Dietary supplementation of fish oils (n-3 PUFA) have been observed to affect insulin action and hence metabolism, affecting the ability to carry out work. Here we examine the effects of fish oil supplementation in conjunction with a glucose load during exertion, on markers of substrate utilization. A pre-test, post-test design was performed on ten healthy young males to assess the effects of 4 weeks fish oil supplementation on muscle metabolism during incremental exertion. Breath-by-breath analysis for respiratory exchange ratio (RER) along with blood lactate and blood glucose were determined at baseline, during exercise following an acute glucose bolus (10 % solution at 4 mL/kg/bw), and again following supplementation of 4.2 g.day-1 (2.2 g EPA, 1.4 g DHA). To examine the effect of fish oil on blood flow, Doppler ultrasound was used to assess femoral blood flow at rest. Following consumption of fish oils, exercising blood glucose and RER were seen to change significantly (4.66 ± 0.44 vs. 4.58 ± 0.31 mmol.L-1 and 0.97 ± 0.03 vs. 0.99 ± 0.04; p<0.05). Resting femoral arterial blood flow was seen to increase significantly (p<0.05) pre- to post- test; 0.26 ± 0.02 - 0.30 ± 0.03 L.min-1. Specific population groups such as those undertaking high-intensity exercise, and clinical groups such as intermittent claudicants, may benefit from the effects of fish oil supplementation.Abstract: Dietary supplementation of fish oils (n-3 PUFA) have been observed to affect insulin action and hence metabolism, affecting the ability to carry out work. Here we examine the effects of fish oil supplementation in conjunction with a glucose load during exertion, on markers of substrate utilization. A pre-test, post-test design was performed on ten healthy young males to assess the effects of 4 weeks fish oil supplementation on muscle metabolism during incremental exertion. Breath-by-breath analysis for respiratory exchange ratio (RER) along with blood lactate and blood glucose were determined at baseline, during exercise following an acute glucose bolus (10 % solution at 4 mL/kg/bw), and again following supplementation of 4.2 g.day-1 (2.2 g EPA, 1.4 g DHA). To examine the effect of fish oil on blood flow, Doppler ultrasound was used to assess femoral blood flow at rest. Following consumption of fish oils, exercising blood glucose and RER were seen to change significantly (4.66 ± 0.44 vs. 4.58 ± 0.31 mmol.L-1 and 0.97 ± 0.03 vs. 0.99 ± 0.04; p < 0.05). Resting femoral arterial blood flow was seen to increase significantly (p < 0.05) pre- to post- test; 0.26 ± 0.02 - 0.30 ± 0.03 L.min-1. Specific population groups such as those undertaking high-intensity exercise, and clinical groups such as intermittent claudicants, may benefit from the effects of fish oil supplementation.