Estimates of trace metal bioavailability to humans ingesting contaminated oysters

Abstract

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.

Introduction

Oyster farming is the primary aquaculture process in France with a production of approximately 130 000 tonnes/year. Thus France is the leading oyster producer in Europe, and among the first four oyster-producing countries in the world (Comité National de la conchyliculture, 2002). Oyster aquaculture represents a very important market which earns 460 millions Euros each year in France (IFREMER, 2002). The safety of food to the consumer is a major concern for our society. Correspondingly there are standards that are intended to ensure the safety of consumers of trace metals in shellfish, by laying down maximum permitted contents of potentially toxic elements therein. Food is considered as unfit for human consumption if it contains trace metal concentrations above these permitted standards. For example, the maximum permitted concentration of cadmium in oysters is 1 mg/kg fresh weight (European Communities, 2001). However, such standards are expressed as total concentrations of metals, whereas the bioavailability of the constituent metals and thus their potential transfer in the trophic web will vary according to the physico-chemical form (speciation) of accumulated storage in the oysters, and their potential breakdown during the digestive process. Until recently it has been particularly the metals present in the soluble fraction of food (prey) items which have been considered as bioavailable to the consumer, but some authors have also studied the bioavailability of metals present in insoluble fractions (Reinfelder and Fisher, 1994; Wang and Fisher, 1996; Wallace and Lopez, 1996, Wallace and Lopez, 1997; Wallace et al., 1998; Wallace and Luoma, 2003). Ettajani and Amiard (1995) showed that, in vitro, acid or enzymatic attacks on organic matter play a dominant role in the release of copper, cadmium and silver. This in vitro process is a model of the process of human digestion, brought about by enzymes (pepsin, chymotrypsin, lipase, trypsin, etc.) combined with various pHs (Ettajani and Amiard, 1995). In the human stomach, the pH of the secretions is lower than 1 but according to Rieutort (1999), after dilution with foods and saliva the pH of the bolus reaches between 2 and 4. This acid pH is likely to be a cause of the mobilisation of metals from the insoluble fraction of food, thereby increasing their bioavailability to the consumer.

The aim of this study was to provide a realistic estimate of the bioavailability, during the gastric digestion, of four trace metals (Ag, Cd, Cu and Zn) in oysters, so that the food standards to be applied to oysters can be set to not only protect consumers but also to avoid unnecessary penalisation of oyster companies. All of these four trace metals have the potential to be toxic above threshold availability, even though copper and zinc are essential to life in low concentrations.