches the kidney where it is filtered and accumulates in the proximal tubule, whose cell possesses transporters for free and bound forms of cadmium and interferes with tubular function.


Cadmium nephropathy is characterized by low molecular weight (LMW) proteinurea due to diminished intra-renal uptake and catabolism of filtered protein. In cadmium nephropathy, proximal tubular dysfunction persists until renal failure supervenes. So are the biochemical events of ultimate renal failure.


Although cadmium nephropathy has been observed in humans exposed to high levels of cadmium, recent data suggest that relatively low levels of cadmium too can cause tubular dysfunction in people living in polluted industrial areas.


Skeletal effects of cadmium intake


Cadmium plays a major role in skeletal health of humans, especially in women over 50 years of age. With exposure to cadmium the chances for demineralization of bones increases which leads to frailty and a great chance of injuries due to broken bones as the population gets older.


High doses of cadmium lead to Itai-Itai disease which is characterized by osteoporosis, osteomalacia (softening of bones) along with renal dysfunction (Satarug et al, 2010). Osteoporosis is a condition that results from having low bone mass density, and it is accompanied by the deterioration of the skeleton that results in an increased possibility of bone fractures (Akesson et al, 2006).


Risk factors of osteoporosis include old age, females, alcohol consumption, smoking, family history, physical activity, low body weight and low calcium and vitamin D intake. Constant exposure to cadmium is the latest addition to this list of risk factors.


Hypertension and cadmium


Hypertension is a condition in which patients blood pressures are raised due to a variety of factors including raised cholesterol levels, stress, and smoking as well as increased cadmium intake (Gallagher & Meliker, 2010). Cadmium is linked to hypertension through constant smoking of cigarettes. This has been confirmed by a 5-year study by Tellez-Plaza et al in 2008 with 14,000 participants. A meta-analysis conducted by Gallaghar and Meliker in 2010 also revealed a positive relationship between cadmium, blood pressure and hypertension.


Cadmium and cancer


Cadmium has been classified as a cancer causing agent by WHO I 1993. Several workers have linked cadmium exposure to pulmonary, renal, breast and prostate cancers (Satarug and Moore, 2004). A 15 year Belgian study has clearly shown the cancerous effect of cadmium on lung cancer. Smoking and inhaling cadmium fumes in smelting factories play a major role here.


Cadmium also has been associated with renal cancer. It is well known that renal system is one of the most – if not the most- affected system when cadmium is introduced into the body.


Breast cancer has also been linked with high cadmium exposures in women. Smoking has been shown to be the major source of cadmium exposure.


Cadmium has been linked to prostate and colorectal cancers mainly due to the effect it has on vitamin D intake and synthesis. It is known that vitamin D has a protective role in prostate cancer but the ingestion or inhalation of cadmium alters vitamin D synthesis, increasing the risk of both prostate and colorectal cancers.


(The writer is a Fellow of National Academy of Sciences, Sri Lanka)


 
 
 
 
 
 
 
 
 
 
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