Mercury in fish:

An evidence-based guide for parents

© 2010-2014 Gwen Dewar, Ph.D., all rights reserved

Recent research confirms that fish are high in nutrients essential for brain growth. Pregnant women who eat fish, and parents who include fish in their children's diets, are providing their families with omega-3 fatty acids that may boost cognitive development (Strain et al 2012).

But unfortunately there are risks. Mercury, which accumulates in the tissues of all ocean life, can sometimes reach levels that are harmful.

This is particularly true for large-bodied, long-lived, predatory fish: Because they are older and exist high on the food chain, these fish have been exposed to higher levels of mercury themselves. And children who consume such fish are at special risk because of their developmental status.

But, as the saying goes, there are more fish in the sea. And the best available evidence suggests that eating low-mercury fish--like salmon, cod, herring, and mackerel--yields net benefits for kids.

So here’s the background:

• why researchers think fish consumption is good for kids

• why parents should be concerned about mercury in fish, and

• how to choose fish that minimizes the risks.

The benefits of eating fish

Fish oil is good for you. It’s high in omega-3 fatty acids, like DHA, that are crucial for brain development. And there is evidence that babies born with higher levels of DHA are better off.

For example, a study tracking Inuit infants measured DHA concentrations in their umbilical cords. Babies with higher cord DHA concentration performed better on visual, motor, and mental tests in the first 12 months after birth (Jacobsen et al 2008).

Another study tracking babies in the United States found that mental scores at age 3 were higher for kids whose pregnant mothers had eaten more than two cans of tuna per week (Oken, Radesky et al 2008).

Are such correlations -- and there are many -- caused by high levels of prenatal DHA? That's not clear.

In randomized, double-blind experiments, women who took fish oil capsules during pregnancy were not more likely to have babies with improved cognitive development (Makrides et al 2010; Campoy et al 2011). And other research suggests that prenatal supplementation is less effective than boosting DHA levels after birth (Makrides 2013).

So maybe the widely-observed links between prenatal DHA and better cognitive development in infants reflects something else, like the tendency for these babies to grow up on postnatal diets that are rich in DHA.

But whatever may be going on, we also know this: High doses of mercury in fish may harm the nervous system and cause impairments in cognition, memory, attention, and other areas (Castoldi et al 2008).

Take, for instance, the United States study mentioned above. Researchers measured how much mercury each child had metabolized (by analyzing hair samples).

The results? Kids with higher mercury levels in their hair performed lower on mental tests. And the association was even stronger when researchers adjusted for maternal fish consumption, which presumably controlled for the positive effects of eating fish (Oken, Radesky et al 2008).

Similar results have been reported for kids living in Granada, Spain—a place where many people eat fish four times a week or more. Kids whose hair had concentrations of mercury exceeding 1.0 microgram/gram scored lower on tests of cognition, memory, and verbal ability (Freire et al 2010).

So fish consumption has benefits and risks.

Higher fish consumption is linked with better mental performance.

Higher mercury consumption is associated with worse performance.

The trick--if you want to enjoy the benefits of fish consumption-- is to seek out fish that is low in mercury.

That’s the message that Cornell University professor Tom Brenna and London Metropolitan University professor Michael Crawford want to get across.

On May 26, 2010, they published an open letter to the U.S. Food and Drug Association, urging that agency to update its 2004 fish consumption advisory to reflect recent research about the benefits of fish eating for fetuses and young children.

Mercury in fish: How much is too much?

That’s controversial. The U.S. Environmental Protection Agency recommends that people avoid fish that contains more than 0.5 parts per million (or 0.5 micrograms per gram) of mercury. And the EPA's current reference dose--the highest dose that a person is supposed to consume--is 0.1 microgram/kg body weight per day.

To some people, that limit seems very conservative. And indeed, the EPA’s reference dose is designed to include a hefty 10-fold safety margin.

But I wonder about the research that analyzed the mercury levels in children’s hair. The Spanish study cited above reported cognitive delays in kids with hair mercury levels over 1.0 microgram /gram. And researchers note that this level of mercury in hair is consistent with consuming mercury just at or above the EPA’s reference dose (Díez et al 2009). Perhaps the EPA’s reference dose isn’t as conservative as it appears.

Mercury in tuna and other fish

It's clear that pregnant women and young children should limit their exposure to mercury in fish. But how?

One good rule of thumb concerns size. The larger the fish, the higher levels of mercury it contains. Mercury is also more concentrated in predatory fish.

So health officials warn pregnant women and young children avoid swordfish, shark, tile fish, and king mackerel. The levels of mercury in these fish approach or exceed 1.0 part per million (ppm), or 1.0 micrograms mercury per gram of fish.

And tuna? In the past, people have assumed that that tuna mercury levels were relatively low. But a recent study led by the University of Nevada’s Shawn Gerstenberger suggests otherwise (Gerstenberger et al 2010).

Gerstenberger and his team went to the local supermarket in Las Vegas and purchased three different brands of canned tuna.

They also examined different varieties of tuna—chunk white, solid white, and chunk light.

When they tested for mercury, they got these results

• chunk white tuna (0.619 ± 0.212 ppm)

• solid white tuna (0.576 ± 0.178 ppm)

• chunk light tuna (0.137 ± 0.063 ppm)

In other words, chunk light tuna met expectations as a relatively low mercury fish. But chunk white and solid white tuna tested at levels that exceed the EPA limit.

