Uncoupling EPA and DHA in Fish Nutrition: Dietary Demand is Limited in Atlantic Salmon and Effectively Met by DHA Alone

Abstract

Due to the scarcity of marine fish oil resources, the aquaculture industry is developing more efficient strategies for the utilization of dietary omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA). A better understanding of how fish utilize EPA and DHA, typically provided by fish oil, is needed. However, EPA and DHA have different physiological functions, may be metabolized and incorporated into tissues differently, and may vary in terms of their importance in meeting the fatty acid requirements of fish. To address these questions, Atlantic salmon were fed experimental diets containing, as the sole added dietary lipid source, fish oil (positive control), tallow (negative control), or tallow supplemented with EPA, DHA, or both fatty acids to ~50 or 100% of their respective levels in the positive control diet. Following 14 weeks of feeding, the negative control diet yielded optimum growth performance. Though surprising, these results support the notion that Atlantic salmon requirements for n-3 LC-PUFA are quite low. EPA was largely β-oxidized and inefficiently deposited in tissues, and increasing dietary levels were associated with potential negative effects on growth. Conversely, DHA was completely spared from catabolism and very efficiently deposited into flesh. EPA bioconversion to DHA was largely influenced by substrate availability, with the presence of preformed DHA having little inhibitory effect. These results clearly indicate EPA and DHA are metabolized differently by Atlantic salmon, and suggest that the n-3 LC-PUFA dietary requirements of Atlantic salmon may be lower than reported and different, if originating primarily from EPA or DHA.

Are we what we eat? Changes to the feed fatty acid composition of farmed salmon and its effects through the food chain

“…DHA in all tissues and EPA in the liver appear to be relatively spared from β-oxidation compared with other FAs (Torstensen et al, 2004a). This difference between EPA and DHA could reflect their respective biological roles, and the extent to which salmon has a specific requirement for EPA has been questioned (Emery et al, 2016;Bou et al, 2017a). Dietary DHA seems to be selectively retained regardless of the feed concentration, whereas EPA is only retained if the dietary concentration is low (Bell and Waagbø, 2008).…”

Thoughts for the Future of Aquaculture Nutrition: Realigning Perspectives to Reflect Contemporary Issues Related to Judicious Use of Marine Resources in Aquafeeds

“…; Emery et al. ). Likewise, providing crustaceans with the correct balance of n‐3 and n‐6 C 18 PUFAs, eicosapentaenoic acid (EPA; 20:5[n‐3], where the number to the left of the colon is the number of carbon atoms, the number to the right of the colon is the number of double bonds, and the number after the hyphen indicates the position of the first double bond from the methyl end), and docosahexaenoic acid (DHA; 22:6[n‐3]) reduces fatty acid requirements, improves utilization of n‐3 LC‐PUFAs, and can yield growth beyond that normally achieved when FO is the primary or only dietary lipid source (Glencross et al.…”

“…; Emery et al. ). These oils present a series of exciting opportunities for the sustainable expansion of the aquaculture sector but also highlight a partial knowledge gap: the dearth of research addressing individual fatty acid requirements.…”

Growth Performance of Atlantic Salmon Smolts Fed Diets Containing Heterotrophic Algal Biomass as Replacement of Fish Oil

“…; Emery et al. ). This points to a potential compensatory mechanism in fish that can retro‐convert DHA to EPA, which has been reported for various fish species, including the Malabar Grouper Epinephelus malabaricus (Wu et al.…”

“…), and Atlantic Salmon (Emery et al. ). A homeostatic effect was observed for EPA in which Atlantic Salmon appeared to maintain EPA levels above 2% in fish fillets and may point to the previously reported retro‐conversion of DHA into EPA.…”