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Large numbers of individuals suffered severe neurological effects from poisoning with methyl mercury from the consumption of fish contaminated by industrial discharge in Minamata in the 1950s and 1960s and from bread made from fungicide-treated wheat in the early 1970s in Iraq. In both episodes, infants exposed in utero showed developmental delays. Data from Iraq permitted the estimation of dose-response relationships and provided evidence of greater sensitivity of the foetal brain compared to adult. Studies in a number of communities regularly consuming fish have sought to determine whether and at what level of prenatal exposure neurodevelopment defects may be detected, but the findings do not present a consistent picture. Benchmark analysis using data from Iraq and the three largest studies (New Zealand, Faroes, and Seychelles) suggest an overlap in estimated dose corresponding to a 10% increase in prevalence of adverse effects above background level. However, the Seychelles study, examining children up to 9 years of age, has not found a consistent pattern of adverse developmental effects; a possible explanation may be the presence of micronutrients in the ocean fish diet that enhance brain development and/or counter the toxic effects of methyl mercury.

This review examines the role of nutrients in child development and outlines the key nutrients identified as potentially important to neurodevelopment among high fish consumers in the Seychelles Child Development Nutrition Study (SCDNS). It describes the clinical assessment of these nutrients in the blood and breast milk samples collected from the cohort of 300 pregnant women who were recruited, at their first antenatal visit, on the SCDNS. These key nutrients include the long chain polyunsaturated fatty acids (LCPUFA), docosohexaenoic acid (DHA) and arachidonic acid (AA), both of which may affect neurodevelopment in the later stages of fetal growth. Only DHA, however, is strongly associated with fish consumption, the predominant source of the neurotoxicant methyl mercury (MeHg). Any benefits of increased selenium status on neurodevelopment are likely to accrue via detoxification of MeHg during fetal growth, while benefits of optimal iodine or thyroid status are likely to be directly related to neurodevelopment during late fetal growth. Unlike LCPUFA, Se, and I, the status of the B vitamins, folate, vitamin B12, vitamin B6, and riboflavin are unlikely to be closely related to fish consumption but the status of each of these B vitamins is likely to impinge on overall status of choline, which is expected to have direct effects on neurodevelopment both prenatally and postnatally and may also impact on MeHg toxicity. Choline status, together with the status of two other candidate nutrients, zinc and copper, which are also likely to have effects on neurodevelopment prenatally and postnatally, are expected to have some correlation with fish consumption.

Evidence from the Seychelles Child Development Nutrition Study suggests that maternal nutritional status can modulate the relationship between prenatal methylmercury (MeHg) exposure and developmental outcomes in children. The aim of this study was to investigate whether maternal PUFA status was a confounding factor in any possible associations between prenatal MeHg exposure and developmental outcomes at 5 y of age in the Republic of Seychelles. Maternal status of (n-3) and (n-6) PUFA were measured in serum collected at 28 wk gestation and delivery. Prenatal MeHg exposure was determined in maternal hair collected at delivery. At 5 y of age, the children completed a comprehensive range of sensitive developmental assessments. Complete data from 225 mothers and their children were available for analysis. Multiple linear regression analyses revealed Preschool Language Scale scores of the children improved with increasing maternal serum DHA [22:6(n-3)] concentrations and decreased with increasing arachidonic acid [20:4(n-6)] concentrations, albeit verbal intelligence improved with increasing (n-6) PUFA concentrations in maternal serum. There were no adverse associations between MeHg exposure and developmental outcomes. These findings suggest that higher fish consumption, resulting in higher maternal (n-3) PUFA status, during pregnancy is associated with beneficial developmental effects rather than detrimental effects resulting from the higher concomitant exposures of the fetus to MeHg. The association of maternal (n-3) PUFA status with improved child language development may partially explain the authors' previous finding of improving language scores, as prenatal MeHg exposure increased in an earlier mother-child cohort in the Seychelles where maternal PUFA status was not measured.

ObjectiveTo assess the nutritional adequacy of Seychellois children in relation to nutrients reported to be important for cognitive development.DesignDietary intakes were assessed by 4 d weighed food diaries and analysed using dietary analysis software (WISP version 3·0; Tinuviel Software, UK). Individual nutrient intakes were adjusted to usual intakes and, in order to investigate adequacy, were compared with the UK Estimated Average Requirements for children aged 4–6 years.SettingChildren 5 years old were followed up as part of the Seychelles Child Development Nutrition Study (SCDNS), located in the high-fish-consuming population of Mahé, Republic of Seychelles.SubjectsAnalysis was carried out on a sample of 229 children (118 boys, 111 girls).ResultsChildren consumed a diet of which fortified cereal and milk products contributed the most to nutrient intakes. The majority (≥80 %) of children met requirements for several nutrients important for child development including Fe, folate and Se. Adjusted dietary intakes of Cu, Zn, iodine, niacin and vitamin A were below the Estimated Average Requirement or Recommended Nutrient Intake. Mean adjusted energy intakes (boys 4769 kJ/d (1139·84 kcal/d), girls 4759 kJ/d (1137·43 kcal/d)) were lower than the estimated energy requirement (boys 5104 kJ/d (1220 kcal/d), girls 5042 kJ/d (1205 kcal/d)) for 88 % of boys and 86 % of girls.ConclusionsNutrition was adequate for most children within the SCDNS cohort. Low intakes of some nutrients (including Zn, niacin and vitamin A) could reflect nutritional database inaccuracies, but may require further investigation. The study provides valuable information on the adequacy of intakes of nutrients which could affect the growth and development of Seychellois children.

