Fishing for answers: is oxidation of fish oil supplements a problem?

Fish oils, rich in n-3 PUFA, have become one of the most popular dietary supplements worldwide with millions of regular consumers (1). Sales in the USA alone exceed US$ 1 billion annually (2). There is a broad range of benefits claimed for n-3 fish oils including: prevention of CVD (3) , reduced cognitive decline (4) , and the improved management of inflammatory diseases (arthritis , inflammatory bowel disease and asthma) (5). However, a series of recent studies has not demonstrated significant benefits , particularly regarding the secondary prevention of CVD (6,7). n-3 PUFA are highly prone to oxidative degradation, making fish oils one of the most labile supplements sold to consumers. Recently in the Journal of Nutritional Science, Jackowski et al. evaluated primary and secondary oxidation in all of the n-3 fish oils available over the counter in retail stores in Canada (8). A total of 171 supplements from forty-nine brands were assessed, with 50 % exceeding voluntary limits for at least one measure of oxidation, and 39 % exceeding the international voluntary safety recommendations for total oxidation (TOTOX) value. These findings are not unique to Canada. In the USA, 27 % of products tested were found to have more than twice the recommended levels of lipid peroxides (9) , while in South Africa (10) and New Zealand (11) more than 80 % of supplements tested exceeded recommended levels. The oxidation of n-3 PUFA is complex, and the degree and rate of oxidation of fish oil are influenced by many factors, including fatty acid composition, exposure to O 2 and light, temperature, antioxidant content, and the presence of water and heavy metals (12). The initial stage of oxidation of fish oils leads to increased levels of hydroperoxides, which decompose into a variety of radicals (12). These react with unoxidised PUFA to form additional hydroperoxides, while also breaking down to form a wide range of possible secondary oxidation products such as volatile ketones and alcohols. These are strongly linked to the rancid smells and off flavours (12,13). While oxidation leads to a complex array of primary and secondary oxidation products, the degree of oxidation can be characterised by just two industry-standard assays. The peroxide value (PV) provides a quantitative measure of hydro-peroxide levels. The most common method to estimate secondary oxidation is the calculation of the anisidine value (AV), which provides a measurement of aldehydic compounds (predominately 2-alkenals and 2,4-alkadienals). By …