In humans, evidence regarding the effects of (omega-3) n-3 supplementation on whole body resting metabolic rate (RMR) and fatty acid (FA) oxidation is limited and controversial. In the first study of this thesis, supplementing with a high dose of fish oil (FO) containing 2 g/d eicosapentaenoic acid (EPA) and 1 g/d docosahexaenoic acid (DHA) for 12 weeks, in healthy young males did not affect RMR and substrate oxidation. However, FA oxidation was increased (36%) with a concomitant decrease (34%) in carbohydrate (CHO) oxidation, during the winter season, independent of supplementation. These data demonstrate that RMR and substrate oxidation were unaffected by n-3 supplementation in healthy young males, and that substrate utilization may be affected by changes in external temperature. The second study of the present thesis investigated whether supplementing with purified EPA or DHA would influence whole body RMR and substrate oxidation, in healthy young females and males. Supplementing with olive oil (OO), EPA or DHA had no effect on RMR and substrate oxidation, in males. However, in females, supplementing with DHA decreased (-7%) RMR with no effect on substrate oxidation, whereas OO and EPA, had no effect on these measurements. These data confirmed previous studies reporting small effects of n-3s in RMR and substrate oxidation in females and males. A previous study, in healthy older females, reported a significant increase in resting oxygen consumption (14%) and fat oxidation (19%), after a 12-week period of FO supplementation. Therefore, we examined the effects of FO on RMR, substrate oxidation and skeletal muscle Na+/K+ ATPase (NKA) and sarcoplasmic reticulum Ca2+ ATPase (SERCA) activity in healthy older adults. There was no effect on RMR, substrate oxidation, and skeletal muscle NKA and SERCA activity and protein content, after a 12-week supplementation period with OO or FO. However, there was a decrease in NKA pump sensitivity to Na+, after FO supplementation. These data suggest, that in healthy older adults, n-3s have no effect on whole body and skeletal muscle resting energy metabolism. Altogether, this thesis supports recent evidence, arguing that the necessity to supplement with EPA and DHA, in healthy individuals has been overemphasized.