Background Dental amalgams contain approximately 50 percent metallic mercury and emit mercury vapor during the life of the restoration. Controversy surrounds whether fetal exposure to mercury vapor resulting from maternal dental amalgam restorations has neurodevelopmental consequences. Methods The authors determined maternal amalgam restoration status during gestation (prenatal exposure to mercury vapor [Hg 0 ]) retrospectively in 587 mother-child pairs enrolled in the Seychelles Child Development Study, a prospective longitudinal cohort study of the effects of prenatal and recent postnatal methylmercury (MeHg) exposure on neurodevelopment. They examined covariate-adjusted associations between prenatal maternal amalgam restoration status and the results of six age-appropriate neurodevelopmental tests administered at age 66 months. The authors fit the models without and with adjustment for prenatal and recent postnatal MeHg exposure metrics. Results The mean number of maternal amalgam restorations present during gestation was 5.1 surfaces (range, 1–22) in the 42.4 percent of mothers who had amalgam restorations. The authors found no significant adverse associations between the number of amalgam surfaces present during gestation and any of the six outcomes, with or without adjustment for prenatal and postnatal MeHg exposure. Results of analyses with the secondary metric, prenatal amalgam occlusal point scores, showed an adverse association in boys only on a letter- and word-identification subtest of a frequently used test of scholastic achievement, whereas girls scored better on several other tests with increasing exposure. Conclusions This study’s results provide no support for the hypothesis that prenatal Hg 0 exposure arising from maternal dental amalgam restorations results in neurobehavioral consequences in the child. These findings require confirmation from a prospective study of coexposure to MeHg and Hg 0 .

Maternal consumption of fish during the gestational period exposes the fetus to both nutrients, especially the long-chain polyunsaturated fatty acids (LCPUFAs), believed to be beneficial for fetal brain development, as well as to the neurotoxicant methylmercury (MeHg). We recently reported that nutrients present in fish may modify MeHg neurotoxicity. Understanding the apparent interaction of MeHg exposure and nutrients present in fish is complicated by the limitations of modeling methods. In this study we fit varying coefficient function models to data from the Seychelles Child Development Nutrition Study (SCDNS) cohort to assess the association of dietary nutrients and children's development. This cohort of mother-child pairs in the Republic of Seychelles had fish consumption averaging 9 meals per week. Maternal nutritional status was assessed for five different nutritional components known to be present in fish (n-3 LCPUFA, n-6 LCPUFA, iron status, iodine status, and choline) and associated with children's neurological development. We also included prenatal MeHg exposure (measured in maternal hair). We examined two child neurodevelopmental outcomes (Bayley Scales Infant Development-II (BSID-II) Mental Developmental Index (MDI) and Psychomotor Developmental Index (PDI)), each administered at 9 and at 30 months. The varying coefficient models allow the possible interactions between each nutritional component and MeHg to be modeled as a smoothly varying function of MeHg as an effect modifier. Iron, iodine, choline, and n-6 LCPUFA had little or no observable modulation at different MeHg exposures. In contrast the n-3 LCPUFA docosahexaenoic acid (DHA) had beneficial effects on the BSID-II PDI that were reduced or absent at higher MeHg exposures. This study presents a useful modeling method that can be brought to bear on questions involving interactions between covariates, and illustrates the continuing importance of viewing fish consumption during pregnancy as a case of multiple exposures to nutrients and to MeHg. The results encourage more emphasis on a holistic view of the risks and benefits of fish consumption as it relates to infant development.

The nutritional status of women during pregnancy influences physiological outcomes in the child, including birth size(1,2), later risk of CVD and diabetes(3,4) and cognitive function(5). Despite some concerns about exposure to methylmercury(6), fish consumption during pregnancy provides an excellent source of dietary protein(7) as well as a number of micronutrients essential for fetal development(8,9) such as Fe, iodine, Zn, Se, choline and long-chain PUFA. Iodine and Fe deficiencies are two of the most common nutritional deficiencies in the world. An estimated 30% of the world’s population inhabits areas of iodine deficiency (10) while Fe deficiency anaemia affects up to 50% of pregnant women in developing(11) countries and up to 25% of children under the age of 3 years, with higher rates observed in developing countries(10). Correction of iodine deficiency in pregnancy can be achieved with supplementation and is associated with improved psychomotor test scores in infants(12). Fe supplementation in children can improve mental and motor scores in standardised developmental assessments(13). Maternal Zn status has been linked with infants’ early behaviour(14), with Zn supplementation during pregnancy associated with increased motor activity in the offspring(15). Se is also a vital component of the maternal diet with essential roles in fetal development(16). Se might influence fetal development directly by interacting with iodine in regulating thyroid function(17) and might also have a protective role in the prevention of methylmercury toxicity(18). Long-chain PUFA play an important structural role in neural tissue, especially the brain and retina(19). Fetal accretion is at its greatest in the third trimester of pregnancy (20) and supplementation with long-chain PUFA in pregnancy has been shown to improve cognitive function (21). Choline, a nutrient that can be synthesised de novo in the body, appears to be a conditionally essential dietary nutrient for optimal brain development both pre- and postnatally(22). Supplementation with choline in animal models has indicated a life-long enhancement in spatial memory(23) and cognitive function(24). The Republic of Seychelles is a small tropical archipelagic state in the Indian Ocean with one of the highest per capita rates of fish consumption in the world(25). The population consumes a traditional diet based around high fish consumption in conjunction with a high intake of fruit and vegetables. Such a diet would be expected to provide optimal nutrient intake with respect to those micronutrients of importance in infant development. Evidence has indicated that fish consumption has decreased by up to one-third over the last two decades as the Seychellois population adopts a more Western-style diet and life-style (26). This has led to increased concern that if these trends continue, micronutrient status may be compromised. However, no study to date has examined in detail dietary patterns during pregnancy in this population. The aims of the current project, therefore, were to characterise the diets of pregnant Seychellois women and to determine the role that fish play in promoting adequate intakes of nutrients important for fetal and neonatal development.

