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Overall, "if we take omega-3 supplements for several years, we may very slightly reduce our risk of heart disease, but balance this with very slightly increasing our risk of some cancers. The overall effects on our health are minimal," Hooper explained in a university news release.
"At this point in time, maintaining a healthy lifestyle with a proper diet, regular aerobic exercise, and good sleep patterns can do more to prevent cancer than fish oils," said Dr. Guy Mintz, who directs cardiovascular health at Northwell Health's Sandra Atlas Bass Heart Hospital, in Manhasset, N.Y.
But he added that the story may differ considerably when it comes to cardiovascular benefits -- especially if a person takes a "prescription strength" form of fish oil.
As Hooper's team explained, small amounts of omega-3 are needed for good health and can be found in foods, including nuts, seeds and fatty fish, such as salmon.
There are also over-the-counter omega-3 supplements, typically fish oil. These are popular because people believe they'll protect against, or even reverse, diseases such as cancer, heart attack and stroke.
The investigators also looked over data from 86 trials that focused on heart disease-related events.
Overall, the trials included more than 100,000 people who either increased their intake of long-chain omega-3 fats (fish oils) or maintained their usual intake for at least a year.
The team found that omega-3 supplements may slightly reduce the risk of death from heart disease, but they also slightly increased the risk of prostate cancer. The investigators also stressed that both the benefits and harms of fish oil supplements appear to be small.
For example, if 1,000 people took omega-3 supplements for about four years, three people would avoid dying from heart disease, six people would avoid a major heart problem such as a heart attack, and three extra people would develop prostate cancer.
The findings were outlined in two reviews published Feb. 28 in the British Journal of Cancer and the Cochrane Database of Systematic Reviews.
And, she suggested, getting fish oil directly from fish -- not in supplement form -- might yield stronger results.
"The evidence on omega-3 mostly comes from trials of fish oil supplements, so health effects of oily fish, a rich source of long-chain omega 3, are unclear," Hooper said. "Oily fish is a very nutritious food as part of a balanced diet, rich in protein and energy as well as important micronutrients such as selenium, iodine, vitamin D and calcium -- it is much more than an omega-3 source."
Mintz also believes that a purified fish oil in prescription form -- a drug called icosapent ethyl (brand name Vascepa) -- has shown a potent effect against heart disease in a recent trial.
When taken along with a cholesterol-lowering statin drug, daily use of the 4-gram dose of Vascepa by heart patients seemed to produce a 25% reduction in outcomes such as death by heart disease, non-fatal heart attack or stroke, and the chest pains known as angina, Mintz said. That study was published last year in the New England Journal of Medicine.
It's not clear why Vascepa had this robust effect while "regular" fish oil supplements do not, Mintz said, but it could be due to the drug's effects on inflammation or cholesterol levels.
Dr. Elena Ivanina is a gastroenterologist at Lenox Hill Hospital in New York City. Reading over the British study, she said that "the take-home is that it's great to get omega-3 fatty acids from natural food sources like nuts, seeds and fatty fish, but there is not enough evidence showing a health benefit to taking omega-3 fatty acid supplements at this time."
Study author Hooper agreed.
"Considering the environmental concerns about industrial fishing and the impact it is having on fish stocks and plastic pollution in the oceans, it seems unhelpful to continue to take fish oil tablets that give little or no benefit," she said.
-- Robert Preidt
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