In recent years, studies had shown that omega-3 (n-3 PUFA) supplementation could have been associated with increased rates of muscle protein synthesis, which might have contributed to greater gains in muscle mass and strength over time. However, unlike studies with the elderly, findings in adults had been contradictory. While greater gains in muscle mass and strength had been observed in response to supplementation in healthy adults in some studies, others had not demonstrated such effects. Thus, it was considered possible that, in conditions where protein synthesis was maximized (i.e., healthy adults engaged in physical exercise and consuming adequate protein), n-3 PUFA supplementation had no additive effect on muscle hypertrophy. Yet, no studies had been found in the literature that tested the effectiveness of n-3 PUFA supplementation in promoting muscle strength and mass gains under these conditions. The aim of the study had been to evaluate the effectiveness of n-3 PUFA supplementation on muscle mass and strength gain in healthy adults undergoing strength training with adequate protein intake. Therefore, a clinical, double-blind, randomized, placebo-controlled study had been conducted with 40 male subjects aged 18 to 40 years. Participants had been divided into two groups in a 1:1 ratio: Omega 3 (5g/day of fish oil) or Placebo (5g/day of sunflower oil). The protocol had lasted 14 weeks, during which all participants had undergone lower limb strength training twice a week. Before and after the intervention, participants had been evaluated for maximal strength (1-RM leg press), body composition (via BodPod), and the cross-sectional area of the rectus femoris and vastus lateralis muscles (via ultrasound). In addition, blood samples had been collected to assess fatty acid incorporation into the blood, and muscle biopsies had been performed to analyze muscle fiber cross-sectional area (fCSA), activation of protein synthesis pathways, and fatty acid incorporation into myocytes (via lipidomic analysis). Furthermore, food intake had been monitored throughout the protocol using 24-hour dietary recalls. REPRESENTATION OF DATASET: This dataset, presented in a spreadsheet, comprehends data from both intervention groups (Omega3 and Placebo), in both study timepoints (pre and post intervention protocol), and includes body composition values - measured via BodPod, maximal strength values (kg) - measured via 1-RM leg press test, cross-sectional area of the rectus femoris and vastus lateralis muscles - measured via ultrasound, muscle fiber cross-sectional area (fCSA) - measured via immunofluorescence protocol, activation of protein synthesis pathways - mTOR, p70s6k and 4EBP, total and phosforilated, and fatty acid incorporation into myocytes (via lipidomic analysis).