Fe is an essential nutrient throughout the life cycle. Maternal Fe status is particularly important during pregnancy, where increased demands are placed on Fe stores in order to supply the growing fetus and placenta and to sustain the associated biological changes(1). Fe requirements and Fe absorption rates, owing to the cessation of menses, are decreased in the first trimester of pregnancy but rise progressively during the second and third trimesters. Fe deficiency remains the most common nutritional deficiency in the world(2) and Fe-deficiency anaemia (IDA) is the most common nutritional deficiency in pregnancy, particularly in developing countries(1,3). The generally accepted cut-off level for Hb and serum ferritin, below which Fe stores are considered to be depleted, are

The Seychelles Child Development Study was designed to test the hypothesis that prenatal exposure to MeHg from maternal consumption of a diet high in fish is detrimental to child neurodevelopment. To date, no consistent pattern of adverse associations between prenatal exposure and children's development has appeared. In a comprehensive review of developmental studies involving MeHg, a panel of experts recommended a more consistent use of the same endpoints across studies to facilitate comparisons. Both the SCDS and the Faeroe Islands studies administered the Bender Visual Motor Gestalt Test. However, the method of test administration and scoring used was different. We repeated the test on the SCDS Main Study children (mean age 10.7 years) using the same testing and scoring procedure reported by the Faeroe studies to obtain Copying Task and Reproduction Task scores. We found no association between prenatal MeHg exposure and Copying Task scores which was reported from the Faeroese study. However, our analysis did show a significant adverse association between MeHg and Reproduction Task scores with all the data (p = 0.04), but not when the single outlier was removed (p = 0.07). In a population whose exposure to MeHg is from fish consumption, we continue to find no consistent adverse association between MeHg and visual motor coordination.

Background:Both folate and betaine are important predictors of total homocysteine (tHcy) during pregnancy. However, studies to date have only been undertaken in populations with Western dietary patterns. Objective: We investigated the predictors of tHcy in pregnant women recruited in the Seychelles, a population where access to fortifiedfoodsislimitedandwherewomenhabituallyconsumediets rich in fish, eggs, rice, and fruit. Design: Pregnant women (n 226) provided blood samples at enrollment,atweek28ofgestation,andatdelivery.Cordbloodwas obtained from a subset of participants (n 135). Results: As in other studies, maternal tHcy was lower during pregnancy than at delivery, whereas folate and vitamin B-12 status declinedsignificantlytodelivery.Despitelowmaternalfolatestatusat delivery (median: 9.0 nmol/L), with 35% of women in the deficient range(serumfolate:6.8nmol/L),cordbloodfolatestatus(median: 40.2 nmol/L) was similar to concentrations reported in Western populations. Folate was a significant predictor of tHcy at all time points (P 0.001). In contrast with previous studies, betaine was only a significant predictor of maternal tHcy (P 0.001) when the essential amino acid methionine was low. Conclusions: The current study reports 2 important findings. First, fetal requirements for folate are paramount, such that cord blood folate statusismaintained,evenwhenmaternalstatusislow.Second,betaine is a significant predictor of tHcy in pregnant women with low serum folate and low serum methionine concentrations. Am J Clin Nutr 2008;87:391–7.

Objective. The grafting of human scalp hair was used as a new application of this method to explore methyl mercury incorporation into human hair and to validate this model for mercury monitoring in hair. Methods. Human scalp grafts were transplanted to athymic BALB/c nude mice. The animals were exposed to methyl mercury either as a single dose i.p. or continuously for 4 months, using ALZET osmotic pumps. The mercury concentration in hair was determined using x-ray fluorescence (XRF) spectrometry by segmental (2 mm) analysis of a single strand, and tissue concentrations were measured by cold vapor atomic absorption analysis. Results. Human scalp hair grown in nude mice showed long-term persistence of human features including the expression of histocompatibility antigens (KAB 3, W 6/32, SF 1-1.1.1) and normal hair morphometry. The disposition of methyl mercury in nude mice followed a one-compartment model with a whole body elimination half-life of 6.7 days (elimination constant, k = 0.1/day). Autoradiographic studies revealed that methyl mercury was rapidly incorporated into areas of the hair follicle undergoing active keratinization. Methyl mercury concentrations in human hair transplanted onto nude mice were two orders of magnitude higher than in blood and attained a mean hair: blood ratio of 217 : 1, similar to ratios reported only in human studies. Conclusions. This study demonstrated that human hair grown on nude mice can record the level of exposure to methyl mercury and can serve as a valuable research tool to study mercury incorporation into human hair. Copyright © 2007 John Wiley & Sons, Ltd.

Several biological media have been used as indicators of the fetal body burden of methylmercury and the levels in the primary target tissue, the developing brain. These media include maternal hair and blood. The relative merits of these media will be considered both with regard to current knowledge of the physiology of mercury disposition in the body and also the practicality of field application with respect to sample, collection, transport, storage and processing.

Thimerosal, which releases the ethyl mercury radical as the active species, has been used as a preservative in many currently marketed vaccines throughout the world. Because of concerns that its toxicity could be similar to that of methyl mercury, it is no longer incorporated in many vaccines in the United States. There are reasons to believe, however, that the disposition and toxicity of ethyl mercury compounds, including thimerosal, may differ substantially from those of the methyl form. The current study sought to compare, in neonatal mice, the tissue concentrations, disposition and metabolism of thimerosal with that of methyl mercury. ICR mice were given single intramuscular injections of thimerosal or methyl mercury (1.4 mg Hg kg(-1)) on postnatal day 10 (PND 10). Tissue samples were collected daily on PND 11-14. Most analysed tissues demonstrated different patterns of tissue distribution and a different rate of mercury decomposition. The mean organic mercury in the brain and kidneys was significantly lower in mice treated with thimerosal than in the methyl mercury-treated group. In the brain, thimerosal-exposed mice showed a steady decrease of organic mercury levels following the initial peak, whereas in the methyl mercury-exposed mice, concentrations peaked on day 2 after exposure. In the kidneys, thimerosal-exposed mice retained significantly higher inorganic mercury levels than methyl mercury-treated mice. In the liver both organic and inorganic mercury concentrations were significantly higher in thimerosal-exposed mice than in the methyl mercury group. Ethyl mercury was incorporated into growing hair in a similar manner to methyl mercury. The data showing significant kinetic differences in tissue distribution and metabolism of mercury species challenge the assumption that ethyl mercury is toxicologically identical to methyl mercury.

