Losing weight is often associated with eating less of certain foods, but a relatively recent study shows that people who add more omega-3s to their diet may shed more calories.
Researchers in Japan found that oil rich in omega-3 fatty acids turned “bad” fat cells into healthy ones that burn calories. That’s great news for people who are in their 40s and older, as “good” fat cells start to decrease around middle-age.
The findings may explain why people who eat a Japanese or Mediterranean diet rich in fish generally lead longer, leaner lives.
Professor Teruo Kawada, who led the study at Kyoto University, said:
“We knew from previous research that fish oil has tremendous health benefits, including the prevention of fat accumulation. We tested whether fish oil and an increase in beige cells could be related. People have long said that food from Japan and the Mediterranean contribute [sic] to longevity, but why these cuisines are beneficial was up for debate.
Now we have better insight into why that may be.” 
When the scientists fed oil containing fatty acids to mice, the rodents gained 5-10% less weight and 15-25% less fat after 4 months compared to mice not given the supplement.
Not All Fat Tissues Store Fat
The finding led researchers to conclude that not all fat tissues actually store fat. The team found that “white” cells store fat in order to preserve energy, while “brown” adipose cells metabolize fat to maintain a stable body temperature and keep a healthy weight.
“Beige” fat cells, which were only recently discovered, function much like brown fat cells. Here is an excerpt from the journal Nature concerning white, brown, and beige fat:
“Adipose tissue, best known for its role in fat storage, can also suppress weight gain and metabolic disease through the action of specialized, heat-producing adipocytes. Brown adipocytes are located in dedicated depots and express constitutively high levels of thermogenic genes, whereas inducible ‘brown-like’ adipocytes, also known as beige cells, develop in white fat in response to various activators.
The activities of brown and beige fat cells reduce metabolic disease, including obesity, in mice and correlate with leanness in humans. Many genes and pathways that regulate brown and beige adipocyte biology have now been identified, providing a variety of promising therapeutic targets for metabolic disease.”
As people approach middle age, these “good” cells decrease, making it harder to lose weight. The researchers believe that taking omega-3 fish oil supplements may fight this process, thereby helping people fight the battle of the bulge as they age.
Supplements with high doses of DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid), compounds found in omega-3 oils, were found to be especially beneficial. 
Omega-3 fish oils have also been shown to protect against cancer and infection, as well as anxiety and inflammation. These healthy oils have also gained a reputation for being anti-aging nutrients.
One caveat worth mentioning is that fish in today’s world are often polluted with toxic chemicals such as PCBs and other heavy metals, so it’s worth doing some research and concluding yourself if fish are worth consuming. There are many individuals who feel the waters are too polluted, while there is some research concluding that some beneficial attributes of fish override the risks.
 Daily Mail