Food Additives and Contaminants: Part B Vol. 4, No. 3, September 2011, 212–217
Mercury and selenium content of Taiwanese seafood G.C. Fanga, D.H. Namb and N. Basub* a
Department of Safety, Health and Environmental Engineering, Hungkuang University, Sha-Lu, Taichung 433, Taiwan; Department of Environmental Health Sciences, University of Michigan, Ann Arbor, MI 48109, USA
b
(Received 22 March 2011; final version received 22 June 2011) Fish consumption is avid in Taiwan (and other Asian nations), but little is known about the mercury and selenium content in local seafood. This paper reports on total mercury, methylmercury and selenium levels from 14 commonly consumed seafood items obtained from Taichung, Taiwan. Mean total mercury concentrations varied nearly 100-fold across species. Fifty per cent of the marlins sampled and 35% of the sharks exceeded the 0.3 mg g1 US Environmental Protection Agency (USEPA) guideline. Methylmercury comprised a majority of the total mercury in all species. In all species studied there was a molar excess of selenium over mercury. The rank order of mean selenium–mercury molar ratios was red tilapia (166.8)4abura (87.9)4river prawn (82.4)4whiteleg shrimp (64.2)4butterfish (44.6)4milkfish (37.0)4tuna (15.6)4grouper (13.9)4ayu (13.4)4 coral hind (13.0)4weever (11.8)4saury (9.0)4shark (7.8)4marlin (4.2). Keywords: fish; mercury; selenium; risk–benefit; selenium health benefit value
Introduction Seafood (fish and shellfish) are a major source of dietary proteins, essential elements and omega-3 fatty acids (Chapman and Chan 2000; Mozaffarian and Rimm 2006). Harvesting seafood is of immense recreational, economical and cultural importance to millions of people worldwide. However, the presence of toxic chemicals in seafood has spurred many people to avoid consuming seafood (Oken et al. 2003). The avoidance of seafood consumption may pose a greater threat to public health than consuming contaminated products (Mozaffarian and Rimm 2006). Accordingly, greater scientific understanding is required to improve our ability to communicate the risks and benefits associated with seafood consumption. Most seafood consumption advisories are driven by the presence of mercury (Hg) (US Environmental Protection Agency (USEPA) 2009). Though a naturally occurring element, Hg is mainly mobilized into the environment by human activities such as fossil fuel combustion (Swain et al. 2007). Upon deposition into aquatic ecosystems, inorganic Hg may be methylated by microorganisms into methyl-Hg (MeHg). As a methylated chemical, MeHg can effectively cross biological membranes and accumulate in organisms, biomagnify through aquatic food chains, and build up in the tissues of predatory animals. The concentrations of MeHg in tissues of predatory animals (such as
*Corresponding author. Email: [email protected] ISSN 1939–3210 print/ISSN 1939–3229 online ß 2011 Taylor & Francis http://dx.doi.org/10.1080/19393210.2011.605526 http://www.tandfonline.com
humans) may be 10 million times greater than ambient levels in the environment (USEPA 1997). As a result, dietary consumption of fish represents the major route of Hg exposure for human (Mergler et al. 2007) and wildlife (Scheuhammer et al. 2007) populations. Once inside the body, MeHg readily crosses the blood–brain barrier via the methionine uptake system (Clarkson and Magos 2006) and accumulate in the central nervous system. Owing to its highly reactive nature and strong affinity for sulfhydryl groups, MeHg can impair multiple neurological pathways and cause neurotoxicity. There exists a strong correlation between Hg exposure and fish consumption, especially the consumption of predatory, long-lived species such as sharks, marlin and swordfish. Consumption can vary according to ethnicity. In the United States, for example, participants in the NHANES survey that identified themselves as Asian had higher blood Hg levels (via fish consumption) that other ethnic groups (Hightower et al. 2006). Though Asians are among the highest seafood consumers worldwide, when compared with North America, several European nations and certain susceptible groups such as Indigenous Peoples (Mergler et al. 2007), much less is known about Hg exposure profiles in Asian countries (except perhaps for Japan) both in terms of Hg levels in fish as well as Hg exposures (via sampling of hair or blood) in people.
Food Additives and Contaminants: Part B The purpose of this study was to report on Hg (both total Hg and MeHg) levels in 14 commonly consumed seafood products from Taiwan. Selenium (Se) values are also reported given the strong belief that this essential nutritional element affords protection against Hg toxicity (Kaneko and Ralston 2007; Ralston et al. 2007). The Hg data are related to guideline values, and the Hg and Se data are compared with each other on a molar basis and indexed through a Se health benefit value (SHBV) equation. By documenting Hg and Se values from same samples and from commonly consumed seafood, public health practitioners may better gauge the risks and benefits.