Background: The Seychelles Child Development Study (SCDS) has been longitudinally following a cohort of over 700 children enrolled in 1989. Their mothers consumed a diet high in fish during pregnancy. Repeated examination of the SCDS cohort at six different ages through age 11 years has shown no pattern of adverse effects. Some early appearing beneficial associations between both prenatal and postnatal hair MeHg and several child development endpoints were noted. We hypothesized these might be related to micronutrients in the fish, but they were not found when the children reached middle school age. These findings suggest that the associations observed between MeHg and developmental outcomes may vary with developmental stage. Method: We examined the main cohort of the SCDS to determine if this might be true using a longitudinal multiple regression analysis design that focused on those endpoints that were repeatedly measured at different ages. The primary endpoint analyzed was global cognition, involving a measure of developmental quotient or IQ. Secondary analyses included other domains such as Reading and Mathematics scholastic achievement, social behavior, and memory. Analyses involved two different approaches, one including incorporation of a passage of time variable, the other including a difference of scores across time points. Results: No significant associations were found between prenatal MeHg exposure and any of the repeatedly measured endpoints. Conclusions: These results suggest that even when individual subject variance is controlled there was no consistent pattern of associations between child development outcomes and prenatal exposures to MeHg from maternal consumption of a diet high in fish. The Seychellois diet contains about 10 times more ocean fish than is typically consumed by US citizens. Our primary focus on IQ should further inform growing scientific interest in the analysis of the risks and benefits of fish consumption on overall cognitive ability.

Methyl mercury (MeHg) is highly toxic to the developing nervous system. Human exposure is mainly from fish consumption since small amounts are present in all fish. Findings of developmental neurotoxicity following high-level prenatal exposure to MeHg raised the question of whether children whose mothers consumed fish contaminated with background levels during pregnancy are at an increased risk of impaired neurological function. Benchmark doses determined from studies in New Zealand, and the Faroese and Seychelles Islands indicate that a level of 4-25 parts per million (ppm) measured in maternal hair may carry a risk to the infant. However, there are numerous sources of uncertainty that could affect the derivation of benchmark doses, and it is crucial to continue to investigate the most appropriate derivation of safe consumption levels. Earlier, we published the findings from benchmark analyses applied to the data collected on the Seychelles main cohort at the 66-month follow-up period. Here, we expand on the main cohort analyses by determining the benchmark doses (BMD) of MeHg level in maternal hair based on 643 Seychellois children for whom 26 different neurobehavioral endpoints were measured at 9 years of age. Dose-response models applied to these continuous endpoints incorporated a variety of covariates and included the k-power model, the Weibull model, and the logistic model. The average 95% lower confidence limit of the BMD (BMDL) across all 26 endpoints varied from 20.1 ppm (range=17.2-22.5) for the logistic model to 20.4 ppm (range=17.9-23.0) for the k-power model. These estimates are somewhat lower than those obtained after 66 months of follow-up. The Seychelles Child Development Study continues to provide a firm scientific basis for the derivation of safe levels of MeHg consumption.

Mercury is ubiquitous in the environment and therefore every human being, irrespective of age and location, is exposed to one form of mercury or another. The major source of environmental mercury is natural degassing of the earth's crust, but industrial activities can raise exposure to toxic levels directly or through the use or misuse of the liquid metals or synthesized mercurial compounds. The aim of this review is to survey differences in human exposure and in the toxicology of different forms of mercury. It covers not only symptoms and signs observed in poisoned individuals by a clinician but also subclinical effects in population studies, the final evaluation of which is the domain of statisticians.

Exposure to environmental mercury may be a factor that contributes to idiosyncratic autoimmune disease. Studies have demonstrated that inorganic, ionic mercury (i.e., Hg2+) modulates several lymphocyte signal transduction pathways, which may be a mechanism whereby Hg2+ dysregulates the immune response. The CD95/Fas apoptotic signaling pathway, which is of critical importance in regulating peripheral tolerance, is disrupted by low and environmentally relevant concentrations of Hg2+. Activation of the cysteine protease caspase-3 is a critical component of both CD95-mediated and TNF-alpha-induced apoptosis. The present work demonstrates that Hg2+ selectively disrupts death receptor mediated caspase-3 activation, where CD95-mediated caspase-3 activation is impaired in Hg2+ treated cells; whereas TNF-alpha-induced caspase-3 activation is not. Using the fluorogenic caspase-3 substrate, Ac-DEVD-7-amino-4-methyl coumarin, to measure caspase-3 enzyme activity as well as Western blotting to track processing of the caspase-3 proenzyme, we have considered the potential direct and indirect effects of Hg2+ on caspase-3. At relatively high concentrations and in a cell-free system, Hg2+ is capable of targeting the active site cysteinyl of caspase-3 resulting in enzyme inhibition. However, at more environmentally relevant exposures, Hg2+ does not gain access in appreciable quantities to the intracellular compartment where caspase-3 resides. Collectively, these data establish that Hg2+ impairs CD95-mediated apoptosis by targeting a plasma membrane proximal signaling event. By measuring the cellular Hg2+ content following various exposure conditions, we have determined that a cellular Hg2+ burden of approximately 50 ng/10(6) cells is sufficient to impair CD95-mediated caspase-3 activation. The present study furthers an understanding of the mechanism whereby relatively low and non-cytotoxic concentrations of Hg2+ may disrupt peripheral tolerance leading to sustained autoimmune disease.

Studies of the association between prenatal methylmercury exposure from maternal fish consumption and neurodevelopmental test scores in the Seychelles Child Development Study have not found adverse effects through age 9 years. The analysis for the most recent 9-year data (Lancet 361 (2003) 1686) employed conventional linear regression models. In this study we reanalyzed the same Seychelles 9-year data using semiparametric additive models with different degrees of smoothing to explore whether nonlinear effects of prenatal exposure were present. Of 21 endpoints in the linear analysis, we chose only those with a two-tailed P value less than 0.2 for the effect of prenatal exposure. Six endpoints met the criterion. A nonlinear effect was identified with the more smooth model for only one endpoint. The test for an overall effect of prenatal exposure was also significant, with a P value of 0.04, while the corresponding P value in the linear regression analysis was 0.08. The nonlinear curve appeared to be nearly flat when the level was below approximately 12 ppm in maternal hair, with a linear trend above that level, suggesting a possible adverse effect in the uppermost range of prenatal exposure included in this cohort. Because of the descriptive nature of semiparametric additive models, the P values are not precise, and certainly there are fewer data above 12 ppm. We conclude that this reanalysis supports the primary linear analysis, showing little evidence for a prenatal adverse effect.

