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Commercial Fish: Eat Up, Despite Low Levels Of Mercury

August 27, 1998
University Of Rochester
Even though the world's fish contain slight amounts of mercury, eating lots of fish carries no detectable health risk from low levels of the substance, even for very young children and pregnant women, concludes the most comprehensive study of the subject yet.

Even though the world's fish contain slight amounts ofmercury, eating lots of fish carries no detectable health riskfrom low levels of the substance, even for very young childrenand pregnant women, concludes the most comprehensive study of thesubject yet.

The findings come from a nine-year University of Rochesterstudy conducted in the Republic of the Seychelles, an islandnation in the Indian Ocean where most people eat nearly a dozenfish meals each week and whose mercury levels are about 10 timeshigher than most U.S. citizens. Indeed, no harmful effects wereseen in children at levels up to 20 times the average U.S. level.The work is published in the August 26 issue of the Journal ofthe American Medical Association.

"We look at the Seychelles people as a sentinel population,"says pediatric neurologist Gary Myers, who examined the children."If somebody who eats fish twice a day does not show effects frommercury exposure, it's unlikely that somebody who eats fish twicea week will be affected. And the fish they eat in the Seychellescontains the same amount of mercury as fish sold at supermarketsand eaten in the United States."

Adds first author Philip Davidson, an expert ondevelopmental disabilities who designed a battery of the mostsophisticated tests available to examine the children: "What wefound in the Seychelles is applicable to every woman, every man,and every child around the world who eats ocean fish."

In the United States the green light applies only to fishbought and sold commercially, at grocery stores, supermarkets,fish shops, and in restaurants. Those fish are already regulatedbased on their mercury levels, and current regulations aresufficient to safeguard frequent fish eaters against mercuryexposure, say the investigators. Consumers still should followadvisories about eating fish caught in lakes and rivers, sincethere are hundreds of polluted waterways whose fish are dangerousto eat in abundance, often because of other pollutants such asPCBs.

The Seychelles study began in 1989, when Rochesterresearchers, with decades of expertise studying mercury exposure,chose the nation of about 65,000 people as an ideal site to studythe effects of mercury exposure (see sidebar). Myers enrolled 779newborn children, about half the births on the islands that year.From the children's mothers, Myers and the team took samples ofhair, which lock in a record of mercury exposure of the childduring gestation.

A neurologist, a childhood development expert, and nursesthen studied the children at 6, 19, 29 and 66 months of age,visiting their homes, talking to their parents, and performingnearly three dozen sensitive developmental and neurological testsdesigned to detect subtle effects of mercury exposure. Theanalysis included noting when the children learned to walk andtalk, measurements of reflexes, word recognition, and socialbehavior, and the best neuropsychological tests yet developed toevaluate children at these ages. At each interval, the results ofthe longitudinal study have been consistent: no ill effects froma high-fish diet. The JAMA paper details the 66-month evaluation,which included 711 of the original children.

Mercury is a deadly neurotoxin that at high levels killsnerve cells, causing blurry vision, lack of coordination, slurredspeech, and even death. Children exposed to high levels of thecompound pre-natally can suffer slowed development, blindness,cerebral palsy, and other birth defects.

While high amounts of mercury are obviously toxic,scientists for years have debated the health effects of lowerlevels. Late last year, the federal Environmental ProtectionAgency (EPA) proposed slashing the amount of mercury that isacceptable for people to ingest from 30 micrograms per day, thelevel recommended both by the World Health Organization and thefederal Agency for Toxic Substances and Disease Registry, to justsix. If the Food and Drug Administration (FDA) follows thisguideline, it will need to slash the current level of mercuryallowable in ocean fish that are sold in the United States belowthe current level of 1 part per million (ppm).

That action would take off the market a significantproportion of the fish now available, especially large predatoryfish like swordfish, shark, and red snapper, and could evenaffect tuna. The team fears that it might also convince consumerswho associate mercury with health dangers to limit their intakeof fish, a remarkably healthy form of nutrition. Under theproposed rules, scientists estimate that the average person wouldbe able to eat only a few ounces of fish per week before bumpingup against the new limit.

