We'd all love a magic bullet for weight control.

And it would be very nice if that same magic bullet helped us build muscle. 

Sadly, no supplement – including the ones hyped in online ads – is a clinically proven weight or muscle aid.
That said, and while it's no magic bullet, omega-3 fish oil may help you reach your personal strength and shape goals.
Likewise, there's evidence that omega-3s help preserve or build muscle: see Omega-3s Aided Aging Muscles & Defenses, Women Strengthened by Omega-3s, and Omega-3s May Reduce Age-Related Muscle Loss.
(You'll find these and related reports in the Omega-3s & Weight/Fitness section of our news archive.)
As Australian researchers wrote six years ago, "Animal studies suggest that increased consumption of omega-3 EPA and DHA [from fish] can protect against the development of obesity in animals … and reduce body fat when already obese.” (Buckley JD et al. 2009)
But they added a cautionary note: "There is also evidence that increased intakes of these [omega-3] fatty acids can reduce body fat in humans, but human studies are relatively few [short, and small] ... making it difficult to draw definitive conclusions.”
Before we examine an encouraging new study, let's review two of those few clinical trials, which had different outcomes. 
Clinical trials showed mixed results; possible reasons why
To date, scientists have conducted very few clinical trials designed to test omega-3s' effects on weight loss.
A 2005 trial from Czechoslovakia found that, compared with women who took placebo capsules, women who took omega-3 fish oil lost more weight: see Omega-3s Boost Weight Loss Benefits of Low-Cal Diets.
An American study conducted six years later did not find any advantage among people who took omega-3 supplements: see Weight Loss Lacking in Omega-3 Trial.
However, the fish oil used in that  trial was unusually high in EPA – one of the two key omega-3s in fish oil – and unusually low in the other omega-3, called DHA (a 5 to 1 ratio, versus the 2 to 1 ratio in most fish oil and fish).
That may have mattered a lot, since prior research has linked high blood levels of DHA to having a leaner body (see Omega-3s Linked to Healthier Weight and Body Composition).
In addition, that unsuccessful clinical trial did not account for the participants' intake of omega-6 fatty acids ... which most Americans consume to an extreme, historically unprecedented excess, largely from cheap vegetable oils (soy, corn, safflower, sunflower, cottonseed).
We know from other research that omega-6 fatty acids promote weight gain: see Excess Omega-6 Fat Intakes Promote Weight Gain and Omega-6 Fats Drive Obesity; Omega-3s Help Avoid It
And a new study in mice supports the idea that diets rich in omega-3s can aid weight control.
Study finds that fish oil transforms fat cells
The new study comes from researchers at Japan's Kyoto University (Kim M et al. 2015).
According to senior researcher Teruo Kawada, "We knew from previous research that fish oil has tremendous health benefits, including the prevention of fat accumulation.”
The Japanese team divided mice into two groups, each of which received a different kind of feed:
  • Fatty mouse chow
  • Fatty mouse chow plus fish oil
Compared with the mice that did not get fish oil, the mice given food with fish oil gained 5-10% less weight and 15-25% less fat.
The Japanese researchers also found that fish oil transformed the animals' fat-storage cells into fat-burning cells.
How do fat-storage cells become fat-burning cells?
Mice and men alike have three different kinds of fat cells: 
  • "White” fat cells store fat as a backup energy supply.
  • "Brown” fat cells burn (metabolize) fat, in part to regulate body temperature.
  • "Beige" cells – which were recently discovered – serve purposes similar to those of brown cells.
Brown and beige fat-burning cells are abundant in babies, but their numbers drop as we grow into adulthood.
Unless we adjust our diets and activity levels to boost the numbers of brown and beige cells, fat accumulates in our white cells, which leads to weight gain.
As in mice, our "sympathetic" nervous system plays a key role in triggering fat burning.
The Japanese researchers found that fish oil stimulated the animals' sympathetic nervous systems, causing fat-storage cells to change into fat-burning cells.
In addition, fish oil activated cell-receptors in the animals' digestive tracts, which may have further promoted this transformation.
"People have long said that food from Japan and the Mediterranean contribute to longevity, but why these cuisines are beneficial was up for debate,” said Dr. Kawada. "Now we have better insight into why that may be.”
Current evidence suggests that best way to gain and maintain a healthy weight is to limit calories, avoid refined carbohydrates, and stay active.
But to give your fitness efforts a nutritional edge, it sure seems wise to get plenty of fish and fish oil ... and avoid cheap, omega-6-laden vegetable oils (soy, corn, safflower, sunflower, cottonseed).
Instead, use oils low in omega-6s ... like olive oil, macadamia nut oil, coconut oil, canola oil, and "high-oleic" versions of safflower and sunflower oil.
  • Buckley JD, Howe PR. Anti-obesity effects of long-chain omega-3 polyunsaturated fatty acids. Obes Rev. 2009 Nov;10(6):648-59. doi: 10.1111/j.1467-789X.2009.00584.x. Epub 2009 May 12. Review. 
  • Garaulet M, Hernandez-Morante JJ, Lujan J, Tebar FJ, Zamora S. Relationship between fat cell size and number and fatty acid composition in adipose tissue from different fat depots in overweight/obese humans. Int J Obes (Lond). 2006 Jun;30(6):899-905.
  • Howe P, Buckley J. Metabolic health benefits of long-chain omega-3 polyunsaturated fatty acids. Mil Med. 2014 Nov;179(11 Suppl):138-43. doi: 10.7205/MILMED-D-14-00154.
  • Kim M, Goto T, Yu R, Uchida K, Tominaga M, Kano Y, Takahashi N, Kawada T. Fish oil intake induces UCP1 upregulation in brown and white adipose tissue via the sympathetic nervous system. Sci Rep. 2015 Dec 17;5:18013. doi: 10.1038/srep18013. 
  • McDonald C, Bauer J, Capra S, Waterhouse M. Muscle function and omega-3 fatty acids in the prediction of lean body mass after breast cancer treatment. Springerplus. 2013 Dec 19;2:681. doi: 10.1186/2193-1801-2-681. eCollection 2013.
  • McDonald C, Bauer J, Capra S. Omega-3 fatty acids and changes in LBM: alone or in synergy for better muscle health? Can J Physiol Pharmacol. 2013 Jun;91(6):459-68. doi: 10.1139/cjpp-2012-0304. Epub 2012 Nov 21. 
  • Raclot T, Groscolas R, Langin D, Ferré P. Site-specific regulation of gene expression by n-3 polyunsaturated fatty acids in rat white adipose tissues. J Lipid Res. 1997 Oct;38(10):1963-72
  • Takahashi Y, Ide T. Dietary n-3 fatty acids affect mRNA level of brown adipose tissue uncoupling protein 1, and white adipose tissue leptin and glucose transporter 4 in the rat. Br J Nutr. 2000 Aug;84(2):175-84.