CONTACT: Tom Rickey, (716) 275-7954, [email protected]
(Embargoed for release at 4 p.m. Tuesday, August 25, 1998, per JAMA embargo)
Commercial Fish: Eat Up, Despite Low Levels of Mercury
Even 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, and 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.
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 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 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.
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, which works its way up the
food chain and accumulates primarily in large predatory fish,
though 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, and 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.
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 pre-
natally 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.
HOW MUCH MERCURY IS TOO MUCH? QUESTIONS AND ANSWERS
How is mercury exposure measured?
Scientists determine a person's exposure to mercury by measuring
the content in a strand of hair. By comparing these levels to the
results of very sophisticated batteries of tests, scientists try
to determine the lowest level that might be harmful.
At what level does mercury become harmful?
The World Health Organization's guidelines maintain that the
lowest level that could possibly be harmful to humans is 5 parts
per million (ppm). This level is based on scientific results from
the 1960s that placed the level at which risk begins at 50 ppm
for most people; WHO then applied a safety factor of 10, deciding
that a level of 5 or less is safe for even the most vulnerable
populations.
Now the University of Rochester team has conducted an extensive
study in the Seychelles Islands of the most sensitive population
-- young children -- where the average level is about 7 ppm,
about 10 times the level of the U.S. population. The scientists
found no harm from mercury at levels up to 15 ppm, nearly twice
the average Seychelles level and about 20 times higher than the
average U.S. level.
How do these numbers translate into people's lives?
Scientists typically calculate how much mercury a person could
ingest each day before bumping up into the danger zone, and then
regulatory agencies issue advisories based on those numbers. In
the past, using the WHO guideline, EPA has recommended that a
person ingest no more than 30 micrograms of mercury per day.
Based on estimates of U.S. fish consumption, FDA recommended that
only commercial fish with less than 1 ppm of mercury be sold.
Nearly all fish caught in the oceans meet this criterion.
Are these recommendations being changed?
Possibly. Late last year the EPA recommended to Congress that the
tolerable daily intake of mercury be dropped to just one-fifth
the current allowable level, to about 6 micrograms per day. For
Americans to reach this level, FDA would need to slash the amount
of mercury acceptable in commercial fish. This would cut the
proportion of some ocean fish that could be sold and would put
pressure on sources of mercury, such as coal-fired power plants,
to reduce mercury emissions.
How does the Seychelles data affect the proposed changes?
The new EPA number is based on data the Rochester group collected
in Iraq after a mercury poisoning event in 1972. The researchers
believe the Seychelles data, which do not support lowering the
recommended daily intake of mercury, is far superior.
"There are many reasons you shouldn't determine human health risk
based on the Iraqi data," says Clarkson. "That was a poisoning
event, much different from a low-level exposure to mercury, and
the mercury source was contaminated grain, not fish, which is how
most people are exposed. In addition, the possible risk at low
levels was determined by just a few cases.
"People have been taking our old data from Iraq and coming
out with very low numbers for tolerable exposure. We who had done
the study knew their numbers were not very solid. That's why we
undertook this extensive Seychelles study."
WHY GO TO THE SEYCHELLES TO STUDY MERCURY EXPOSURE?
Some factors that make the Seychelles an ideal site to study
the effects of mercury exposure:
*The people eat a lot of fish, oftentimes twice a day.
Mothers in the study ate an average of 12 fish meals per week.
*There are no local sources of mercury pollution. The
population's exposure to mercury comes almost completely from
ocean fish, which have about the same amount of mercury
throughout the world.
*The nation boasts high-quality free medical care and
education.
*Most people born on the islands stay there, making the
population easy to track over a number of years. (Investigators
were able to evaluate about 90 percent of the original children
five and one-half years after the study began.)
*Women drink and smoke very little, eliminating those habits
as factors in their children's pre-natal development.
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