Risks and benefits of copper in light of new insights of copper homeostasis

Copper is an essential micronutrient involved in a variety of biological processes indispensable to sustain life. At the same time, it can be toxic when present in excess, the most noticeable chronic effect being liver damage. Potent, efficient regulatory mechanisms control copper absorption in the digestive tract and copper biliary excretion; absorption ranges between 12 and 60% in humans, depending on Cu intake, presence of other factors in the diet that may promote or inhibit its absorption and on the copper status of the individual. Current evidence suggests that copper deficiency may be more prevalent than previously thought, while copper toxicity is uncommon under customary daily life conditions. Menkes syndrome and Wilson disease are genetic conditions associated with severe copper deficiency and severe copper toxicity, respectively. Effects of milder degrees of copper deficiency and excess copper exposure are not well described, mainly due to lack of sensitive and specific indicators; serum copper concentration and ceruloplasmin are the most frequently used indicators, but they only detect rather intense changes of copper status. Of the many proteins assessed as potential markers of copper status the chaperone of Zn–Cu superoxide dismutase (CCS1) has yielded promising results; data on its performance under different conditions are needed to confirm its use as an indicator of early copper deficiency. Defining copper requirements and upper safe limits of consumption (UL) is a complex process since there are adverse health consequences from both copper deficiency and copper excess (U shape curve). The regulatory framework for risk assessment of essential trace elements introduced by the International Programme on Chemical Safety (IPCS) has proposed a homeostatic model to determine the Adequate Range of Oral Intake (AROI) of essential trace elements; the nadir of the resulting U shape curve serves to define the AROI. At this range of intake physiological mechanisms allow for normal homeostasis and basically, there are no detectable adverse effects. At present, Recommended Dietary Intakes (DRIs) and Adequate Intakes (AIs) are used to recommend copper intakes at different ages and life situations. Evidence obtained in humans and non-human primates presented here suggest that current copper UL should be re evaluated. Developing the scientific basis for a copper UL and evaluating the relevance of copper deficiency globally are future key challenges for copper researchers.