Methods Sample collection and preparation Twenty edible samples of 14 different seafood species were obtained from local restaurants and markets in the town of Taichung, Taiwan. Samples are considered to be commonly consumed representative items. Common names and scientific names are provided (Table 1). Samples were kept frozen at 20 C until processed at Hungkuang University. Samples were oven dried at 60 C for 3 days and then pulverized into a powder. Moisture content was calculated for each sample (Table 1). All concentrations in this paper are reported on a wet weight basis.
Analysis of total mercury (THg) and methylmercury (MeHg) Concentration of THg in each sample (n ¼ 20 per species) was measured in a Direct Mercury Analyser 80 (DMA-80, Milestone Inc., Shelton, CT, USA) according to USEPA Method 7473 as previously described by Nam and Basu (2011). About 10–30 mg of dried
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sample were weighted in a nickel sampling boat and placed into the DMA-80. Following decomposition of the sample at 800 C, liberated Hg was next trapped using gold and then subsequently desorbed, carried to an absorbance cell and quantified spectrophotometrically. For MeHg, eight to ten samples for each species were analysed according to methods outlined by Nam and Basu (2011). Briefly, approximately 50 mg of dried samples were homogenized in 50 mM Tris-HCl buffer (pH 8.5) which contained protease. The homogenate was next incubated at 50 C for 1 h, and then sequentially mixed with 40% NaOH, 1% cysteine, 25 mM CuSO4, acidic NaBr and toluene. The mixture was centrifuged at 13 000g for 5 min, and the top toluene layer was transferred to a test tube and mixed twice with of Na2S2O3 (5 mM) to permit back-extraction of MeHg into an aqueous phase. The aqueous layer was placed into another test tube for analysis of MeHg by direct analysis in the DMA-80.
Selenium analysis For Se, eight to ten samples for each species were analysed according to methods outlined previously (Nam et al. 2011; Nam and Basu 2011). Concentrated HNO3 (0.9 ml, 70% trace element grade) was added to dried samples in a Teflon tube, capped and allowed to digest overnight in a fume hood. After gentle shaking, tubes were heated for 3–4 h at 100 C until digestion was complete. After samples were cooled, the tube was rinsed with MilliQ water and the digest was transferred into clean test tubes. Se concentrations were determined by graphite furnace AAS (Varian AAS 220FS) equipped with a deuterium corrector to minimize background signals. Into a graphite tube, 20 ml of standard solution, tissue samples or blanks were combined with 20 ml of nickel nitrate modifier
Table 1. Mercury (total and MeHg) and selenium concentrations in 14 commonly consumed Taiwanese seafoods. Values represent mean standard deviation. Common name Marlin Shark Tuna Grouper Weever Pacific saury Coral hind Ayu sweetfish Japanese butterfish Milkfish Abura Penaeid Shrimp River Prawn Red Tilapia
Scientific name
Total Hg (mg g1)
% MeHg
Se (mg g1)
Mokaira mazara Carcharhinus sp. Thunnus albacares Epinephelus hexagonatus Lateolabrax japonicus Cololabis saira Cephalopholis miniata Plecoglossus altivelis altivelis Psenopsis anomala Chanos chanos Johnius distinctus Penaeus vannamei boone Macrobrachium rosenbergii Oreochromis hybrids
0.329 0.267 0.305 0.256 0.158 0.083 0.091 0.028 0.083 0.051 0.079 0.048 0.077 0.042 0.047 0.007 0.029 0.048 0.016 0.004 0.014 0.009 0.013 0.006 0.010 0.011 0.004 0.001
80.4 31.2 82.8 33.1 89.3 15.0 93.2 7.9 97.0 6.4 81.3 11.8 92.1 7.8 97.8 7.0 83.9 7.1 71.8 14.1 92.4 11.0 102 2.9 80.8 21.6 99.7 0.8
0.424 0.285 0.457 0.340 0.959 0.913 0.500 0.245 0.241 0.059 0.265 0.104 0.331 0.189 0.248 0.090 0.330 0.141 0.230 0.111 0.402 0.103 0.256 0.167 0.261 0.052 0.229 0.075
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(6 mg ml1). Samples were then dried at 85–120 C, heated slowly to 1000 C and atomized at 2600 C. Standard curves and detection limits were determined using a series of diluted Se standards.
equation: SHBV ¼ ½ðm mol Se kg1 Þ ðSe=HgÞ ½ðm mol Hg kg1 Þ ðHg=SeÞ
Quality control Analytical accuracy and precision were determined through the use of Standard Reference Materials (DOLT-3, DOLT-4 and TORT-2 from the National Research Council of Canada) and intermittent analysis of duplicate samples. Average recovery rates of SRMs for THg, MeHg and Se were 95.7% 7.6%, 94.2% 8.4%, and 90.1% 7.9%, respectively, and deemed to be excellent. For analyses of THg, MeHg and Se, the relative standard deviation (% RSD) was lower than 9% for all replicate measures. The detection limit for the direct Hg analyser was 0.021 ng and ranged from 0.011 to 0.028 ng. For Se, the limit of measurement by graphite furnace AAS was 0.001 mg g1 of Se. All concentrations are expressed as mg g1 (ppm) wet weight.
