Fish oil may be helpful in preventing the type of weight gain that comes with middle age, a preliminary study finds.

Fat tissue is made up of different cell types. So-called "white" cells store fat in order to maintain energy supply, while "brown" cells metabolize fat to maintain a stable body temperature. Brown cells are abundant in babies but decrease in number with maturity into adulthood.

A third type of fat cell -- "beige" cells -- have recently been found in humans and mice, and have shown to function much like brown cells. Beige cells also reduce in number as people approach middle age; without these metabolizing cells, fat continues accumulating for decades without ever being used.

A Japanese research team fed a group of mice fatty food, and the other group fatty food with fish oil additives. The mice that ate food with fish oil, they found, gained five-to-10 percent less weight and 15-to-25 percent 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.  Now we have better insight into why that may be,” said researcher Teruo Kawada about the findings, which appear in Scientific Reports.