In the interest of full disclosure, I'm a marine biologist specializing the population dynamics and conservation of tuna fish in the Pacific Ocean. My expertize in chemistry is 30 years out of date and is pretty much a dim memory. Nevertheless, people often ask "Is it safe to eat tuna?" when I tell them my profession. Around 1997, these questions became more common, and I set out to educate myself on the issue.
I'll fess up to a bit of a priori skepticism at the start. Lots of people around the Pacific in California, Japan, Hawaii, and other Pacific Islands eat fish or canned tuna several times a week. These places are not generally populated by folks with serious neurological afflictions. Perhaps my acquaintance network is biased, but I don't know anyone who might be suffering from mercury poisoning. Most of my geeky school friends and I ignored our mothers' advice and made coins shine with mercury salvaged from broken thermometers. So, applying the common sense test, if chronic low-level mercury exposure have adverse long-germ effects, they are going to be hard to detect.
National Academy of Sciences Report
In 2000, National Research Council (NRC) published a report of the findings of a United States National Academy of Sciences review of methyl mercury (MeHg) toxicity. The review was commissioned and funded by the US Environmental Protection Agency (EPA). The NRC report is a good starting place in a search for information. The report leaves no doubt that MeHg is pretty scary stuff. Two infamous examples, contamination of seafood by industrial pollution in Japan (see Minimata Disease) and the consumption of bread baked from seed grain preserved with mercury in Iraq, provide ample evidence of the horrific effects of acute MeHg toxicity. Evaluating the effects of chronic exposures to low doses of a toxin, however, is a classic "hard problem" in environmental toxicology. The case of MeHg is no exception.The NRC report reviews several studies attempting to find a link between maternal mercury consumption and neurological development of children. Three of these studies, conducted in the late 1980s and 1990s, are judged to be of highest scientific quality: Faroe Islands (North Atlantic), Seychelles (Indian Ocean), and New Zealand (South Pacific). One of the tasks set before the NRC panel was to recommend to the EPA which of these studies to use in preparing recommendations on the appropriate reference dose for Hg exposure.
These studies estimate maternal exposure by measuring mercury concentration in hair or umbilical cord samples. They estimate effects on neurological development by subjecting children a battery of tests meant to gauge neurological development. Not surprisingly, the results were highly variable and required serious statistical manipulation to reach conclusions. In short, the Seychelles study found no adverse effects of maternal mercury consumption on child development. The Faroes study found clear evidence of lower performance related to maternal mercury consumption on one test. The New Zealand study produced ambiguous results. One interesting, but little remarked, result was that in many of the studies reviewed by the NRC there were occasional results where the test results appeared to be improved by maternal mercury consumption.
Such variability is expected since these studies were conducted in quite different environments. The people concerned are ethically heterogeneous, and their diets are drastically different. Faroe Islanders eat significant amounts of pilot whale meat, know to have 10 times the mercury content of most fish.
The lack of definitive results and the occasional counter intuitive result suggest to me that we are missing part of the picture.
Got Selenium?
Selenium is an essential element with important biochemical functions. Selenium containing enzymes appear to be particularly important as anti-doxidants, for brain function, and for the proper development and function of both the immune and nervous systems. Mercury and selenium combine chemically, atom for atom, to form a highly stable compound. Mercury selenide is found in nature as the mineral Tiemannite. One hypothesis for the mechanism of mercury toxicity is that the mercury combines with the selenium causing a selenium deficiency.Concern over mercury in tuna and other fish is not new. It was well known in the 1960s that the mercury content of tuna and swordfish exceed guidelines. One study from that period (Ganther et al 1972) showed that laboratory animals survived better when either selenium or tuna was added to diets laced with MeHg.
More recently, Nicholas Ralston and colleagues from the University of North Dakota published a results of and EPA funded study that showed that dietary selenium protects rats against mercury toxicity. They showed that mercury decreases the ability of the blood to delver selenium to the brain, possibly through the formation HgSe complexes. Increasing selenium in the diet allows more selenium to be delivered to the brain. Ralston and his colleagues conclude
" ... that seafood safety evaluations of health risks associated with Hg exposure would be improved by concurrent assessments of Se availability. Criteria that incorporate Hg to Se molar relationships will provide more comprehensive evaluations that will enhance environmental health ssessments and improve public safety."
