Is there a limit on how much tuna you can safely eat?

Mercury levels put a limit on how much fish we can safely eat. But where does canned tuna stand? Find out what experts say.

Aussies love tuna, whether it’s on top of crackers, in salads or in lunchbox wraps.

And with over 14 million Australian adults eating canned tuna, it has certainly earned its pantry staple status.

But as we eat tuna and other fish, we are also consuming mercury, a naturally occurring element that may wreak havoc on our health when consumed in excess.

Before you clear out those tins of tuna, read on — we ask experts whether mercury toxicity is worth worrying about, and how this heavy metal comes to be in one of our fish favourites.

What is mercury?

While mercury is naturally found in soil, air and water, it isn’t a natural nutritional component of fish.

“It is a mineral contaminant and pollutant in our environment,” nutritionist Kristen Beck says.

Whether we eat fish or not, Kristen says we are all exposed to low levels of mercury; and problems can arise if we are exposed to too much of it.

“High concentrations of mercury are toxic, particularly for our brain and nervous system,” she says.

“Mercury exposure in humans can also lead to kidney, gastrointestinal, genetic, cardiovascular and developmental disorders.”

Fish take up mercury from oceans and streams while feeding — so when we consume these fish, we are also consuming mercury.

“The amount of mercury in the fish we eat has less to do with the species of fish and more to do with the lifespan of the fish, the types of smaller fish they eat, and the amount of mercury in the ocean area where the fish comes from,” Kristen says.

However, the body will naturally reduce levels of heavy metals, including mercury, through detoxification methods such as sweating and urination.

Which fish contain the most mercury?

Predatory fish typically contain the most mercury.

“Because mercury builds up in fish over time, the highest amounts are found in fish that are large, with long lifespans and … (that) are predatory,” dietitian and nutritionist Jemma O’Hanlon says.

“These include orange roughy (deep sea perch), catfish, shark (often known as flake) and billfish (swordfish or broadbill and marlin).”

Food Standards Australia New Zealand (FSANZ) recommends adults consume these types of fish once a week, with no other fish that week.

When it comes to canned tuna, Kristen says mercury content varies between brands and depends on where the tuna is sourced from.

“The mercury content of fish has more to do with the levels of mercury in the ocean that it came from,” Kirsten explains.

“In Australia, tinned tuna generally has a lower mercury content than tuna fillets — this is because smaller, younger tuna is generally used for tinned tuna.

“The tinning process does not impact the mercury content either way (neither does freezing or cooking).”

How much canned tuna is safe to eat?

Mercury toxicity is something most of us don’t need to worry about.

“Generally speaking, we don’t need to be concerned about mercury levels, as we’d need to be eating very large amounts of big sea fish frequently to have an impact on our health,” Jemma says.

“Most of the fish we eat on a more regular basis like salmon and tuna tend to have lower levels of mercury, so it’s safe to enjoy these two to three times a week.”

While there is no specific recommendation for tinned tuna, the NSW Government states that it is generally safe for all population groups to consume 2-3 servings of any type of tuna or salmon a week, canned or fresh.

Can you eat tuna fish when pregnant?

Unborn babies are particularly vulnerable to the effects of mercury, which can pass through the placenta and harm the development of their central nervous system.

Canned tuna, however, is low in mercury and the FSANZ guidelines are based more around the consumption of large fish that contain more mercury.

“It’s only the high-mercury fish that are recommended to be reduced during pregnancy,” Jemma says.

“For women who are pregnant or planning a pregnancy, stick to one portion (150g) of orange roughy or catfish and no other fish that week, or one portion (150g) of shark or billfish per fortnight, and no other fish that fortnight.

“Raw and undercooked fish should be avoided due to the listeria risk.”

Jemma says canned tuna is safe to eat during pregnancy.

“Fish, including tuna, is recommended during pregnancy as it’s a valuable source of nutrients including protein, omega-3s, vitamin B12 and iodine, which are important for a baby’s brain development,” she explains.

“Most pregnant women need about three-and-a-half serves of protein-rich foods each day, which include lean meat, poultry, fish, eggs, tofu, legumes, nuts and seeds.”

Is canned tuna healthy?

Protein power

In its convenient canned form, tuna is an affordable and versatile source of protein.

“Adding a small tin of tuna to salads, wraps and pasta dishes can significantly improve the nutritional value of your meal, fuel your brain and muscles, and keep you feeling fuller for longer,” Kristen says.

Cost-wise, tinned tuna is also a winner in comparison to other protein options such as red meat, eggs and fresh fish, she notes.

Other key nutrients

Tuna — be that canned, fresh or frozen — is packed with key vitamins and omega-3’s that Jemma says can support brain and heart health, lower blood pressure and reduce cholesterol.

“We know that people who eat fish regularly have a lower risk of heart disease, stroke, macular degeneration and dementia — so the benefits are widespread.”

Tuna is also rich in B vitamins.

“Tuna contains a range of vitamins including niacin (vitamin B3) and vitamin B12, which provide energy to the body and support nerve and brain function,” Jemma says.

It is also rich in selenium, which supports thyroid function and fertility, and prevents cell damage, she notes.

What about sodium?

Some flavoured tinned tuna can be high in sodium, so Jemma recommends checking the food label or sticking to tuna in springwater for a healthier option.

“Choosing tuna in springwater is the best way to choose the lowest sodium option,” she says.

“Some tinned tuna contains added salt but generally, in the portions we tend to eat, we don’t need to be concerned.”

Want more flavour? Jemma suggests adding fresh herbs, spices or a squeeze of fresh citrus juice to naturally liven up your tuna.

Or, opt for tinned tuna that is packed in healthy oils, such as olive oil or sunflower oil, she suggests.

“Some tinned tuna have a blend of both sunflower and olive oils, which is also a good option,” Jemma says.

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Written by Hayley Hinze.