Should I worry about the mercury in the fish I eat?

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1. Troubled waters

The fish we eat, such as tuna and salmon, all contain small amounts of mercury – a pollutant that can be highly toxic to our nervous systems.

As a medical doctor, I’m conscious about what I eat. And a diet that includes fish comes with some great health benefits. Seafood is a good source of protein and, unlike many meats, it's typically low in saturated fat. It’s also a great source Vitamin D and Omega-3 – a type of fatty acid we can’t live without.

However some fish contain more mercury than others. I want to know how mercury can harm my body and whether I should limit the type and amount of fish I eat.

2. Discovering the problem

The presence of mercury in fish didn’t become a health concern until the 1950s – when an incident in Minamata, a coastal city in Japan, brought it to global attention.

1/5: On 21 April 1956, a five-year old girl was taken to a hospital in Minamata with convulsions and walking difficulties. In the following weeks, more patients were admitted with similar symptoms – but doctors could not identify a cause.

Photo: two young victims

© Shisei Kuwabara

2/5: Scientists were called in to investigate and soon discovered cats in the area had been having convulsions too. Seabirds were found crouching in the bay, unable to fly. This made them think the disease must be transmitted by eating fish.

© Shisei Kuwabara

3/5: They suspected the fish were being contaminated by wastewater, pumped into the bay by a local chemical factory. In 1962, they corroborated the claim with evidence that water near the factory pump contained extraordinarily high levels of mercury.

© Aileen Archive & S. Kuwabara

4/5: People showing symptoms of the disease – whose diets consisted primarily of fish – were also tested. The maximum mercury level they recorded was 705mg/kg, compared to an average level of 4mg/kg recorded in people living outside the area.

© Shisei Kuwabara

5/5: Patients lost control of their muscles. In extreme cases they were left paralysed or fell into comas. More than 1,780 people died from eating fish heavily contaminated with mercury.

© Mick Stetson

3. WATCH: How mercury gets into fish

Click or tap below to find out.

4. CLICKABLE: Harmful effects

Click or tap the image below to discover how high mercury levels can damage our nerves, brain and heart.

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5. Catch of the day

Mercury in fish

Data source: Karimi R, Fitzgerald TP and Fisher NS (2012). Around 95% of the mercury found in fish occurs as methylmercury.

Some types of fish contain much more mercury, on average, than others. However, mercury levels can vary between populations of the same species depending on where they reside and migrate.

6. Radha's review

The Minamata disaster shows high doses of methylmercury can cause irreparable damage to our bodies.

However the effect of regular, low doses in a typical diet containing fish is less well understood. That’s partly because seafood also contains beneficial substances such as Omega-3 fatty acids, which are good for the nervous system and may help counteract the adverse effects of methylmercury.

Pregnant women and babies

For example, there is evidence methylmercury can inhibit the development of an unborn child’s nervous system. However, the Omega-3s in fish may also promote the development of a baby’s brain.

That's why the NHS advises pregnant and breast-feeding women to continue eating fish – but to limit the amount and type they consume (click the link below for more information).

It's also important to note it takes months to excrete the methylmercury absorbed from fish. This means a human foetus can be exposed to some of the mercury consumed by the mother before she falls pregnant.

Non-pregnant adults and children

Studies show a clear relationship between high, frequent fish consumption and the amount of mercury in our bodies. Yet defining the amount of fish we should eat to avoid harm is complex because methylmercury affects everyone differently depending on their weight, age and genetics (Silbernagel S. et al, 2011).

Many dietary advisories advocate that the benefits of eating fish generally outweigh the risks of mercury exposure. For example, although some studies found that heart disease increases with mercury exposure, others have shown fish oils may reduce the risk of cardiac death (Mozaffarian D., 2009).

The Food Standards Agency (FSA) suggests that eating species relatively low in mercury is the most effective way to minimise excessive exposure. Fish that are low in mercury but high in Omega-3s include Atlantic mackerel, salmon and sardines.