Studies of the effects of environmental exposures on human health typically require estimation of both exposure and outcome. Standard methods for the assessment of the association between exposure and outcome include multiple linear regression analysis, which assumes that the outcome variable is observed with error, while the levels of exposure and other explanatory variables are measured with complete accuracy, so that there is no deviation of the measured from the actual value. The term measurement error in this discussion refers to the difference between the actual or true level and the value that is actually observed. In the investigations of the effects of prenatal methylmercury (MeHg) exposure from fish consumption on child development, the only way to obtain a true exposure level (producing the toxic effect) is to ascertain the concentration in fetal brain, which is not possible. As is often the case in studies of environmental exposures, the measured exposure level is a biomarker, such as the average maternal hair level during gestation. Measurement of hair mercury is widely used as a biological indicator for exposure to MeHg and is the only indicator that has been calibrated against the target tissue, the developing brain. Variability between the measured and the true values in explanatory variables in a multiple regression analysis can produce bias, leading to either over or underestimation of regression parameters (slopes). Fortunately, statistical methods known as measurement error models (MEM) are available to account for measurement errors in explanatory variables in multiple regression analysis, and these methods can provide an (either “unbiased” or “bias-corrected”) estimate of the unknown outcome/exposure relationship. In this paper, we illustrate MEM analysis by reanalyzing data from the 5.5-year test battery in the Seychelles Child Development Study, a longitudinal study of prenatal exposure to MeHg from maternal consumption of a diet high in fish. The use of the MEM approach was made possible by the existence of independent, calibration data on the magnitude of the variability of the measurement error deviations for the biomarker of prenatal exposure used in this study, the maternal hair level. Our reanalysis indicated that adjustment for measurement errors in explanatory variables had no appreciable effect on the original results.

Summary Introduction Exposure to methylmercury (MeHg) before birth can adversely affect children's neurodevelopment. The most common form of prenatal exposure is aternal fish consumption, but whether such exposure harms the fetus is unknown. We aimed to identify adverse neurodevelopmental effects in a fish-consuming population. Methods We investigated 779 mother-infant pairs residing in the Republic of Seychelles. Mothers reported consuming fish on average 12 meals per week. Fish in Seychelles contain much the same concentrations of MeHg as commercial ocean fish elsewhere. Prenatal MeHg exposure was determined from maternal hair growing during pregnancy. We assessed neurocognitive, language, memory, motor, perceptual-motor, and behavioural functions in children at age 9 years. The ssociation between prenatal MeHg exposure and the primary endpoints was investigated with multiple linear regression with adjustment for covariates that affect child development. Findings Mean prenatal MeHg exposure was 6·9 parts per million (SD 4·5ppm). Only two endpoints were associated with prenatal MeHg exposure. Increased exposure as associated with decreased performance in the grooved pegboard using the non-dominant hand in males and improved scores in the hyperactivity index of the Conner's teacher rating scale. Covariates affecting child development were appropriately associated with endpoints. Interpretation These data do not support the hypothesis that there is a neurodevelopmental risk from prenatal MeHg exposure resulting solely from ocean fish consumption.

The historical record of clinical cases of methyl mercury poisoning dates back to the 19th century when the first chemical synthesis occurred. The potent fungicidal properties of both methyl and the closely related ethyl mercury compound were subsequently discovered, which led to widespread agricultural application for prevention of fungal infection in seed grain. Several catastrophic outbreaks of poisoning occurred in the mid-20th century when the treated seed grain was mistakenly used to prepare homemade bread. The largest outbreak took place in rural Iraq in the early 1970s. Human poisonings also occurred in Japan due to the release of methyl mercury into bodies of fresh and ocean water. The most infamous outbreak occurred in the area of Minamata Bay: methyl mercury, which was unwittingly discharged into the ocean water, avidly accumulated in the aquatic food chain to such an extent that people who consumed fish were severely poisoned. Today, human exposure to methyl mercury occurs from consumption of fish and sea mammals. Inorganic mercury that is present in aquatic sediments is methylated by microorganisms and accumulates in the aquatic food chain. Although no cases of clinical poisoning have been reported, a number of epidemiological studies have been carried out that raise the possibility of prenatal damage. Previous studies (especially the Iraq outbreak) indicate that the prenatal stage of the life cycle is the most vulnerable. However, ongoing epidemiological studies of heavy fish consumers of the Seychelles Islands in the Indian Ocean do not reveal adverse effects. To the contrary, the results of some developmental tests that were conducted on prenatally exposed children indicate beneficial outcomes that correlate with mercury levels during pregnancy. This article discusses the potential role of micronutrients in fish as a plausible explanation for these findings.

This article discusses three examples of delay (latency) in the appearance of signs and symptoms of poisoning after exposure to methylmercury. First, a case is presented of a 150-day delay period before the clinical manifestations of brain damage after a single brief (

Mechanisms of methylmercury (MeHg) and inorganic mercury (Hg) uptake were examined in HepG2 cells, a human hepatoma-derived cell line. MeHg uptake was faster when it was present as the l -cysteine complex, as compared to the glutathione (GSH), CysGly, γ-GluCys, d -cysteine, N-acetylcysteine, l -penicillamine, or albumin complexes. Uptake of MeHg- l -cysteine was independent of Na+, stereoselective, and was inhibited by the amino acid transport system l substrates l -leucine, l -valine, and l -phenylalanine (5 mM). Moreover, [3H] l -leucine uptake was inhibited by MeHg- l -cysteine, suggesting that MeHg- l -cysteine is transported into HepG2 cells by an l -type amino acid carrier. Uptake of MeHg as the GSH complex (MeHg-SG) was dependent on the extracellular GSH concentration, and was diminished when cellular γ-glutamyl transpeptidase activity was inhibited. Inorganic mercury uptake was slower than that of MeHg, but was also sensitive to the type of thiol ligand present. These findings demonstrate that mercury uptake by HepG2 cells is dependent on the chemical structure of the mercury compound, the thiol ligand, and the activity of γ-glutamyl transpeptidase. γ-Glutamyl transpeptidase appears to play a key role in the disposition of MeHg-SG by facilitating the formation of MeHg- l -cysteine, which is readily transported into the cells on an amino acid-type carrier.

