Évaluation des facteurs de modulation de l’exposition du consommateur de poissons et de mammifères marins au méthylmercure : utilisation d’approches in vitro, in vivo et probabiliste

Fish flesh and marine mammals is an important source of proteins and nutrients, such as selenium (Se), vitamin E and long chain polyunsaturated fatty acids. However, flesh of those animals may bioaccumulate the organic form of mercury (Hg), methylmercury (MeHg). This contaminant has been the subject of various epidemiological studies, namely because of its neurotoxicity through in utero exposure, closely related to highly MeHg contaminated fish consumption. In response to this toxicological risk, health authorities have set fish consumption guidelines in order to protect the population. Still, those guidelines present limits that are related to premises and omissions of the determinist equation used by Health Canada in order to assess the exposure to MeHg: (1) it considers that 100% of Hg in fish flesh is MeHg, (2) it supposes that MeHg is homogeneously distributed within fish flesh, (3) it takes for granted that 100% of MeHg will be absorbed by the consumer and (4) by using a determinist approach, they omit the potential uncertainty and intra-population variability in the data. The aim of this thesis was to address these limits, in order to better understand the exposure of MeHg for consumers. We first assessed the distribution of MeHg, Se (MeHg antagonist) and arsenic (As, Se antagonist) within fish musculature, as a function of its biomolecule composition (proteins and lipids). Our results demonstrated that the concomitant presence of white and red muscle induces a large gradient of protein and lipid within the muscular apparatus of the same individual. This in turn causes on average a variation by 2.2-fold regarding MeHg, Se and As bioaccumulation, which are distributed according to their biochemical affinity. Those results confirmed that MeHg can distribute heterogeneously within fish muscle, which could lead to an under- or overestimation of MeHg exposure for consumers, as function of the part of the fish consumed. Subsequently, we focused on the hypothesis stipulating that 100% of MeHg is absorbed by the consumer. A method used to assess the fraction of MeHg that would be available to be further absorbed by the gut wall consists of measuring the bioaccessibility, i.e. the soluble fraction of MeHg, using an in vitro digestion model. Several studies that assessed the bioaccessibility of cooked fish flesh observed a decreased of MeHg solubility, that would potentially diminish its intestinal absorption. However, those results have not been yet validated in vivo. To that end, we conducted an in vivo experience using the pig model and, in parallel, we used an in vitro digestion model. According to the pig’s blood profile, the oral bioavailability of MeHg from cooked tuna flesh is not less bioavailable than the MeHg from the raw tuna. Contrasting results have been found with the in vitro model, with a decrease of MeHg bioaccessibility observed when fish flesh is cooked. Our results demonstrated that in vitro digestion models are not optimized to be directly used in MeHg exposure calculus as recently proposed in the literature. Finally, we tested the impact of adding variables to the deterministic equation currently used by Health Canada to assess consumer exposure to MeHg, by conducting a probabilistic risk assessment. We considered (1) the proportion of Hg that is MeHg, (2) the MeHg bioaccessibility and (3) the increased of MeHg level after the cooking of fish flesh due to moisture loss. Our results showed that each individual variable significantly increases or decreases the calculated exposure. Thereby, it highlights the sensitivity of the equation used to assess the exposure of MeHg. It strongly suggests that more research is needed to improve Hg exposure calculation to avoid underestimating the potential health risks of MeHg exposure. This thesis presents important results regarding the exposure of MeHg through fish and marine mammals’ consumption. This thesis shows that the recommendations are difficult to generalize since the flesh of fish and mammals has different properties, depending on the animal species considered. The conclusions of this thesis demonstrate that the premises and omissions of the deterministic equation used by Health Canada in the assessment of exposure to Hg should be better investigated by the scientific sphere, especially in the case of marine mammals.

Keyphrases: arsenic, Methylmercury, Fish, Muscle type, Bioaccessibility, Oral bioavailability, Probabilistic risk assessment, Country food