Scientists from Gettysburg College in Pennsylvania conducted a study that looked at the effects of
fish oil supplementation
on resting metabolic rate (RMR), body composition, and cortisol levels in healthy men and women. In this doubled blinded study, baseline (pre-supplementation) measures of RMR,
body composition, and cortisol levels were completed after fasting. Subjects were randomly put into one of two groups who received 4 capsules (4 grams) per day of either safflower oil (containing only omega-6 fatty acid) or fish oil (containing only omega-3 fatty acid). They were then instructed to take 2 capsules with breakfast and 2 capsules with dinner for 6 weeks; with no change in their duet. After 6 weeks of supplementation, subjects went back to the lab to be tested again for RMR, body composition, and cortisol levels.
With no other intervention, those who took fish oil had a greater than 1lb increase in lean mass and a 1lb loss in fat mass, where as the safflower oil group saw no changes. Interestingly, the authors reported no change in RMR or body mass in either group, but cortisol levels tended to be lower in the fish oil group. The tendency for lower cortisol in the fish oil group correlated with their increase in lean mass and decrease in fat mass.
Noreen EE, Sass MJ, Crowe ML, Pabon VA, Brandauer J, Averill LK. Effects of supplemental fish oil on resting metabolic rate, body composition, and salivary cortisol in healthy adults. J Int Soc Sports Nutr. 2010 Oct 8;7:31.