Protection Against Low Levels of Cadmium
Cadmium is acutely toxic: The lethal dose is about 1 g. Humans are protected against chronic exposure to low levels of cadmium by the presence of the sulfur-rich protein metallothionein, the usual function of which is the regulation of zinc metabolism. Because it has many sulfhydryl groups, metallothionein can complex almost all ingested Cd2+; the complex is subsequently eliminated in the urine. If the amount of cadmium absorbed by the body exceeds the capacity of metallothionein to complex it, the metal is stored mainly in the liver and kidneys. Indeed, there is evidence that chronic exposure to cadmium eventually leads to an increased chance of acquiring kidney disease.
The average cadmium burden in humans is increasing. Although cadmium is not biomagnified, it is a cumulative poison since, if not eliminated quickly (by metallothionein, as discussed above), its lifetime in the body is several decades. The geographic areas at greatest risk from cadmium exposure are Japan and central Europe; in both regions, the pollution of the soil by cadmium is particularly high due to contamination from industrial operations. Rice grown in many areas of Japan is often contaminated with rather high cadmium levels. As a consequence, the dietary intake of cadmium by residents of Japan is substantially greater than for peoples of other developed countries. Indeed, in Japan the average daily amount of ingested cadmium is beginning to approach the maximum level recommended by health authorities, although this limit has a large built-in safety factor relative to levels at which health effects would occur.
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