Ralston then teamed up with John Kaneko, a Honolulu seafood safety consultant, to examine the selenium content of fish caught in the Hawaii-based longline fishery in a study funded by the National Oceanic and Atmospheric Administration. They found that the selenium content equaled or exceed the mercury content in all fish sampled, except mako shark. Here is Figure 2 from their report.
The National Institutes of Health recognize the health benefits of selenium and provide a table of natural sources. The top two sources of dietary selenium are Brazil nuts (a one ounce serving will give you 8 times your daily value) and canned tuna (a three ounce serving will supply 95% of your daily value).
Eat your fish
Joseph Hibblen and a team of scientists for the United States and the United Kingdom, analysed data from a large study of maternal and child health in the UK. This study, the Avon Longitudinal Study of Parents and Children (ALSPAC), involved some 14,000 mothers enrolled during pregnancy in 1991 and 1992. The health and development of their children has been followed in great detail ever since. The statistical analysis by Hibblen et al was supported by the Intramural Research Program of the National Institute on Alcohol Abuse and Alcoholism, National Institutes of Health and National Institute of Mental Health.Seafood consumption was estimated in this study by sending questionnaires to a sample of the ALSPAC mothers. The self-repoted estimates of seafood consumption were validated by correlation with biochemical markers, including ironically, umbilical cord mercury content. The authors concude
"... we recorded no evidence to lend support to the warnings of the US advisory that pregnant women should limit their seafood consumption. By contrast, we noted that children of mothers who ate small amounts (< 340 g per week) of seafood were more likely to have suboptimum neurodevelopmental outcomes than children of mothers who ate more seafood than the recommended amounts." [emphasis added]
Who to believe?
In a comment on the Hibbeln article in Lancet, Gary Myers (one of the authors of the Seychelles study), notes that"A survey in the USA showed that two-thirds of Americans believe that 1000–100 000 US children are poisoned by mercury from eating fish every year. In fact, there has never been even one child with prenatal mercury poisoning from consuming fish documented outside Japan."
At the other extreme, conservationist groups more interested in protecting turtle health and fish populations than in protecting human health, claim that eating tuna is outright dangerous.
One thing is certain: you should not follow medical advice from marine biologists and conservationists. Look at the sources cited below. Consult someone who is actually committed to protecting your health.
Information Sources:
The FDA website is populated with information and advisories on mercury in fish. For example,
FDA, EPA Revise Guidelines on Mercury in Fish
Committee on the Toxicological Effects of Methylmercury, Board on Environmental Studies and Toxicology, National Research Council, "Toxicological Effects of Methyl Mercury", National Academy Press, Washington D.C., 2000.
The Mercury Morass on the University of Wisconsin's Why Files site, generally a pretty good site for the "science behind the news".
H. E. Ganther; C. Goudie; M. L. Sunde; M. J. Kopecky; P. Wagner; Sang-Hwan Oh; W. G. Hoekstra, 1972. Selenium: Relation to Decreased Toxicity of Methylmercury Added to Diets Containing Tuna.
Science, 175(4026):1122-1124.
Nicholas V. C. Ralston, J. Lloyd Blackwell III and Laura J. Raymond. 2007. Importance of Molar Ratios in Selenium-Dependent Protection Against Methylmercury Toxicity. Biol Trace Elem Res 119:255–268. DOI 10.1007/s12011-007-8005-7.
J. John Kaneko and Nicholas V. C. Ralston. 2007. Selenium and Merecury in Pelagic Fishe in the Central North Pacific Near Hawaii. Biol Trace Elem Res 119:242-254, DOI 10.1007/s12011-007-8005-8.
Joseph R Hibbeln, John M Davis, Colin Steer, Pauline Emmett, Imogen Rogers, Cathy Williams, Jean Golding. 2007. Maternal seafood consumption in pregnancy and neurodevelopmental outcomes in childhood (ALSPAC study): an observational cohort study. Lancet 369: 578–85.
Gary J Myers, Philip W Davidson. 2007. Maternal fish consumption benefits children’s development. Lancet 369:357-58.
1 comment:
that was well balanced and interesting. Keep up the good work. However, your post did contain some typos that you may wish to correct.
Rgds
Agger Fish Corp.
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