"Eating lots of ocean fish isn't much of a hazard comparedto missing out on the benefits from not eating fish," says ThomasClarkson, professor of environmental medicine and aninternationally recognized authority on mercury. Clarkson isprincipal investigator of the study, which is being funded by theNational Institutes of Health, the Food and Drug Administration,and the Republic of the Seychelles.

"A slew of scientific reports have shown that eating fishhelps protect against cardiovascular disease and enhances braindevelopment before and after birth. Fish is a rich source of low-fat protein and is full of fatty acids known to lowercholesterol. Overstating the almost negligible risk of mercurycould adversely affect millions of people who face the risk ofheart disease," says Clarkson. He adds that FDA's currentguideline already helps people avoid excessive mercury exposure,which would be a danger primarily for someone eating frequentmeals of fish like swordfish and shark.

Fish are the primary source of exposure to mercury aroundthe world. Scientists estimate that about half the mercury in theEarth and its atmosphere originates from natural sources such asvolcanoes that belch massive quantities of the substance. Man-made sources include coal-fired power plants, smoke from burningcigarettes, and incinerators that burn items like fluorescentbulbs, batteries, and mercury thermometers. Mercury vapor entersthe atmosphere and falls in rainwater to the Earth. Then, in apoorly understood process in the oceans and other bodies ofwater, microbes play a key role, transforming the mercury into asubstance known as methyl mercury, which works its way up thefood chain and accumulates primarily in large predatory fish,though methyl mercury is found in virtually all fish around theglobe.

While the study focused on healthy fish from ocean waters,its implications spill over into the freshwater arena too.Mercury is one of many pollutants that limit consumption of fishfrom lakes and rivers across the nation, and individual statesrely on federal guidelines when developing recommendations on howmany fish can be eaten per week or month. If federal agencieslower the level of mercury they say is acceptable in the diet,that would likely force states to recommend that residents eatfewer fish from local waters.

The Rochester team is continuing the study and is currentlyanalyzing the same group of children at eight years of age. Thescientists are also working with nutrition experts from theUniversity of Ulster in Northern Ireland to explain an unexpectedfinding: As mercury levels in the children went up, so did theirperformance on tests.

That link could be due to several factors, scientists say."Certainly no one thinks that the increased performance is due tomercury," says Davidson. The scientists caution that the mostobvious explanation -- that fish is so nutritious that thosechildren who ate more were healthier than those who didn't --hasn't been established because the study was not designed tolook at such a link. But these results do show that the tests theteam used are sensitive enough to detect very subtle neurologicaland psychological effects in children, says Davidson.

The Rochester findings are in contrast to those by a teamfrom the University of Odense in Denmark. That team recentlystudied a population in the Faroe Islands, near Iceland, that isexposed to mercury mainly by eating whales as well as fish. Thosescientists found that children who were exposed to mercury pre-natally had slight abnormalities in development at age seven.

The Rochester scientists feel those findings may be relevantto people who eat whale meat but are not convinced they apply topeople eating fish and not whale. Whale meat contains othertoxins and pollutants, like PCBs, and is higher in mercury thanfish. Another key difference is that a community often eats anentire whale in a short period of time, causing a spike inmercury levels that may affect the body differently than lowerlevels.

The White House has organized a meeting for November wherethe two teams and other scientists are expected to discuss thevarying results.

Besides Clarkson, Davidson, and Myers, the team alsoincluded Christopher Cox, associate professor of biostatistics;University of Rochester researchers Catherine Axtell, JeanSloane-Reeves, Elsa Cernichiari, Anna Choi, and Yining Wang;Conrad Shamlaye of the Republic of Seychelles Ministry of Health;Larry Needham of the U.S. Centers for Disease Control andPrevention; and Maths Berlin of the University of Lund inSweden.

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University Of Rochester. (1998, August 27). Commercial Fish: Eat Up, Despite Low Levels Of Mercury. ScienceDaily. Retrieved February 24, 2017 from
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