As a result, say the researchers, pregnant women and kids should be careful about tuna consumption. If we follow the guidelines of the U.S. EPA reference dose (0.1 microgram/kg body weight per day) and make calculations based on an average mercury value of 0.619 ppm, then "a 25-kg (or 55-lb) child may consume 75 g (or 2.6 ounces) of canned chunk white tuna only once every 18.6 days."

But that’s assuming that the chunk white tuna is the only substantial source of mercury in the child’s diet. If he’s eating other seafood, then he should reduce his tuna consumption accordingly.

And younger, smaller kids? It seems pretty obvious that, based on the recommendations above, these children can eat only very small amounts of chunk white tuna without exceeding the EPA’s recommended dose.

And let’s consider chunk light tuna--the low mercury tuna. If chunk light tuna has 0.137 micrograms mercury/ gram, then according to the EPA, a 10 kg (22 lb) child shouldn’t eat more than about 29 grams (or one ounce) of chunk light tuna every 4 days.

That’s the equivalent of one 6-ounce can of chunk light--the low-mercury tuna-- every 24 days.

And again, that’s assuming no other sources of mercury.

Safer choices?

You can check out this table prepared by the FDA, “Mercury levels in commercial fish and shellfish.”

It was last updated in 2006, so it doesn’t reflect the 2010 research on tuna. But it gives you an overview of previous findings, and, as you’ll see, many fish are listed as having very low levels of mercury (under 0.05 ppm or 5 micrograms / gram).

Some of these are also ecologically sustainable and are very high in omega-3 fatty acids:

• salmon

• herring

• sardines

• anchovies

Other ecologically sustainable fish listed on the FDA’s low mercury list include

• Cod (used in “fish and chips,” “fish fingers,” and “fish sticks”)

• Atlantic haddock (another “fish and chips” fish)

• North Atlantic mackerel

• Pollock (used to make artificial crab)

Interestingly, salmon, cod, herring, and mackerel were the fish most commonly eaten by pregnant Danish women in a study that tracked more than 25,000 babies from gestation to 18 months postpartum.

In that study, Emily Oken and colleagues found that higher maternal fish intake during pregnancy and breastfeeding was linked with better development at 18 months (Oken, Østerdal et al 2008).

Mercury in fish oil supplements

What about fish oil supplements? Check and see what fish is the source. In many cases, children’s supplements use small, low mercury fish—like sardines—as their source. But I’ve seen at least one brand that claims to use tuna as a source. The label didn’t specify what kind.

How you can estimate the amount of mercury in fish

Using the FDA chart, you can do your own calculations. It’s a three step process.

1. Calculate the total mercury in fish for a given meal

Multiply the mercury concentration (in ppm OR micrograms/gram) by the total grams of fish = ______

Example: Mario wants to eat a 6 ounce (170 gram) can of solid white tuna. According to the latest data (above), that amount contains a total of 0.576(170)= 97.92 micrograms of mercury.

2. Determine how much mercury your child can consume per day

Multiply the child’s weight (in kilograms) by 0.1 = ______

Example: Mario weights 15 kg (33 lbs). To stay within the EPA reference dose limits, he should consume (on average) no more than 15(0.1)= 1.5 micrograms of mercury per day.

3. Divide the total mercury in the meal by the total mercury permitted per day for the child

Example: 97.92 / 1.5 = 65.28 days. To stay within the EPA’s reference dose, Mario should eat no more than 6 ounces of solid white tuna every 65 days.

More information

As noted above, you can access the FDA’s 2004 report on mercury in fish here.

You can also read Tom Brenna’s and Michael Crawford’s open letter to the FDA to revise their recommendations about fish consumption.

Does it make a difference how you cook fish? Perhaps not, not if you are trying to reduce levels of mercury in fish (Perello et al 2008). But one study found that deep frying increased the concentration of mercury in fish (Burger et al 2003).

And what about shellfish? If you check out the FDA’s report, you’ll notice that most shellfish are very low in mercury. But, with a few exceptions, they also tend to be low in omega-3 fatty acids.

As for other health issues, it’s true that mercury isn’t the only concern when you consume fish and seafood. Some kids are allergic to shellfish. And, like land-based foods, seafood is sometimes contaminated with pollutants (like PCBs) or disease-causing microorganisms. For example, PCB levels in salmon depend on what the salmon eat, which varies locally (Sprague et al 2010). Presently, it’s not clear that average levels have detrimental developmental effects (Moon et al 2009). For a critical look at the PCB salmon story, check out out Trevor Butterworth’s article on STATS.

Copyright © 2006-2021 by Gwen Dewar, Ph.D.; all rights reserved.
For educational purposes only. If you suspect you have a medical problem, please see a physician.


References: Mercury in fish

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Campoy C, Escolano-Margarit MV, Ramos R, Parrilla-Roure M, Csábi G, Beyer J, Ramirez-Tortosa MC, Molloy AM, Decsi T, and Koletzko BV. 2011. Effects of prenatal fish-oil and 5-methyltetrahydrofolate supplementation on cognitive development of children at 6.5 y of age. Am J Clin Nutr. 94(6 Suppl):1880S-1888S.

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Strain JJ, Davidson PW, Thurston SW, Harrington D, Mulhern MS, McAfee AJ, van Wijngaarden E, et al 2012. Maternal PUFA status but not prenatal methylmercury exposure is associated with children's language functions at age five years in the Seychelles. J Nutr. 142(11):1943-9.

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Content of "Mercury in fish" last modified 3/14