Now a major new study suggests the advice was wrong. Scientists who reviewed no fewer than 89 studies of omega 3 fats, the key constituent of fish oils thought to protect against heart disease, found no clear evidence that they are of any use at all.
Researchers from the University of East Anglia and eight other institutions say that when the results were pooled they showed no strong evidence that omega 3 fats had an effect on overall deaths, heart disease, stroke or cancer.
The finding, if confirmed, will place fish oils at the top of the list of medical shibboleths that turned out to be myths. Among them are claims that fibre can prevent bowel cancer, vitamin C can halt colds, spinal manipulation can cure back pain, tranquillisers can cure anxiety and removing tonsils can prevent throat infections.
All have held sway, in some cases for decades, leading patients to treat themselves or seek treatment that turned out to be worthless.
Sales of fish oil capsules have soared on the back of the advice - for the millions who find oily fish unpalatable. Eggs high in omega 3 fats and margarine enriched with them have appeared on the market in recent years, in response to consumer demand.
The health value of these products is now in doubt. Yesterday the British Heart Foundation responded to the unexpected result by calling for more research. The Health Supplements Information Service, representing manufacturers of fish oil capsules, suggested omega 3 fats might affect different people differently. Only further studies could supply the answer, it said.
For their review of omega 3 research, the scientists from the University of East Anglia selected studies that involved a treatment group and a control group, and had investigated the effect of consuming extra omega 3 fats on health for at least six months. Differences in the quality of the studies were taken into account to minimise bias. In 48 of the trials, the omega 3 fats were taken in the form of dietary supplements such as capsules.
Until three years ago, cumulative evidence showed a benefit from omega 3 fats, but a major study called the DART-2 trial published in 2003 changed the overall picture. It included more than 3,000 men and showed a higher heart death rate in those taking fish oil capsules.
Many consumers take fish oil capsules, which are enriched with vitamins A, D and E, for their joints, skin and hair, but the study did not examine their effect on these.
The potential of omega 3 fats taken as supplements for preventing memory loss and dementia in elderly people is currently being tested, with results due in 2008.
In addition to oily fish, which contain long chain omega 3 fats, shorter chain omega 3 fats found in some plant oils are also thought to be good for health. But the review found no effect of either kind.
The findings, by researchers from the School of Medicine at the University of East Anglia, are published in the online edition of the British Medical Journal today.
The authors say the findings do not rule out the possibility that omega 3 fats have an important effect. Current UK guidelines advising people to eat more oily fish should continue, but the evidence should be reviewed regularly, they say.
The British Heart Foundation speculated that the puzzling finding could be linked to mercury levels in fish, a possibility raised by the authors. Mercury is harmful to health and builds up in oily fish in waters contaminated with the metal.
Mike Knapton, director of prevention and care at the British Heart Foundation, said: "Until now medical research has demonstrated a benefit from omega 3 fats. More research is needed to establish why some studies have shown a slightly increased risk associated with eating very high amounts of oily fish, which is possibly related to mercury levels.
"The current Food Standards Agency advice for most people is to eat no more than four portions of oily fish a week. This is still sensible advice. It is worth remembering that eating lots of one type of food is rarely best for your health."
Claims for the oil
* JANUARY 1993
Scientists claim that substances in fish oil may aid cancer prevention as well as help the battle against heart disease. Dutch doctors claim that eating fish once or twice a week can cut the risk of dying from a stroke. The Danish medical establishment maintains fish oil may help to avert premature births.
* JANUARY 1998
Further studies add asthma and inflamed bowels to the roster of ills cured by fish oils.
* MARCH 1998
Professor Michael Crawford, of the Queen Elizabeth Hospital for Children in London, says that lack of omega 3s during foetal growth can hinder brain and eye development.
* APRIL 1999
More studies suggest that eating oily fish could help to prevent skin cancer.
* MAY 2000
Failure to eat enough oily fish "could be sending evolution into reverse by causing a decline in brain power", says Professor Crawford. A fish diet was key to our ancestors developing more powerful brains than other primates, he says.
* JUNE 2001
More research leads The Independent to conclude: "Eating sardines twice a week halves risk of contracting prostate cancer ... and greatly reduce their risk of dying from the disease."
* SEPTEMBER 2003
The Sun wants to know: "Can fish really stop your children turning to crime?" Researchers in Mauritius have found that "children who eat lots of oily fish are 64 per cent less likely to have a criminal record by the time they reach 23". Other benefits are beating wrinkles, minimising the prospects of getting Alzheimer's, and soothing the painful skin condition psoriasis.
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