Obesity is a growing concern leading to many cardiovascular and metabolic diseases, while further creating a strain on health care systems. Fish oil has been shown to have an effect on body weight and fat reduction when tested in animals. Previous research has shown that fish oils can improve lipid profiles and cardiovascular health in humans, but little is known about the fat reduction potential.
A recent meta-analysis examined the current data on this topic of fish oil use and weight or fat reduction. Results concluded that fish oils alone were not associated with weight reduction, changes in BMI, or changes in waist circumference. However, additional weight loss effects were seen when fish oils were combined with a weight loss program. Furthermore, it has been shown to improve waist to hip ratio, which is an indicator for health risk. Although more research is needed to explore the health benefits, preliminary research suggests it could be an effective component of a weight reduction program for those with obesity.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4646500/