While fish oil helps prevent obesity and reduce inflammation, research in animals shows that these effects are prevented by sugar and other high-glycemic carbohydrates. While fish oil would normally prevent weight gain in mice, sugar increases obesity when given with fish oil.
High-glycemic index carbohydrates abrogate the antiobesity effect of fish oil in mice. (Link)
Sucrose counteracts the anti-inflammatory effect of fish oil in adipose tissue and increases obesity development in mice. (Link)