Methylmercury (MeHg) is a neurotoxicant whose high-dose effects first became known following a number of poisoning outbreaks that occurred worldwide. The primary human exposure is low dosage from fish consumption. Studies of fish-eating populations have not found a consistent pattern of association between exposures and outcomes. Therefore, examining specific areas of cognitive functioning has been suggested as an important approach to determine whether more subtle effects of MeHg exposure are present. In the Seychelles longitudinal study of prenatal and postnatal MeHg exposure from fish consumption and development, the McCarthy Scales of Children's Abilities (MSCA) were administered to children at age 66 months. No association between MeHg exposure and performance on the MSCA General Cognitive Index was identified. We analyzed these data further to determine whether associations were present on specific subscales of the MSCA. The standard MSCA subscales were analyzed. Then, more specific subscales of the MSCA were defined and analyzed utilizing a neuropsychological approach. The subscales were recombined to approximate the domains of cognitive functioning evaluated in the Faroes and New Zealand studies. Analyses of both the standard and the recombined MSCA subscales showed no adverse associations with MeHg exposure and neuropsychological endpoints. A positive association between postnatal MeHg exposure and performance on the MSCA Memory subscale was found. These findings are consistent with previous reports from the Seychelles study in that no adverse effects of MeHg exposure from fish consumption can be detected in this cohort.

Methylmercury is a neurotoxin at high exposures, and the developing fetus is particularly susceptible. Because exposure to methylmercury is primarily through fish, concern has been expressed that the consumption of fish by pregnant women could adversely affect their fetuses. The reference dose for methylmercury established by the U.S. Environmental Protection Agency was based on a benchmark analysis of data from a poisoning episode in Iraq in which mothers consumed seed grain treated with methylmercury during pregnancy. However, exposures in this study were short term and at much higher levels than those that result from fish consumption. In contrast, the Agency for Toxic Substances and Disease Registry (ATSDR) based its proposed minimal risk level on a no-observed-adverse-effect level (NOAEL) derived from neurologic testing of children in the Seychelles Islands, where fish is an important dietary staple. Because no adverse effects from mercury were seen in the Seychelles study, the ATSDR considered the mean exposure in the study to be a NOAEL. However, a mean exposure may not be a good indicator of a no-effect exposure level. To provide an alternative basis for deriving an appropriate human exposure level from the Seychelles study, we conducted a benchmark analysis on these data. Our analysis included responses from batteries of neurologic tests applied to children at 6, 19, 29, and 66 months of age. We also analyzed developmental milestones (age first walked and first talked). We explored a number of dose-response models, sets of covariates to include in the models, and definitions of background response. Our analysis also involved modeling responses expressed as both continuous and quantal data. The most reliable analyses were considered to be represented by 144 calculated lower statistical bounds on the benchmark dose (BMDLs; the lower statistical bound on maternal mercury hair level corresponding to an increase of 0.1 in the probability of an adverse response) derived from the modeling of continuous responses. The average value of the BMDL in these 144 analyses was 25 ppm mercury in maternal hair, with a range of 19 to 30 ppm. Images Figure 1

Controversy exists concerning the fetal risk associated with exposure to low-dose methylmercury from maternal fish consumption. Previous studies of the effects of acute prenatal mercury exposure identified delays in achieving developmental milestones among exposed children. This led to public health concern that prenatal low-dose exposure from fish consumption could adversely affect the fetus. We evaluated the effects of prenatal methylmercury exposure (through maternal fish consumption) on the age that children walked and first said words in the main study cohort of the Seychelles Child Development Study. We used semiparametric generalized additive models to identify nonlinearities in the relationships between prenatal exposure and developmental outcomes, after adjusting for covariates, and to evaluate their importance. Very slight delays (

Ingestion of fish or grain contaminated with methylmercury resulted in epidemics of severe neurotoxicity and death in Japan in the 1950s and 1960s1 and in Iraq in 1972.2 The World Health Organization and other organizations have warned of the dangers of methylmercury compounds to the environment and to scientific researchers.1,3–6 Dimethylmercury may be even more dangerous than methylmercury compounds. The physical properties of dimethylmercury permit transdermal absorption, and the volatility of this liquid permits toxic exposure through inhalation. Since dimethylmercury is lethal at a dose of approximately 400 mg of mercury (equivalent to a few drops, or about . . .

This article describes the health effects of the two principal forms of mercury to which humans are exposed, namely mercury vapor and methylmercury compounds. Mercury vapor is the monatomic gas that vaporizes from liquid metallic mercury. It has a long history of human exposure dating back to ancient times. The earliest life forms on this planet may have received substantial exposure and developed defense mechanisms that we inherit to this day. Severe forms of poisoning from inhalation of mercury vapor are now rare. More commonly observed in occupational exposures are subtle neurobehavioral effects and preclinical changes in biochemical markers of kidney function. Methylmercury compounds may also date back to Archean times. The methylation of inorganic mercury by methanogenic bacteria is believed to be a protective mechanism for these cells that appeared early in evolution. Unfortunately, for life forms dependent on a central nervous system, what is protective for simple cells becomes a threat to the more complex life forms. Today, the main source of human exposure is from methylation of inorganic mercury in bodies of fresh and ocean water, the ensuing bioaccumulation in the aquatic food chain, and the consumption of fish or sea mammals by humans. Methylmercury selectively damages the brain. In adult humans, damage is focal, affecting specific cell types in certain anatomical areas such as the visual cortex and the cerebellum. Prenatal exposure disrupts the normal developmental processes of the fetal brain. Indeed, the prenatal period is widely believed to be the most susceptible stage of the life cycle. At present, two large epidemiological studies are underway to determine human health risks from methylmercury in fish. J. Trace Elem. Exp. Med. 11:303–317, 1998. © 1998 Wiley-Liss, Inc.

