The negative interaction of metal ions is one of the major dietary factors that causes low bioavailability of these nutrients. Interactions of nutritional significance include sodium-potassium, calcium-magnesium, manganese-iron, iron-copper, and zinc-copper. These interactions reach potential importance when the first metal of each pair listed above is in excess and the other is at the lower limit of requirement. The trace element interaction of highest practical significance in human nutrition is the negative effect of excess zinc on copper bioavailability.