GJ, PW Davidson, C Cox, CF Shamlaye, D Palumbo, E Cernichiari, J
Sloane-Reeves, GE Wilding, J Kost, L-S Huang, and TW Clarkson. 2003.
Prenatal methylmercury exposure from ocean fish consumption
in the Seychelles child development study. Lancet
study challenges previous findings that fetal exposure to relatively
low levels of methyl mercury resulting from maternal consumption
of contaminated fish can harm neurological development.
research team, led by Dr Gary Myers from the University of Rochester
Medical Center, reports that methyl mercury in the womb does not
harm development of a baby's brain, at least at
levels experienced because of ocean fish consumption. The
weight of the evidence on mercury's impacts, however, still warrants
strong protections, and justifies strengthening current US FDA standards
to be consistent with the US EPA and National Academy of Sciences.
did they do? Myers et al. carried out a series
of mental and motor tests on 9-yr old children born on the Seychelles
Women living on the Seychelles eat ocean-going fish carrying methyl
mercury levels comparable to that found in comparable fish in the
US (roughly 0.3 µg/g),
but the fish consumption rate in the Seychelles is significantly
higher. Hence babies in the womb in the Seychelles are exposed to
more methyl mercury than is typical for US babies.
tested children had been enrolled in this ongoing research project
at birth, when samples of their mother's hair were taken to determine
mercury levels experienced in pregnancy. During infancy and childhood,
the research team conducted a series of psychomotor tests on the
subjects, at ages 6, 19, 29 and 66 months of age, none of which
had revealed developmental effects of mercury.
current measurements, conducted when the enrolled children were
9 years old, tested a series of psychomotor endpoints, including
neurocognitive, memory, motor, perceptual-motor and behavioral functions.
Other variables known to affect cognitive development, for example
the home environment, maternal IQ and socioeconomic status were
also assessed and treated as potentially confounding variables in
the statistical analyses the team conducted.
did they find? Myers's group obtained measurements from
779 infants whose mothers ate an average of 12 fish meals per week.
pattern linking higher methyl mercury exposures to adverse neurocognitive
scores emerged in the analysis. Two measured variables at first
appeared to be related to mercury levels, but statistical considerations
indicated that the relationships were most likely due to chance.
research team did confirm, however, consistent associations between
some of the confounding variables (e.g., maternal IQ) and neurocognitive
development, thus indicating their testing methodologies could have
detected mercury effects, were they to have occurred.
does it mean? Viewed in isolation, these results might
be interpreted to ease concerns about low level mercury exposures
associated with ocean-going fish consumption and the possible impact
on fetal development, at least beneath current US FDA exposure standards.
According to the lead scientist, Dr. Gary Myers, current FDA standards
"are reasonable." Existing standards, for example, recommend
that pregnant women avoid certain highly contaminated fish like
swordfish altogether, and limit consumption of certain other species
to no more than 12 ounces per week.
is good news if it means that ongoing mercury contamination has
not yet been sufficient to force all ocean-going fish, an important
source of nutrition, out of diets.
one possible interpretation of the results is that the positive
effects of eating fish oils may be protective against adverse effects
of mercury. The research group led by Myers is now attempting to
test this hypothesis.
two other extensive studies of mercury impacts on fetal development
have shown mercury to have deleterious effects. One of
these studies is of children born on the Faroe
mothers ate whale meat and blubber. The other is from New Zealand,
where the exposure is via consumption of contaminated shark meat
used in fish and chips.
least four possible explanations for the conflicting results are
now being explored.
focuses on the fact that the shark muscle and whale meat/blubber
on the Faroe Islands and in New Zealand carry much higher methyl
mercury concentrations than does the fish eaten on the Seychelles.
Thus even if the Seychelle Islanders ate cumulatively as much
mercury as the other study populations, because fish formed a
larger and more consistent portion of the diet, mercury would
have reached fetal tissue in a relatively low-level continuous
stream on the Seychelles, as compared to a more burst like pattern
with brief but higher exposure levels in the Faroes and New Zealand.
These higher burst exposures may have different effects than more
continuous but lower exposures. Recent analysis has suggested,
however, that the Faroe Island exposures are relatively continuous
instead of burst-like.
second builds from the observation that the whale meat consumed
on the Faroe Islands not only contains mercury but also a collection
of persistent chlorinated compounds, including PCBs, and that
these may either cause the effect or interact with mercury to
influence neurological development. This interpretation, however,
would not explain the New Zealand results, because the shark meat
consumed there does not contain significant PCBS. Moreover,
the Faroe Island study took great care in separating the effects
of mercury from the effects of PCBs. Their mercury effect is most
likely real, and independent of simultaneous PCB exposures. This
interpretation is supported by independent analysis by the U.S.
National Academy of Sciences.
Faroe Island study measured mercury in umbilical cord blood and
hair whereas the Seychelles study used hair samples. Other research
has suggested that hair samples may be less accurate. Hence it
may be that had the Seychelles work measured mercury in cord blood,
they would have been able to find effects at lower levels. This
interpretation, however, is not universally accepted.
a combination of reasons related to study design and execution,
the Seychelles study does not have sufficient statistical power
to detect small differences.
at the moment these studies collectively do not provide clear signals
for how parents or public health officials should make decisions
about consuming fish contaminated with low levels of mercury. The
current study suggests existing standards are adequate; the others
suggest they should be tightened.
issues of public health, particularly where irreversible damage
may be at stake, when two well-designed and well-executed
studies conflict in their conclusions, the prudent course is to
structure public health regulations based on the study that shows
adverse effects rather than those showing no effect. The
Faroe Island and New Zealand studies both indicate that current
FDA standards remain too lax.
wishing to exercise caution might reasonably:
continue to keep ocean fish such as Pacific salmon, flounder and
haddock in family diets because of the nutritional benefits;
limit consumption to no more than one meal a month of moderately
contaminated fish like canned tuna, mahi mahi, blue mussel, cod,
pollock, salmon from the Great Lakes; and
species known to be relatively high in mercury, including swordfish,
tilefish, king mackeral and tuna steaks.
that data from studies by the Mobile Register in Alabama suggest
that special circumstances in the Gulf of Mexico related to disposal
of oil drilling muds have lead to unexpectedly high methyl mercury
contamination in predatory sport fish in these waters. For access
to these studies see (1) the introductory
exposé published in the Mobile Register on 22 July 2001;
and (2) a list
of all articles in the series.