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Iron is an essential constituent of haemoglobin, cytochrome, and other components of respiratory enzyme systems. It constitutes approximately 4g of average adult body weight.
It is the most abundant metal element on earth, yet iron deficiency is the most common form of anaemia throughout the world.
The apparent paradox occurs because iron is usually oxidised - in the ferric form - which is insoluble and so physiologically unavailable. However, soluble ferrous iron is highly toxic. Iron binding proteins such as haemoglobin and ferritin render iron soluble but safe.