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Is it safe to eat oily fish?

Bupa investigative news - 5 December 2002
written by Rachel Newcombe, Bupa health news reporter

Oily fish is recognised as having health benefits, due to the omega-3 fatty acids it contains. In particular, it helps reduce the risk of heart attacks, keeps the circulation healthy and even provides help for those suffering from arthritis. New research, however, suggests that eating certain types of fish could be harmful, as they contain high levels of mercury and may actually increase the risk of a heart attack. But, how big is the risk, what types of fish are affected and what types can be eaten safely?

KEY POINTS

Oily fish (dark fish) are known to reduce the risk of coronary heart disease and stroke.

Oily fish's protective effect comes from the omega-3 fatty acids they contain. These fatty acids are also called eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).

Oily fish include herring, mackerel, sardines, salmon, fresh tuna, trout, marlin, kippers and pilchards.

One study suggests that eating fish contaminated with mercury can raise a person's heart attack risk.

Experts say that, with most types of fish, these risks are small and are outweighed by the benefits of eating oily fish.

What were the headlines?

Fish and mercury contamination made the news with reports that some fish raise the risk of heart attacks, a condition that eating oily fish has been shown to protect against. Stories were featured in most of the major newspapers and websites with headlines such as "Warning over fish mercury levels", "Mercury in fish linked to risk of heart disease" and "Fish linked to heart attack risk".

As the main message in the past has been to eat fish as part of a balanced diet, the news that certain fish could be harmful has caused some confusion.

"Oily fish are a great source of what we call essential fatty acids. The body can't produce them, so you need to have a dietary source. They help reduce the risk of heart disease, blood clotting and arthritis, as well as being good for overall health and disease prevention. People should have at least one portion of oily fish per week. The type of oily fish that most people eat, like salmon, pilchards and sardines, tend to have a lower amount of mercury, so should be okay."

Rhoda Sutherland,
state registered dietician
Bupa Hospitals Chalybeate

What is the bigger picture?

The reports stemmed from an international study published in the New England Journal of Medicine (NEJM). Researchers examined the mercury levels in over 1,400 men from eight European countries, including Britain, and Israel. To find their mercury levels, the researchers analysed clippings of the men's toenails, as this has proven to be a reliable method of determining long-term mercury exposure. In total 684 men who had been diagnosed with a heart attack for the first time were compared with 724 men who were of the same age and geographic location, but had no history of heart disease.

The team discovered that men who had survived a heart attack had mercury levels that were 15 per cent higher than those without a history of heart problems. Because eating certain fish is believed to be the primary source of mercury exposure, the researchers suggest caution at eating fish that are contaminated with high levels of mercury.

Fish become contaminated with mercury due to environmental pollution by industrial processes. The mercury enters the water, is converted into a toxic form called methylmercury and is then absorbed at low levels by small fish. High levels of methylmercury can build up in large predatory fish, such as sharks, because they eat large numbers of the smaller fish.

Elisio Guallar, one of the researchers, said, "Our findings suggest that mercury found in fish may counteract the benefits of the omega-3 fatty acids. Exposure to methylmercury is a concern in specific high-risk groups, such as pregnant women and women of childbearing age who may become pregnant, but this warning perhaps should be extended to the general population."

Professor Rudolph A. Riemersma, from the University of Edinburgh Cardiovascular Research Unit, another researcher, was keen to stress that it's not necessary to give up eating fish. "We do not advise people to stop eating fish," he explained. "Our analyses are consistent with a protective effect of dietary fish, as long as it is not heavily contaminated by mercury."

Although it was an international study, the total number of people studied was small, only consisted of men and the UK participants only came from one city, which may not provide representative evidence. Plus, the amount of contaminated fish differs according to geographical location; for example it is known that the Baltic Sea is twice as contaminated as the North Sea.

Additionally, no information was gathered on the sources of mercury or the amount and type of fish eaten by the participants - so, in theory, it's possible that the men's levels of mercury were due to another source other than fish.

Interestingly, another study published in the same issue of the NEJM, also looking at heart disease and mercury levels in men, found no association between mercury exposure and risk of heart disease.

ACTION POINTS

Adults should eat two portions of fish, one oily, per week.

If you're pregnant, trying for a baby, or are feeding infants and young children, avoid consuming shark, swordfish and marlin.

The fish with the highest levels of mercury include swordfish, king mackerel and shark.

Fish with intermediate levels of mercury include tuna, trout, pike and bass.

Cod liver oil is another way of obtaining omega-3 fatty acids.

Other ways of reducing heart problems include exercise, not smoking and eating a balanced diet.

What does this mean?

This isn't the first time the mercury-fish link has been highlighted. A study carried out by the Food Standards Agency (FSA) earlier this year found shark and swordfish have the highest levels of mercury - on average, 1.5mg mercury per kilogram of fish. As a result, they advised that pregnant women, women trying for a baby, under 16-year-olds and infants should avoid eating those types, along with marlin.

The current study also mentioned high levels in shark and swordfish but, as Bupa Hospitals dietician Rhoda Sutherland says, these are white fish not oily fish. According to Dr Frankie Robinson, from the British Nutrition Foundation, "In this country, we tend to eat very little of them anyway," so they're unlikely to prove too dangerous. Tuna, trout, pike and bass have "intermediate concentrations" of mercury, but the levels - 0.1 to 0.5 micrograms of mercury per gram of fish - are small.

Belinda Linden, Head of Medical Information at the British Heart Foundation, described the study as "interesting" as it "shows the potential long-term harm caused by environmental pollution", but says more evidence is needed.

The FSA are taking note of the study, but for the moment their advice remains the same. "We recommend that people should eat two portions of fish per week, one of which should be oily, along with a balanced and varied diet," said a spokesperson.

What does this mean to me?

"These new findings do not affect our current advice for people to try to eat fish twice a week (including one portion of oily fish such as sardines and mackerel)," said Belinda Linden. "This quantity should offer the benefits of omega-3 oils in fish while avoiding any potential harmful effects."

Dr Frankie Robinson, agrees: "For the general public, I'd think that having a portion of oily fish a week is going to be more beneficial to their health than avoiding it."

According to Rhoda Sutherland, "It's helpful to think in terms of having a weekly intake of 5 to 10g of omega-3. Tinned salmon has 1.8g of omega-3s per 100g, whilst sardines, mackerel and pilchards all have 2.5g. So, one or two oily fish meals per week should provide all the benefit you need."

For those who don't like fish, but still want the cardiovascular benefits, fish oil supplements are useful, says Rhoda, with, "one teaspoon of cod liver oil providing about 1g of omega-3".

Summary

Oily fish are well known for providing a multitude of health benefits, particularly in relation to cardiovascular disease. Although mercury has been found in some fish, current expert opinion suggests the benefits of eating one portion of low-mercury oily fish on a weekly basis outweighs the risks.

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