We knew from previous research that

We knew from previous research that fish oil has tremendous health benefits, including the prevention of fat accumulation," says senior author Teruo Kawada. "We tested whether fish oil and an increase in beige cells could be related."
The team fed a group of mice fatty food, and other groups fatty food with fish oil additives. The mice that ate food with fish oil, they found, gained 5-10% less weight and 15-25% less fat compared to those that did not consume the oil.
They also found that beige cells formed from white fat cells when the sympathetic nervous system was activated, meaning that certain fat-storage cells acquired the ability to metabolize.
"People have long said that food from Japan and the Mediterranean contribute to longevity, but why these cuisines are beneficial was up for debate," adds Kawada. "Now we have better insight into why that may be."