Mercury in fish: The trouble with tuna and other seafood

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

Recent research confirms that fish are loftier in nutrients essential for brain growth. Pregnant women who eat fish, and parents who include fish in their children'due south diets, are providing their families with omega-3 fat 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 achieve levels that are harmful.

This is particularly true for large-bodied, long-lived, predatory fish: Considering 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 run a risk considering of their developmental status.

Only, as the maxim goes, there are more fish in the bounding main. And the best available evidence suggests that eating low-mercury fish–like salmon, cod, herring, and mackerel–yields cyberspace benefits for kids.

And then here'south the background:

• why researchers think fish consumption is adept 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 y'all. It's high in omega-3 fatty acids, like DHA, that are crucial for brain evolution. And there is evidence that babies born with college levels of DHA are amend 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 nativity (Jacobsen et al 2008).

Another study tracking babies in the U.s. institute that mental scores at historic period 3 were higher for kids whose significant mothers had eaten more than 2 cans of tuna per week (Oken, Radesky et al 2008).

Are such correlations — and there are many — caused past high levels of prenatal DHA? That'south not clear.

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

Then possibly the widely-observed links between prenatal DHA and amend cognitive development in infants reflects something else, like the tendency for these babies to grow upwards 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, retention, attending, and other areas (Castoldi et al 2008).

Accept, for instance, the Usa study mentioned in a higher place. 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 furnishings of eating fish (Oken, Radesky et al 2008).

Like 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 noesis, memory, and verbal ability (Freire et al 2010).

So fish consumption has benefits and risks.

College fish consumption is linked with ameliorate mental operation.

Higher mercury consumption is associated with worse performance.

The fob–if you lot want to enjoy the benefits of fish consumption– is to seek out fish that is low in mercury.

That's the bulletin that Cornell University professor Tom Brenna and London Metropolitan University professor Michael Crawford desire to get across.

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

Mercury in fish: How much is too much?

That's controversial. The U.S. Ecology Protection Bureau recommends that people avert fish that contains more than than 0.5 parts per million (or 0.five micrograms per gram) of mercury. And the EPA's current reference dose–the highest dose that a person is supposed to consume–is 0.ane microgram/kg body weight per day.

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

Merely I wonder about the research that analyzed the mercury levels in children's pilus. The Spanish study cited above reported cerebral delays in kids with hair mercury levels over 1.0 microgram /gram. And researchers note that this level of mercury in pilus is consequent with consuming mercury only at or above the EPA's reference dose (Díez et al 2009). Perhaps the EPA'southward reference dose isn't every bit bourgeois 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 information technology contains. Mercury is also more concentrated in predatory fish.

And so health officials warn pregnant women and immature children avoid swordfish, shark, tile fish, and king mackerel. The levels of mercury in these fish approach or exceed ane.0 part per one thousand thousand (ppm), or 1.0 micrograms mercury per gram of fish.

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

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

They too examined different varieties of tuna—clamper white, solid white, and chunk low-cal.

When they tested for mercury, they got these results

• clamper white tuna (0.619 ± 0.212 ppm)

• solid white tuna (0.576 ± 0.178 ppm)

• chunk calorie-free tuna (0.137 ± 0.063 ppm)

In other words, clamper light tuna met expectations as a relatively low mercury fish. Just 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 brand calculations based on an average mercury value of 0.619 ppm, so "a 25-kg (or 55-lb) child may consume 75 m (or 2.6 ounces) of canned chunk white tuna merely in one case every 18.six days."

Just that'due south assuming that the chunk white tuna is the only substantial source of mercury in the kid'southward diet. If he's eating other seafood, then he should reduce his tuna consumption appropriately.

And younger, smaller kids? It seems pretty obvious that, based on the recommendations above, these children can swallow simply very pocket-sized amounts of chunk white tuna without exceeding the EPA'south recommended dose.

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

That's the equivalent of i 6-ounce can of chunk light–the low-mercury tuna– every 24 days.

And again, that's bold no other sources of mercury.

Safer choices?

You can cheque out this table prepared by the FDA, opens in a new window"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 v micrograms / gram).

Some of these are also ecologically sustainable and are very high in omega-3 fat 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 college 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 near fish oil supplements? Check and come across what fish is the source. In many cases, children's supplements use minor, low mercury fish—like sardines—as their source. But I've seen at least one brand that claims to utilize tuna as a source. The label didn't specify what kind.

How you lot can estimate the amount of mercury in fish

Using the FDA chart, you lot can do your own calculations. Information technology's a 3 footstep procedure.

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 consume a 6 ounce (170 gram) can of solid white tuna. Co-ordinate to the latest information (above), that amount contains a full of 0.576(170)= 97.92 micrograms of mercury.

two. Determine how much mercury your child tin can swallow per mean solar 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.ane)= ane.5 micrograms of mercury per day.

3. Carve up the full 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'southward reference dose, Mario should consume no more than half-dozen ounces of solid white tuna every 65 days.

More than data

Every bit noted above, y'all can admission the FDA's 2004 written report on mercury in fish hither.

