# Chemical Compositional Changes in Over-Oxidized Fish Oils
## 1 Introduction
The experimental conditions used to create the over-oxidized hoki liver oil involved continuous exposure to oxygen gas over a 30-day period under a fluorescent lamp at room temperature. This approach to create rancid oil does not realistically reflect even the poorest storage conditions of any fish oil retail product. Although the results did not apply to the human consumption of fish oil, statements made in the publication may have created unnecessary bias toward the perceived poor quality of fish oil products on the market, as well as dissuaded women from consuming good quality fish oil supplements during pregnancy. Furthermore, the authors assumed that the developmental disruption and effects on a mother's metabolism were caused by the oxidation of omega-3 fatty acids in the oil. However, fish oils also contain other fatty acids and lipids that are susceptible to oxidation and that could potentially contribute to the observed outcome. A major newspaper perpetuated the study's conclusion by stating, \"Giving pregnant rats the unoxidized fish oil did not increase mortality rates in their babies, indicating that the lethal effect on newborns came from the chemicals that omega-3 fatty acids break down into during oxidation\" [30]. This conclusion is premature due to the lack of detailed analysis of the composition of the over-oxidized oil. The objective of the present study was to evaluate the chemical compositional changes occurring during the over-oxidation of hoki liver oil under conditions that very closely replicate those employed in the study by [PERSON] et al. [28]. The results may also provide new insight into the formation of specific oxidation products that could have contributed to the observed developmental and maternal toxicity. In order to determine if the observations could be relevant to other fish oils, refined anchovy oil was also exposed to the same conditions. Anchovy oil is a much more commonly used omega-3 ingredient oil globally for finished products consumed by adults, including during pregnancy [31]. Furthermore, to learn if oil type-specific susceptibility to rancidity development and the associated chemical changes would be distinct under different oxidation conditions, both the hoki liver oil and the anchovy oil were also submitted to thermal oxidation in the absence of light [32]. This condition better reflects the oxidation conditions that may occur in poorly stored finished products, such as encapsulated or bottled fish oils that are further packaged and hence fully protected from light. The oxidative parameters of edible oils were measured to assess the temporal-chemical changes that occurred over the 30-day period, including PV, p-AV, TOTOX, acid value, oligomers, moisture content, EPA and DHA content, fatty acid composition, octopherol content, and induction time. This study also focused on understanding changes in the levels of specific classes of secondary oxidation products. The reason for this attention is because in the study by [PERSON] and colleagues, a low level of secondary oxidation, i.e., a p-AV of only 4.5, was reported. This value is well within even the strictest quality guidelines on the oxidative quality of edible oils. The oxidation of esterified EPA and DHA is typically accompanied with a substantial formation of secondary oxidation products. In contrast to the relatively high level of primary oxidation that was achieved after 30 days of accelerated oxidation (PV was 48.8 meq O\({}_{2}\)/kg oil), attaining such a low value of _para_-anisidine-reactive fatty aldehydes suggests that a large portion of volatile secondary oxidation products were lost to the incubation. Secondary oxidation products include fatty aldehydes and dialdehydes with relatively low boiling points, which originate from the chain cleavage of fatty acid peroxides formed in primary oxidation. Some of these volatiles contain conjugated aldehydes that are reactive with cellular proteins and can have cytotoxic effects [33; 34; 35].