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Ask Dr. Sue
SEAFOOD AND MERCURY
Dear Dr. Sue,
My toddler loves seafood, especially shellfish. I
remember hearing something quite awhile ago about the
danger of mercury in seafood. Might it be a
problem for someone her age?
Concerned Mom
Dear Mom,
Mercury can be a concern for growing brains, but only if
mercury-containing foods are consumed in really large
amounts.
Mercury is a substance that occurs normally in the
environment. Human beings have added to the
existing levels by some of our activities; mainly the
burning of household and industrial wastes.
(Interestingly, though, museum specimens of tuna caught
from 1879 to 1909 have mercury levels about the same as
those caught recently, so scientists feel that most of
the mercury we are dealing with is the naturally
occurring portion.) Mercury vapor is continually
released into the environment, from where it is
deposited on land and water. Small amounts of the
mercury actually dissolve in the water. This
fraction of mercury can be acted on by bacteria in the
water to become methyl mercury, which is the dangerous
form. Fish living in the water then take the
methyl mercury in through their gills, and it deposits
throughout their bodies, including the muscle that we
eat. They also absorb more mercury through preying
on other fish; therefore, the older and larger a
predator-type fish is, the more mercury is it likely to
contain.
Commercially available fish, including that being
imported from other countries, are tested for mercury
levels. If the mercury present exceeds the FDA's
limit (1 part per million, or 1 ppm), it is rejected.
Sport fishing bypasses this checkpoint, of course.
If you have family members or friends who routinely
offer you their catch, check with the local or state
Health Department for up-to-date information about local
bodies of water and local species. (You can also
call the 24 hour FDA Seafood Hotline at 1-800-FDA-4010
for more information.)
We know what can happen when people consume extremely
high amounts of mercury. Unfortunately, there have been
tragic incidents in Japan in which fish were consumed in
large amounts from areas that were heavily polluted with
mercury. Severe brain damage and death did occur
in a number of people. The average mercury content
of fish samples from those areas contained from 9 to 24
parts per million per fish, and some had levels as high
as 40 ppm.
In the U.S., there has been only one known case of
mercury toxicity from eating fish. This was in a
woman who was on a fad diet, who ate 12 ounces of
swordfish daily for 10 months. She developed
symptoms suggestive of mild mercury poisoning, though it
took awhile for the proper diagnosis to be made.
Caution is still warranted, however, because methyl
mercury crosses the placenta easily. In fact,
levels can be up to 30% higher in the red blood cells of
a fetus than in the mother. There is a concern
about exactly how much more sensitive the growing brain
of an unborn baby might be, and there is no answer
available at this time. Studies are underway in
the Seychelles Islands, in the Indian Ocean. Fish
is the major source of protein for the women of
childbearing age there, and these women are being
followed through their pregnancies, as their children
are born and then as they are weaned. The children
are then watched closely for any sign of nervous system
disorders. The results of this study will help us to
understand whether we need to take other steps to
protect unborn babies from this element.
Meanwhile, the FDA has issued an advisory to pregnant
women and women of childbearing age who COULD become
pregnant about the dangers of eating shark, swordfish,
king mackerel, and tilefish. These are the longer-lived,
larger predator fish with the highest levels of methyl
mercury. It would be wise at this time to avoid
feeding these fish to young children, also. These
same groups of people are advised to eat only an average
of 12 ounces of most other fish per week. I would
advise that you limit your daughter to this amount until
more is known about the effect of methyl mercury on
growing brains. Of course, the average toddler
serving size is usually pretty small; I would be
somewhat surprised if she was eating more than this
amount now.
The FDA did specifically exclude the top 10 seafood
species from the above limits, since the mercury content
in these species is less than 0.2 ppm. These 10
species make up about 80 percent of the seafood consumed
in the U.S., and include canned tuna, shrimp, pollock,
salmon, cod, catfish, clams, flatfish, crabs and
scallops. If your daughter eats these varieties,
she can have all she wants. See the chart below
for some more specific levels (sampling was done between
October 1992 and September 1994):
Fish Name: Species Range (ppm)
(Average (ppm))
Domestic Samples:
-
Catfish: ND - 0.16
(ND=not detected, Or less than 0.10)
-
Cod: ND - 0.17
(0.13)
-
Crab: ND - 0.27
(0.13)
-
Flounder: ND
-
Hake: ND
-
Halibut: 0.12 - 0.63
(0.24)
-
Pollock: ND
-
Salmon (canned): ND
-
Salmon (fresh or frozen): ND
-
Shark: 0.30 - 3.52
(0.84)
-
Swordfish: 0.36 - 1.68
(0.88)
-
Tuna (canned): ND - 0.34
(0.20)
-
Tuna (fresh or frozen): ND -
0.76 (0.38)
Import Samples:
-
Pollock: ND - 0.78
(0.16)
-
Shark: ND - 0.70
(0.36)
-
Swordfish: 0.80 - 1.61
(0.86)
-
Tuna (canned): ND - 0.39
(0.14)
-
Tuna (fresh or frozen): ND -
0.75 (0.27)
 
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