You can also read opens in a new windowTom Brenna'south and Michael Crawford's open up letter to the FDA to revise their recommendations near fish consumption.

Does it make a difference how you cook fish? Possibly non, not if y'all are trying to reduce levels of mercury in fish (Perello et al 2008). Only i study institute that deep frying increased the concentration of mercury in fish (Burger et al 2003).

And what about shellfish? If you cheque 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 exist low in omega-iii fatty acids.

As for other health issues, it'south 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 boilerplate levels have detrimental developmental effects (Moon et al 2009). For a critical look at the PCB salmon story, check out opens in a new windowout Trevor Butterworth's article on STATS.

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References: Mercury in fish

Burger J, Dixon C, Boring CS, and Gochfeld M. 2003. Effect of deep-frying fish on run a risk from mercury. J Toxicol Environ Health A. 66(9):817-28

Castoldi AF, Johansson C, Onishchenko North, Coccini T, Roda E, Vahter M, Ceccatelli Due south, and Manzo 50. 2008. Human developmental neurotoxicity of methylmercury: impact of variables and risk modifiers. Regul Toxicol Pharmacol. 51(ii):201-xiv.

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

Díez S, Delgado S, Aguilera I, Astray J, Pérez-Gómez B, Torrent M, Sunyer J, and Bayona JM. 2008. Prenatal and early childhood exposure to mercury and methylmercury in Spain, a high-fish-consumer state. Curvation Environ Contam Toxicol. 56(3):615-22.

Freire C, Ramos R, Lopez-Espinosa MJ, Díez Due south, Vioque J, Ballester F, and Fernández MF. 2010. Hair mercury levels, fish consumption, and cognitive development in preschool children from Granada, Spai. Environ Res. 110(one):96-104.

Gerstenberger SL, Martinson A, and Kramer JL. 2010. An evaluation of mercury concentrations in three brands of canned tuna. Environmental Toxicology and Chemical science 29(2): 237-242.

Jacobson JL, Jacobson SW, Muckle Chiliad, Kaplan-Estrin M, Ayotte P, and Dewailly E. 2008. Beneficial effects of a polyunsaturated fat acid on infant development: evidence from the inuit of arctic Quebec. J Pediatr. 152(3):356-64.

Mahaffey KR, Clickner RP, and Jeffries RA. 2008.Methylmercury and omega-iii fat acids: co-occurrence of dietary sources with emphasis on fish and shellfish.Environ Res. 107(1):20-9.

Makrides M, Gibson RA, McPhee AJ, Yelland L, Quinlivan J, Ryan P; DOMInO Investigative Team. 2010. Issue of DHA supplementation during pregnancy on maternal depression and neurodevelopment of young children: a randomized controlled trial. JAMA. 304(fifteen):1675-83.

Oken E, Østerdal ML, Gillman MW, Knudsen VK, Halldorsson TI, Strøm M, Bellinger DC, Hadders-Algra M, Michaelsen KF, and Olsen SF. 2008. Associations of maternal fish intake during pregnancy and breastfeeding elapsing with attainment of developmental milestones in early on childhood: a report from the Danish National Birth Cohort. Am J Clin Nutr. 88(3):789-96.

Oken Due east, Radesky JS, Wright RO, Bellinger DC, Amarasiriwardena CJ, Kleinman KP, Hu H, and Gillman MW. 2008. Maternal fish intake during pregnancy, blood mercury levels, and child knowledge at age three years in a United states of america accomplice. Am J Epidemiol. 167(10):1171-81.

Moon HB, Kim HS, Choi M, Yu J, and Choi HG. 2010. Human being health risk of polychlorinated biphenyls and organochlorine pesticides resulting from seafood consumption in Republic of korea, 2005-2007. Food Chem Toxicol. 47(8):1819-25

Perelló M, Martí-Cid R, Llobet JM, and Domingo JL. 2008. Effects of various cooking processes on the concentrations of arsenic, cadmium, mercury, and lead in foods.J Agric Food Chem. 56(23):11262-nine.

Sprague M, Bendiksen EA, Dick JR, Strachan F, Pratoomyot J, Berntssen MH, Tocher DR, and Bell JG. 2010. Effects of decontaminated fish oil or a fish and vegetable oil blend on persistent organic pollutant and fatty acrid compositions in diet and mankind of Atlantic salmon ( Salmo salar). Br J Nutr. 103(10):1442-51

Strain JJ, Davidson Pw, Thurston SW, Harrington D, Mulhern MS, McAfee AJ, van Wijngaarden E, et al 2012. Maternal PUFA status only non prenatal methylmercury exposure is associated with children'due south linguistic communication functions at age five years in the Republic of seychelles. J Nutr. 142(11):1943-9.

Valent F, Mariuz K, Bin M, Little D, Mazej D, Tognin V, Tratnik J, McAfee AJ, Mulhern MS, Parpinel M, Carrozzi M, Horvat M, Tamburlini G, and Barbone F. 2013. Associations of prenatal mercury exposure from maternal fish consumption and polyunsaturated fat acids with child neurodevelopment: a prospective accomplice report in Italy. J Epidemiol. 23(five):360-lxx.

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