<p> <b>Background:</b> Age-related sarcopenia leads to frailty, physical disability and loss of independence. Although exercise is an effective strategy to counteract the prevailing loss of muscle mass, older adults exhibit blunted anabolic responses, and are often unable to adopt an active lifestyle due to comorbidities associated with aging. Long chain polyunsaturated fatty acids (n-3 PUFA), eicosapentaenoic (EPA) and docosahexaenoic (DHA) acid, are non-pharmaceutical nutrients which have surfaced for their potential anabolic properties on skeletal muscle and may be particularly beneficial in the context of sarcopenia. </p><p> <b>Objective:</b> First, to determine if EPA and DHA increase muscle protein synthesis in older adults. Second, to determine if n-3 PUFA increase the anabolic response to an acute resistance exercise stimulus in older adults. Third, to assess if their effect is mediated through improved mitochondrial function, which is known to be impaired with aging. </p><p> <b>Methods:</b> Twelve old, sedentary, healthy women and men (65-85 years) were given 3.9 grams/day purified EPA/DHA for 4 months. 12 young adults (18-35 years) were included as a comparison group for baseline measurements. Muscle protein fractional synthesis rate (FSR) was measured before and after treatment for mixed muscle, and subcellular fractions of myofibrillar, mitochondrial and sarcoplasmic proteins. We infused a stable isotope tracer of [ring-¹³C₆] phenylalanine and monitored incorporation of the amino acid into muscle proteins, at the fasting, post absorptive state, and 16 hours following an acute bout of unaccustomed resistance exercise, using mass spectrometry. Muscle mitochondrial function was assessed <i>ex vivo </i> from skeletal muscle biopsies. Further mechanistic information was generated through large scale and individual mRNA gene expression, inflammatory markers, and protein phosphorylation signaling of the anabolic pathway. </p><p> <b>Results:</b> Protein synthesis was similar between age groups at baseline and post exercise, despite the robust decline in mRNA gene expression with aging. EPA/DHA supplementation increased total lean mass, and increased mitochondrial and sarcoplasmic FSR at baseline. Following acute exercise, mixed muscle and subcellular FSR did not change significantly, but participants were segregated into responders and non-responders. EPA/DHA further potentiated the anabolic response of mitochondrial FSR to levels greater than that in the young. There was no improvement in mitochondrial oxidative capacity and efficiency, but there was a significant decrease in ROS emissions. </p><p> <b>Conclusion:</b> In healthy older adults, EPA/DHA exhibited significant anabolic effect in baseline skeletal muscle mitochondrial and sarcoplasmic FSR, which was dissociated from mitochondrial oxidative capacity. The anabolic response to exercise was variable between responders and non-responders where some individuals presented with marked increase in mixed muscle and subcellular FSR. This observation sets the ground for identifying the phenotypic traits of the elderly who are likely to benefit from the therapeutic use of n-3 PUFA to combat sarcopenia of aging.</p>