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 ven though the world's fish contain slight
amounts of mercury, eating lots of commercial 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.
 The findings come from a nine-year
University of Rochester study conducted in the Republic of the Seychelles,
an island nation in the Indian Ocean where most people eat nearly a dozen
fish meals each week and whose mercury levels are about 10 times higher
than most U.S. citizens. Indeed, no harmful effects were seen 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 of the 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 from mercury exposure, it's unlikely that somebody who
eats fish twice a week will be affected. And the fish they eat in the Seychelles
contains the same amount of mercury as fish sold at supermarkets and eaten
in the United States."
Adds first author Philip Davidson,
an expert on developmental disabilities who designed a battery of the most
sophisticated tests available to examine the children: "What we found
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 fish bought and sold commercially, at grocery stores, supermarkets,
fish shops, and in restaurants. Those fish are already regulated based on
their mercury levels, and current regulations are sufficient to safeguard
frequent fish eaters against mercury exposure, say the investigators. Consumers
still should follow advisories about eating fish caught in lakes and rivers,
since there are hundreds of polluted waterways whose fish are dangerous
to eat in abundance, often because of other pollutants such as PCBs.
The Seychelles study began in 1989,
when Rochester researchers, with decades of expertise studying mercury exposure,
chose the nation of about 65,000 people as an ideal site to study the effects
of mercury exposure (see sidebar). Myers enrolled 779 newborn children,
about half the births on the islands that year. From the children's mothers,
Myers and the team took samples of hair, which lock in a record of mercury
exposure of the child during gestation.
A neurologist, a childhood development
expert and nurses then studied the children at 6, 19, 29 and 66 months of
age, visiting their homes, talking to their parents, and performing nearly
three dozen sensitive developmental and neurological tests designed to detect
subtle effects of mercury exposure. The analysis included noting when the
children learned to walk and talk, measurements of reflexes, word recognition,
social behavior and the best neuropsychological tests yet developed to evaluate
children at these ages. At each interval, the results of the longitudinal
study have been consistent: no ill effects from a high-fish diet. The JAMA
paper details the 66-month evaluation, which included 711 of the original
children. |
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Mercury is a deadly neurotoxin that
at high levels kills nerve cells, causing blurry vision, lack of coordination,
slurred speech, and even death. Children exposed to high levels of the compound
prenatally 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
lower levels. Late last year, the federal Environmental Protection Agency
(EPA) proposed slashing the amount of mercury that is acceptable for people
to ingest from 30 micrograms per day, the level recommended both by the
World Health Organization and the federal Agency for Toxic Substances and
Disease Registry, to just six. If the Food and Drug Administration (FDA)
follows this guideline, it will need to slash the current level of mercury
allowable in ocean fish that are sold in the United States below the current
level of 1 part per million (ppm).
That action would take off the market
a significant proportion of the fish now available, especially large predatory
fish like swordfish, shark, and red snapper, and could even affect tuna.
The team fears that it might also convince consumers who associate mercury
with health dangers to limit their intake of fish, a remarkably healthy
form of nutrition. Under the proposed rules, scientists estimate that the
average person would be able to eat only a few ounces of fish per week before
bumping up against the new limit.
"Eating lots of ocean fish isn't
much of a hazard compared to missing out on the benefits from not eating
fish," says Thomas Clarkson, professor of environmental medicine and
an internationally recognized authority on mercury. Clarkson is principal
investigator of the study, which is being funded by the National Institutes
of Health, the Food and Drug Administration, and the Republic of the Seychelles.
"A slew of scientific reports
have shown that eating fish helps protect against cardiovascular disease
and enhances brain development before and after birth. Fish is a rich source
of low-fat protein and is full of fatty acids known to lower cholesterol.
Overstating the almost negligible risk of mercury could adversely affect
millions of people who face the risk of heart disease," says Clarkson.
He adds that FDA's current guideline already helps people avoid excessive
mercury exposure, which would be a danger primarily for someone eating frequent
meals of fish like swordfish and shark. |
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Fish are the primary source of exposure
to mercury around the world. Scientists estimate that about half the mercury
in the Earth and its atmosphere originates from natural sources such as
volcanoes that belch massive quantities of the substance. Man-made sources
include coal-fired power plants, smoke from burning cigarettes and incinerators
that burn items like fluorescent bulbs, batteries, and mercury thermometers.
Mercury vapor enters the atmosphere and falls in rainwater to the Earth.
Then, in a poorly understood process in the oceans and other bodies of water,
microbes play a key role, transforming the mercury into a substance known
as methyl mercury. Methyl mercury subsequently works its way up the food
chain and accumulates primarily in large predatory fish, although methyl
mercury is found in virtually all fish around the globe.
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
fish from lakes and rivers across the nation. Individual states rely on
federal guidelines when developing recommendations on how many fish can
be eaten per week or month. If federal agencies lower the level of mercury
they say is acceptable in the diet, that would likely force states to recommend
that residents eat fewer fish from local waters.
The Rochester team is continuing
the study and is currently analyzing the same group of children at eight
years of age. The scientists are also working with nutrition experts from
the University of Ulster in Northern Ireland to explain an unexpected finding:
As mercury levels in the children went up, so did their performance on tests.
That link could be due to several
factors, scientists say. "Certainly no one thinks that the increased
performance is due to mercury," says Davidson. The scientists caution
that the most obvious explanation that fish is so nutritious that those
children who ate more were healthier than those who didn't hasn't been established
because the study was not designed to look at such a link. But these results
do show that the tests the team used are sensitive enough to detect very
subtle neurological and psychological effects in children, says Davidson. |
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The Rochester findings are in contrast
to those by a team from the University of Odense in Denmark. That team recently
studied a population in the Faroe Islands, near Iceland, that is exposed
to mercury mainly by eating whales as well as fish. Those scientists found
that children who were exposed to mercury prenatally had slight abnormalities
in development at age seven. The Rochester scientists feel those findings
may be relevant to people who eat whale meat but are not convinced they
apply to people eating fish and not whale. Whale meat contains other toxins
and pollutants, like PCBs, and is higher in mercury than fish. Another key
difference is that a community often eats an entire whale in a short period
of time, causing a spike in mercury levels that may affect the body differently
than lower levels.
The White House has organized a meeting
for November where the two teams and other scientists are expected to discuss
the varying results.
Besides Clarkson, Davidson, and Myers,
the team also included Christopher Cox, associate professor of biostatistics;
University of Rochester researchers Catherine Axtell, Jean Sloane-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 and Prevention; and Maths Berlin of the University of Lund
in Sweden.  |
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