Estimates of trace metal bioavailability to humans ingesting contaminated oysters

Oysters, as very popular food items in France, are subject to rules concerning the maximum acceptable contents of trace metals in foods. The food standards for the quantities of metals permitted are always based on total metal concentrations, and not on the metal concentrations that are potentially bioavailable to the consumer (man). In order to estimate the percentages of accumulated trace metals (i.e. Ag, Cd, Cu and Zn) that are potentially bioavailable during consumption, we have used a simple chemical digestion simulation on the insoluble fraction of oysters. These quantities have been added to the soluble fraction, assuming that metals in this fraction are completely bioavailable. Our experiments were conducted on oysters Crassostrea gigas sampled from five sites on the French Atlantic coast and on oysters Ostrea edulis sampled from Restronguet Creek in the United Kingdom. These sites are characterised by various degrees of metal contamination. This study has allowed us to gain a better estimate of the real concentrations of metals bioavailable to the consumer. Only a part of the total metal present is bioavailable: 36-68% for silver, 44-75% for cadmium, 26-80% for copper and 50-80% for zinc. These new estimates have the potential to contribute to any re-evaluation of the food standards for metals.