Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) have been found to stimulate protein synthesis with little information regarding their effects on protein breakdown. Furthermore whether there are distinct effects of EPA and DHA remains to be established. The aim of the current study was to determine the distinct effects of EPA and DHA on protein synthesis, protein breakdown and signalling pathways in C2C12 myotubes. Fully differentiated C2C12 cells were incubated for 24 h with 01% ethanol (control), 50 p,M EPA or 50 mu M DHA prior to experimentation. After serum (4 h) and amino acid (1 h) starvation cells were stimulated with 2 mM L-leucine and protein synthesis measured using H-3-labelled phenylalanine. Protein breakdown was measured using 3H-labelled phenylalanine and signalling pathways (Akt, mTOR, p70S6k, 4EBP1, rps6 and FOX03a) via Western blots. Data revealed that after incubation with EPA protein synthesis was 25% greater (P < 0.05) compared to control cells, with no effect of DHA. Protein breakdown was 22% (P < 0.05) lower, compared to control cells, after incubation with EPA, with no effect of DHA. Analysis of signalling pathways revealed that both EPA and DHA incubation increased (P < 0.05) p70s6k phosphorylation, EPA increased (P < 0.05) FOX03a phosphorylation, with no alteration in other signalling proteins. The current study has demonstrated distinct effects of EPA and DHA on protein metabolism with EPA showing a greater ability to result in skeletal muscle protein accretion. (C) 2013 Elsevier Inc. All rights reserved.