The secret story of fish: decreasing nutritional value due to pollution?

Fish, especially fatty fish, have long been viewed as a healthy dietary component because of their unique content of longchain n-3 PUFA (n-3 fatty acids). An observation in 852 male residents of Zutphen, The Netherlands, aged 40–59 years in 1960 indicated that fish intake was inversely associated with the incidence of CHD over 20 years of follow-up. On the other hand, fish may also contain diverse environmental pollutants such as heavy metals and persistent organic pollutants (POP), including organochlorine pesticides, polychlorinated biphenyls (PCB), dioxins, polybrominated diphenylether (PBDE) and perfluorinated compounds (PFCO). Therefore, different studies evaluating potential risks v. benefits of fish consumption, especially with respect to CVD, have been performed. Although various results have been reported, it has generally been suggested that the benefits of fish eating outweigh the risks. Compared with CVD, the situation for diabetes has remained largely unexplored. Recently, several prospective studies have documented that the consumption of fish was associated with a higher later occurrence of type 2 diabetes, a disease strongly linked to heart disease and stroke, yet that has a much distinct pathophysiology linked to glucose dysregulation and microvascular deterioration. In this issue of the British Journal of Nutrition, a meta-analysis performed by Zhou et al. summarises these and other studies, concluding that fish and n-3 fatty acid consumption is associated with a significantly increased risk of type 2 diabetes. In two other meta-analyses of the same topic and of largely the same data, it has been concluded that there was no benefit from fish intake in terms of reduced diabetes risk. How can we explain these findings? Have the health effects associated with fish consumption been overestimated? At an experimental level, several pieces of evidence support the idea that the health effects of fish consumption vary depending on the presence or absence of POP. Rats exposed to contaminated salmon oil (containing background levels of POP) developed metabolic complications linked to type 2 diabetes, whereas animals exposed to decontaminated salmon oil (treated to achieve very low levels of POP) did not show such disturbances. Furthermore, mice fed commercially available farmed salmon fillet with common POP levels were found to develop insulin resistance, glucose intolerance, visceral obesity, fatty liver and chronic low-grade inflammation, in contrast to mice fed farmed salmon fillet containing lower levels of POP, which showed a better metabolic profile. In addition, consumption of salmon protein hydrolysate containing less than 0·2 % of lipids, and therefore very low concentrations of POP, was found to protect rats against insulin resistance induced by a high-fat diet containing lard and ‘corn oil’. Taken together, these findings emphasise that background levels of POP, which many people consider to be at safe levels, can completely counteract the potential benefits of n-3 fatty acids and other nutrients present in fish, in particular leading to the serious metabolic features which often precede type 2 diabetes. Thus, these animal feeding studies are consistent with the recent human prospective and cross-sectional studies showing an association between type 2 diabetes and POP. Previously, Kaushik et al. reported that, whereas fish and n-3 fatty acid intake increases the risk of diabetes, the consumption of n-3 fatty acid supplements did not. Interestingly, the oil found in most n-3 fatty acid supplements differs considerably from the oil found in fatty fish, in that most lipophilic pollutants such as POP have been extracted through decontamination processes. On the other hand, the oil present in fatty fish has not been decontaminated, and often contains many POP because these pollutants are omnipresent in aquatic environments, making fatty fish one of the most important sources of human exposure to POP. Furthermore, the presence of POP in fish, compared with other food products, is still poorly regulated. In the European Union, the levels of organochlorine pesticides, PCB and PBDE in fish and seafood are, for instance, still unregulated. Thus, the exposure to POP through fatty fish intake could have contributed to an enhanced risk of diabetes. These findings also pinpoint that the common practice to extrapolate the results obtained with n-3-derived supplements to fatty fish, or seafood in general, has probably led to a serious misinterpretation and should be done with extreme caution. The concentrations and types of POP mixtures may vary substantially in fatty fish depending on the food consumed, the time and the geographic area of the fish. Similarly, although n-3-derived supplements may contain lower levels of POP than fatty fish, the levels of POP in these supplements may fluctuate considerably due to differing methods used by the industry to refine the oil, including activated carbon adsorption, short-path distillation and deodorisation. This varying quality of fish and n-3-derived supplements has probably contributed to the different findings about the British Journal of Nutrition (2012), 108, 397–399 doi:10.1017/S0007114512002048 q The Authors 2012