The major physical forms of mercury to which humans are exposed are mercury vapor, Hg0, and methylmercury compounds, Ch3HgX. Mercury vapor emitted from both natural and anthropogenic sources is globally distributed in the atmosphere. It is returned as a water-soluble form in precipitation and finds its way into bodies of fresh and ocean water. Land run-off also accounts for further input into lakes and oceans. Inorganic mercury, present in water sediments, is subject to bacterial conversion to methylmercury compounds that are bioaccumulated in the aquatic food chain to reach the highest concentration in predatory fish. Human exposure to mercury vapor is from dental amalgam and industries using mercury. Methylmercury compounds are found exclusively in seafood and freshwater fish. The health effects of mercury vapor have been known since ancient times. Severe exposure results in a triad of symptoms, erethism, tremor, and gingivitis. Today, we are concerned with more subtle effects such as preclinical changes in kidney function and behavioral and cognitive changes associated with effects on the central nervous system. Methylmercury is a neurological poison affecting primarily brain tissue. In adults, brain damage is focal affecting the function of such areas as the cerebellum (ataxia) and the visual cortex (constricted visual fields). Methylmercury also at high doses can cause severe damage to the developing brain. Today the chief concern is with the more subtle effects arising from prenatal exposure such as delayed development and cognitive changes in children.

To reach its target tissue, methylmercury must traverse brain capillary endothelial cells, the site of the blood-brain barrier. Methylmercury uptake from blood plasma into these cells is mediated in part by an amino acid carrier that transports the methylmercury-L-cysteine complex; however, the mechanism by which it is released from the endothelial cells into brain interstitial space is unknown. Using bovine brain capillary endothelial cells in culture, the present study examined the hypothesis that methylmercury is transported out of these cells as a glutathione (GSH) complex. GSH concentration in cultured bovine brain capillary endothelial cells was 13.1 +/- 3.3 nmol/mg protein. Depletion of intracellular GSH in [203Hg]methylmercury-preloaded cells by exposure to 1-chloro-2,4-dinitrobenzene or diethyl maleate decreased the rate of [203Hg]methylmercury efflux. Incubation of [203Hg]methylmercury-preloaded cells with high concentrations of S-methylglutathione, S-ethylglutathione, S-butylglutathione, and sulfobromophthalein-glutathione inhibited [203Hg]methylmercury efflux. The GSH analogs gamma-glutamylglycylglycine and ophthalmic acid also inhibited [203Hg]methylmercury efflux, but to a lesser degree than the glutathione S-conjugates, whereas L-leucine, L-methionine, and L-alanine had no effect. Efflux was not affected by depletion of intracellular ATP with 2-deoxyglucose or antimycin A. These results indicate that complexation with GSH and subsequent transport of the complex by an ATP-independent mechanism may be involved in the transport of methylmercury out of brain capillary endothelial cells.

In order to establish guidelines for exposure of astronauts to iodine, used as a water disinfectant in space, we studied the usefulness of hair, saliva, and urine for biological monitoring in humans and in the human hair/nude mouse model. The monitoring of iodine in patients that received 150 mCi of Na131I (carrier-free) showed similar patterns of elimination for blood, saliva, and urine. The mean correlation coefficient (r) between iodine elimination for blood/saliva was 0.99, for blood/urine, 0.95, and for saliva/urine, 0.97. The absolute value of iodine concentrations in urine revealed marked variability, which was corrected by adjusting for creatinine levels. The autoradiographic studies of human hair demonstrated that iodine is rapidly incorporated into external layers of the hair root and can be removed easily during washing. These data were confirmed after iodine exposure using the human hair/nude mouse model. Hair does not provide satisfactory information about exposure due to unstable incorporation of iodine. The most useful medium for biological monitoring of astronauts exposed to high doses of iodine in drinking water is urine, when adjusted for creatinine, and saliva, if quantitative evaluation of flow rate is provided. Images Figure 1. Figure 2. Figure 3. Figure 4.

Both terrestrial and aquatic food chains are capable of accumulating certain environmental contaminants to toxic concentrations. This article focuses on the aquatic food chain because we have less control over contaminant entry into this chain than we have for the terrestrial chain. In general, at least three special properties are required for a contaminant to bioaccumulate in an aquatic food chain: 1) a high octanol-water partition coefficient, 2) chemical and metabolic stability in water and in organisms in the food chain, and 3) a low toxicity to organisms in the chain so that the chain is not broken by loss of an intermediate species. Few of the thousands of chemicals produced by human industry meet these requirements. In terms of organic chemicals, the best known examples of bioaccumulation in aquatic food chains are the polychlorinated biphenyls (PCBs), dioxins, and organochlorine pesticides such as dichlorodiphenyltrichloroethane (DDT). Few examples exist of bioaccumulation of metal compounds. Methylmercury is arguably the most dramatic and best documented example of high bioaccumulation.

Metals have been mined and used since ancient times. The industrial era has seen a sharp increase in both the amounts and variety of metals that find applications in industry. The inadvertent release of metals, such as from fossil fuel consumption, also adds to the global burden. A number of catastrophic outbreaks have alerted us to the occupational and environmental health risks. Life on this planet has evolved in the presence of metals. Cells learned to make use of the more abundant metals in the Archean oceans as an integral component in their structure and function. Today, we inherit these as the essential metals. At the same time, evolving life must have developed means of coping with the potentially toxic actions of metals. The appearance of oxygen in the atmosphere in the Precambrian period also resulted in cells both using and developing protective mechanisms against what must have been a highly toxic, reactive gas. Atmospheric oxygen must have increased the solubility of many metals as insoluble metal sulfides were oxidized to the more soluble sulfates. It may be no coincidence that the protective mechanisms for oxygen are also used to protect against a number of toxic metals. Selected examples are given on the role of evolution in metal toxicology, specifically, examples where the normal function of essential metals is deranged by competition with nonessential metals. Examples are also given of protective mechanisms that involve enzymes or cofactors involved in the oxygen defense system.

