Omega-3 oils have been linked to everything from boosting brain power to relieving joint aches. But many experts claim that some of the purported benefits were shown in small, badly designed studies, and so the evidence is not convincing.
So what do the findings show in connection with the following diseases?
HEART DISEASE AND STROKE
By far the strongest evidence in favour of fish oils is for heart health.
The latest study from the U.S. provides compelling evidence that they help to protect against repeat heart attacks in people with established heart disease.
Previous research has also shown fish oils help prevent blood clotting and lower blood pressure.
There are other heart benefits. A large recent Japanese study of 18,000 patients with high cholesterol found having 1.8g of EPA a day, as well as statins, reduces the risk of cardiac problems by about 20 per cent. Likewise, heart attack survivors in an Italian study found taking fish oil supplements daily cut their risk of death by half after four months.
But this does not mean fish oils are an effective preventative measure for everyone.
'Most studies have confirmed that fish oils are useful only to people with existing cardiovascular problems, who have already suffered a heart attack, for instance,' says Bridget Benelam.
The new study also seems to contradict one three years ago in the British Medical Journal which suggested there is little evidence that fish oils cut the risk of stroke, cancer or heart disease.
Another Australian study found giving daily fish oil supplements did not reduce the risk of stroke in patients who'd already suffered one.
'Fish oils are not as good as people claim them to be,' said the lead researcher at the American Stroke Association's International Stroke Conference last year.
BOTTOM LINE: Helps to protect patients with existing heart problems; might protect otherwise healthy people.
ALZHEIMER'S AND INTELLIGENCE
Brainy: Taking fish oil is widely believed to improve intelligence, but experts believe it won't improve your IQ so you might not be the next Carol Vorderman
Some researchers have suggested that low intake of omega-3 during pregnancy can result in children with more behavioural problems such as ADHD, and researchers at the University of Tel Aviv found evidence that too little led to depression.
But it's bad news for parents who routinely feed their children fish oil supplements to boost intelligence and exam performance.
'There are emerging findings that fish oil supplements are helpful for children with behavioural problems such as ADHD, but there is little proof that they are useful for other children,' says Bridget.
'Studies showing they boost mental performance in schoolchildren have attracted a lot of media attention, but they've been ill-designed research programmes and the findings haven't been considered reliable by scientists.'
BOTTOM LINE: Helpful for foetal brain development and might help protect against degenerative brain disease, but won't boost your IQ.
The evidence for fish oils protecting against cancer is mixed.
A major review of studies involving more than 700,000 patients published three years ago found fish oils didn't generally protect against cancers. However, there is evidence a regular consumption of omega-3 oils in the form of oily fish could help to prevent bowel cancer.
Earlier this year, Professor John Wittee, from the University of California, also reported that a high intake of omega-3 fatty acids could reduce the risk of men developing prostate cancer by about 60 per cent, and could also reverse the effects of an inherited gene known to increase the risk of aggressive prostate cancer.
Men who ate oily fish one to three times a month had a 36 per cent lower risk of getting the disease than those who never ate any.
Dr Jodie Moffat, senior health information officer at Cancer Research UK, says eating a Mediterranean-style diet containing lots of fish has been shown to help reduce the risk of cancer, but adds: 'The links between diet and cancer risk are far too complex to state that fish oils have a direct benefit.'
BOTTOM LINE: Fish oils may be helpful in preventing some forms of cancer such as prostate and bowel.
Although fish oils are widely taken for joint pain relief, the scientific data on this particular health effect is mixed.
A study by the prestigious Harvard Medical School found omega-3s helped with rheumatoid arthritis. They effectively 'put the brakes on' the overactive immune response that causes the disease, the researchers reported.
However, another major American study concluded that the link was far from proven, and many of the clinical trials have been conducted 'using very large amounts of omega-3 that would be inappropriate to take in a daily diet'.
Many people take fish oil supplements for osteoarthritis, and research appears to back this up.
Professor Bruce Caterson, at Cardiff University, has found that the supplements 'turn off inflammatory responses' that cause much of the inflammation. This, in turn, helps prevent the destruction of cartilage that leads to bones rubbing together and the result of osteoarthritis.
BOTTOM LINE: The evidence for fish oils easing joint pain is inconclusive.
EYE PROBLEMSSome experts think eating oily fish at least once a week could protect against age-related macular degeneration (AMD), the most common cause of blindness in older people.
A study last year by the London School of Hygiene & Tropical Medicine found those who regularly consumed oily fish were 50 per cent less likely to get the disease.
It followed an earlier study involving more than 4,500 men and women which showed diets rich in omega-3s lead to a 40 per cent reduction in AMD.
Omega-3s might reduce inflammation and promote healthy blood vessel function, said Professor Astrid Fletcher, who led the study.