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Some health claims for fish oil are, well, fishy

In an article appearing online today in New Scientist magazine, psychologist and science writer Sanjida O’Connell discusses the evidence for — and against — various health claims regarding omega-3 fatty acids.

Many of those claims, under the harsh glare of the existing evidence, are, well, fishy, she reports.

Omega-3 is found primarily in fish but also in some nuts (especially walnuts) and certain vegetable oils (such as flaxseed, canola, olive and soybean). Headlines and ads (from food and supplement manufacturers) claim it can ward off all sorts of ailments, including heart disease, cancer and depression. Its enthusiasts also believe it can boost brainpower in both kids and the elderly.

No wonder fish oil has become a growing multibillion-dollar business. “Sales of over-the-counter fish oil supplements rose 18% [in 2008] to $739 million [and] Americans buy another $1.8 billion worth of foods (such as margarine and peanut butter) fortified with extra omega-3s,” reported Forbes magazine last year.

In fact, a 2007 survey by the National Institutes of Health’s National Center for Complementary and Alternative Medicine reported that 37 percent of American adults and 31 percent of children had taken omega-3 supplements for health reasons within the 30 days prior to the survey. That makes these supplements second only to vitamins and minerals as the most commonly used “natural product” in the United States.

“Omega-3 supplements first appeared in the early 1980s,” O’Connell writes. “Given they are still going strong 30 years later, you would be forgiven for thinking that claims of their beneficence have all been substantiated. Yet several new studies, as well as recent reviews of existing evidence, call this received wisdom into question.”

Here’s a quick summary of her take on the studies. (For her full discussion of the studies, be sure to read the article.)

  • Heart disease. This is where the evidence in favor of omega-3’s protective role is strongest. But two recent analyses of previous studies show that any benefit is modest: One review found that when people who’ve already had a heart attack eat oily fish or take fish-oil supplements for about two years they reduce their relative risk of a second heart attack by about 18 percent. Another review of randomized controlled studies found that fish and/or fish-oil supplements reduce the relative risk of developing any type of heart disease by about 11 percent. On the other hand, a 2006 review found that omega-3 consumption had no clear effect on preventing deaths from heart disease — or cancer. And scientists agree that they still don’t know the biological reasons why omega-3s might protect the heart.
  • Rheumatoid arthritis. A 2007 review of 17 randomized controlled studies found evidence that omega-3’s anti-inflammatory properties can probably help reduce joint pain caused by rheumatoid arthritis. But taking omega-3 supplements does not slow down the progression of the disease.
  • Depression.  Studies suggesting omega-3 can help alleviate depression have been “vastly over-hyped,” according to the experts O’Connell interviewed. That’s because such studies have shown only an associative, not a cause-and-effect, relationship between fish consumption and depression. (For example, Germans eat less fish than the Japanese. Germans are more depressed than the Japanese. You can't conclude from this that eating fish helps ease depression.) Studies showing that omega-3 reduces murder rates and aggression in prisoners suffer from the same limitations.
  • Brainpower. One study of 300 British kids found a significant improvement in the children’s ability to write, read and concentrate when they took omega-3 fish oil supplements — but the improvement was seen in less than half of the children taking the supplements. Another study that got better results is deemed “virtually worthless” by one of O’Connell’s experts because it had no placebo control group. As for omega-3 helping adults, a two-year double-blind placebo-controlled study published last year found no improvement in the cognitive skills of older people (aged 70 to 80) who took the supplements.

If you want to take omega-3 supplements …
The American Heart Association (AHA) recommends eating two 3.5 servings of fatty fish each week — a recommendation they acknowledge is complicated by the fact that many fatty fish contain high levels of mercury, polychlorinated biphenyls (PCBs), dioxins and other environmental toxins. Pregnant women and children need to be particularly careful about consuming contaminated fish. (For the Environmental Defense Fund’s list of fish that are high in omega-3 fatty acids and low in environmental contaminants, go here.) People who want to get their omega-3s through supplements should first talk with their physician, the AHA adds.  High amounts of such supplements can cause bleeding in some people.

Of course, as one of O’Connell’s experts points out, there are other, more effective ways of protecting yourself against heart disease. “The bottom line, as with many nutrition-related questions,” concludes O’Connell, “is to simply maintain a balanced diet.”

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