Dental amalgams contain approximately 50 percent metallic mercury and emit mercury vapor during the life of the restoration. Controversy surrounds whether fetal exposure to mercury vapor resulting from maternal dental amalgam restorations has neurodevelopmental consequences.The authors determined maternal amalgam restoration status during gestation (prenatal exposure to mercury vapor [Hg(0)]) retrospectively in 587 mother-child pairs enrolled in the Seychelles Child Development Study, a prospective longitudinal cohort study of the effects of prenatal and recent postnatal methylmercury (MeHg) exposure on neurodevelopment. They examined covariate-adjusted associations between prenatal maternal amalgam restoration status and the results of six age-appropriate neurodevelopmental tests administered at age 66 months. The authors fit the models without and with adjustment for prenatal and recent postnatal MeHg exposure metrics.The mean number of maternal amalgam restorations present during gestation was 5.1 surfaces (range, 1-22) in the 42.4 percent of mothers who had amalgam restorations. The authors found no significant adverse associations between the number of amalgam surfaces present during gestation and any of the six outcomes, with or without adjustment for prenatal and postnatal MeHg exposure. Results of analyses with the secondary metric, prenatal amalgam occlusal point scores, showed an adverse association in boys only on a letter- and word-identification subtest of a frequently used test of scholastic achievement, whereas girls scored better on several other tests with increasing exposure.This study's results provide no support for the hypothesis that prenatal Hg(0) exposure arising from maternal dental amalgam restorations results in neurobehavioral consequences in the child. These findings require confirmation from a prospective study of coexposure to MeHg and Hg(0).

Introduction People worldwide depend upon daily fish consumption as a major source of protein and other nutrients. Fish are high in nutrients essential for normal brain development, but they also contain methylmercury (MeHg), a neurotoxicant. Our studies in a population consuming fish daily have indicated no consistent pattern of adverse associations between prenatal MeHg and children's development. For some endpoints we found performance improved with increasing prenatal exposure to MeHg. Follow up studies indicate this association is related to the beneficial nutrients present in fish. Objectives To determine if the absence of adverse outcomes and the presence of beneficial associations between prenatal MeHg and developmental outcomes previously reported persists into adolescence. Methods This study was conducted on the Main Cohort of the Seychelles Child Development Study (SCDS). We examined the association between prenatal MeHg exposure and subjects’ performance at 17 years of age on 27 endpoints. The test battery included the Wisconsin Card Sorting Test (WCST), the California Verbal Learning Test (CVLT), the Woodcock-Johnson (W-J-II) Achievement Test, subtests of the Cambridge Neuropsychological Test Automated Battery (CANTAB), and measures of problematic behaviors. Analyses for all endpoints were adjusted for postnatal MeHg, sex, socioeconomic status, maternal IQ, and child's age at testing and the child's IQ was added for problematic behavioral endpoints. Results Mean prenatal MeHg exposure was 6.9 ppm. There was no association between prenatal MeHg and 21 endpoints. Increasing prenatal MeHg was associated with better scores on four endpoints (higher W-J-II math calculation scores, reduced numbers of trials on the Intra-Extradimensional Shift Set of the CANTAB), fewer reports of substance use and incidents of and referrals for problematic behaviors in school. Increasing prenatal MeHg was adversely associated with one level of referrals to a school counselor. Conclusions At age 17 years there was no consistent pattern of adverse associations present between prenatal MeHg exposure and detailed domain specific neurocognitive and behavioral testing. There continues to be evidence of improved performance on some endpoints as prenatal MeHg exposure increases in the range studied, a finding that appears to reflect the role of beneficial nutrients present in fish as demonstrated previously in younger subjects. These findings suggest that ocean fish consumption during pregnancy is important for the health and development of children and that the benefits are long lasting.

Rat pheochromocytoma PC12 cell sublines with increasing resistance to methylmercury (MeHg) were isolated by exposure to MeHg in two sequential steps. The strongest resistance (PC12/TM) among the clones obtained by the first cloning was characterized. PC12/TM cells exhibited about an 8- to 10-fold increase in resistance compared with parent PC12 cells on the bases of the concentration required for 50% inhibition of growth and colony-forming activity. PC12/TM cells accumulated smaller amounts of MeHg (one-half to one-fifth) than parent PC12 cells. This reduced MeHg accumulation in PC12/TM cells resulted from the slow uptake and rapid efflux. A close correlation between reduced MeHg accumulation and MeHg resistance was found among seven sublines of PC12 cells with different sensitivity to MeHg. The reduced retention of MeHg in PC12 sublines was also well correlated with the sensitivity to MeHg. Phenol-3,6-dibromophthalein disulfonate inhibited MeHg efflux from PC12/TM cells and increased its accumulation. These results suggest that efflux of MeHg from PC12/TM cells is associated with glutathione.

Methylmercury is among the most potentially toxic species to which human populations are exposed, both at high levels through poisonings and at lower levels through consumption of fish and other seafood. However, the molecular mechanisms of methylmercury toxicity in humans remain poorly understood. We used synchrotron X-ray absorption spectroscopy (XAS) to study mercury chemical forms in human brain tissue. Individuals poisoned with high levels of methylmercury species showed elevated cortical selenium with significant proportions of nanoparticulate mercuric selenide plus some inorganic mercury and methylmercury bound to organic sulfur. Individuals with a lifetime of high fish consumption showed much lower levels of mercuric selenide and methylmercury cysteineate. Mercury exposure did not perturb organic selenium levels. These results elucidate a key detoxification pathway in the central nervous system and provide new insights into the appropriate methods for biological monitoring.

_ This review discusses the basic principles of risk assessment as used in general toxicology and in monitoring side-effects of therapeutic treatments. It also outlines how these principles may apply to assessment of biological reactions to dental restorative materials. Mercury exposure from amalgam fillings is used as an example. The calculations performed are intended only as illustrations, and many other factors must be taken into account. Thus, the calculations are not intended as the last word in risk assessment of amalgam fillings.