Increasing omega-3 fatty acids in the diet — whether by eating fish or taking supplements — has little or no effect on reducing the risk of developing type 2 diabetes, according to a major systemic review published in The BMJ.

“Our previous research has shown that long-chain omega 3 supplements, including fish oils, do not protect against conditions such as heart disease, stroke or death. This review shows that they do not prevent or treat diabetes either,” said Lee Hooper, the study’s senior author and an epidemiologist at Great Britain’s University of East Anglia, in an interview with Medicalresearch.com.

People “should not be encouraged” to take omega-3 supplements for diabetes prevention or treatment, she added.

In the United States, more than 30 million adults have diabetes, and more than 90 percent of them have type 2 diabetes. This form of diabetes, which develops over many years, occurs when the body either doesn’t produce enough insulin or becomes resistant to it.

Insulin is a hormone that is produced in the pancreas. It regulates how sugar (glucose) gets into the body’s cells.

Although you don’t have to be overweight to develop type 2 diabetes, having excess body fat, particularly around the abdomen, is a major risk factor for the condition.

Diabetes claims more than 83,000 lives in the United States each year, making it the seventh leading cause of death in the country. It is also a leading cause of disability, for the disease is associated with many serious complications, including heart disease, kidney damage, nerve damage (and resulting amputations of the feet or legs) and blindness.

Previous studies have suggested that polyunsaturated fatty acids (PUFAs) derived from oily fish (long-chain omega-3) and from plants (alpha-linoleic acid and omega-6) might protect or even reverse type 2 diabetes by helping with glucose metabolism and reducing insulin resistance. Recently, however, concerns have been raised that consuming high doses of PUFAs, particularly in the form of supplements, might make glucose control more, not less, difficult.

The current study, which was funded by the World Health Organization, set out to look more closely at these issues.

Dozens of studies 

For the study, Hooper and her colleagues reviewed 83 randomized controlled trials (considered the gold standard of medical research) involving 121,000 people in countries around the world. The studies randomly assigned half their participants to increase their consumption of PUFAs by eating more fish or other foods naturally enriched with the fats or by taking supplements. The rest of the participants were instructed to continue their usual dietary habits.

At the end of the studies, which lasted for at least six months, the participants’ glucose metabolism was measured and compared to what it had been before the studies began. The studies also noted how many participants had been newly diagnosed with diabetes.

After analyzing the combined results of all those studies, Hooper and her colleagues found no evidence that omega-3 fats had any effect — good or bad — on glucose metabolism or on the risk of developing diabetes, even among the participants in the longer running studies.

“The lack of effect was not due to low doses or the trials being too short to see effects,” said Hooper.

The analysis did uncover some evidence, however, that a daily dose of more than 4.4 grams of omega-3 might be harmful by interfering with glucose metabolism.

As for the effects of alpha-linoleic acid and omega-6 on the risk of diabetes, the studies produced too little good quality evidence on those PUFAs to draw any conclusions.

“So we don’t know whether these fats are neutral, protective or harmful as regards diabetes risk,” said Hooper.

Limitations and implications

The review comes with several caveats. Most notably, some of the studies were missing data or were designed in ways that may have introduced bias into their findings. Also, the majority of the omega-3 studies involved supplements, “which precluded fair assessment of effects of increasing oily fish consumption,” the researchers point out.

Future studies are needed to assess the effects on diabetes of eating more oily fish, not just supplements, Hooper said.

Still, as she and her co-authors point out, this is the most comprehensive review to date on the effects of PUFAs on glucose metabolism and the risk of diabetes.

“This is definitive — taking fish oil supplements does not protect against diabetes, but neither is it harmful except possibly at high doses,” said Hooper.

The lack of harm found in this study for lower dose omega-3 supplements doesn’t mean, however, that people should take them.

As Hooper told BBC News reporter Caroline Parkinson: “This is really expensive stuff. If somebody’s at risk of diabetes, there are much better things to spend money on, like physical activity — or oily fish.”

FMI: You can read the study on The BMJ’s website.