Data analyses Statistical operations were performed using SPSS (v11.5, Chicago, IL, USA). Data analysis included the tabulation of descriptive statistics for all measurements. To relate Hg values to suggested fish consumption guidelines by the World Health Organization (WHO) (1 mg g1 in fish; Food and Agriculture Organization of the United Nations (FAO) 1991) and USEPA (0.3 mg g1 in fish; USEPA 2001), the percentage of samples exceeding values was calculated. An SHBV for each sample was calculated according to Kaneko and Ralston (2007) based on the following
Results and discussion Hg (total, MeHg) was detected in each sample. The mean THg concentration (mass as microgram per gram, molar as micromole per kilogram) for each of the 14 species studied is presented in Tables 1 and 2. Mean concentrations across seafood varied nearly 100fold and ranged from 0.004 mg g1 for red tilapia to 0.329 mg g1 for marlin (Figure 1). The WHO uses a THg guidance value of 1 mg g1 wet weight in fish (FAO 1991). Mean THg concentrations were below 1 mg g1 for all species, and only one shark sample exceeded 1 mg g1 (¼1.046). The USEPA uses a THg guidance level of 0.3 mg g1 wet weight to protect public health (USEPA 2001). Here, mean THg values in marlin and shark both exceeded this 0.3 mg g1. Fifty per cent of the marlins sampled and 35% of the sharks exceeded 0.3 mg g1. The THg values reported here in shark are similar to those reported previously on Taiwanese sharks (0.73 0.54 mg g1) (Chien et al. 2007). Further, THg in tuna and milkfish reported here are similar to values reported previously from Taiwan, though tilapia concentrations reported here are lower than values reported by Chien et al. MeHg comprised a majority of the THg in all species, but the proportion varied (Table 1). In milkfish, MeHg comprised 71.8% of the THg, whereas MeHg comprised495% of the total in weever, ayu, whiteleg shrimp and red tilapia (Figure 1). Understanding and reporting MeHg levels in fish is
Table 2. Molar concentrations of mercury and selenium, as well as molar ratios and calculated selenium health benefit value. Values represent mean standard deviation.
Species
Hg (mmol kg1)
Se (mmol kg1)
Hg/Se molar ratio
Se/Hg molar ratio
Se HBV
Marlin Shark Tuna Grouper Weever Pacific saury Coral hind Ayu sweetfish Japanese butterfish Milkfish Abura Penaeid shrimp River prawn Red tilapia
1.64 1.33 1.53 1.28 0.79 0.41 0.46 0.14 0.42 0.26 0.40 0.24 0.39 0.21 0.23 0.03 0.14 0.24 0.08 0.02 0.07 0.04 0.06 0.03 0.05 0.05 0.02 0.01
5.36 3.61 6.21 4.40 12.14 11.56 6.32 3.09 3.06 0.74 3.36 1.31 4.19 2.39 3.14 1.14 4.17 1.78 2.92 1.41 5.09 2.44 3.24 2.12 3.30 0.66 2.89 0.95
0.46 0.27 0.24 0.12 0.13 0.13 0.10 0.10 0.06 0.03 0.01 0.02 0.02 0.01
4.19 7.83 15.57 13.86 11.77 9.01 12.95 13.40 44.58 36.98 87.90 64.21 82.44 166.77
26.74 54.64 295.68 104.93 37.19 33.72 67.41 47.89 214.51 134.54 540.00 251.14 289.40 537.43
Food Additives and Contaminants: Part B
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Figure 1. Total mercury concentrations (mg g1) in 14 commonly consumed Taiwanese seafoods. Bars represent the mean standard deviation.
Figure 2. Molar ratios of mercury:selenium in 14 commonly consumed Taiwanese seafoods. All means are below a 1 : 1 molar ratio. Bars represent the mean standard deviation.
relevant given that this form of Hg is readily absorbed into the body and crosses protective barriers for the brain and placenta (Clarkson and Magos 2006). The mean total Se concentration (mass as microgram per gram, molar as micromole per kilogram) for each of the 14 species studied is presented in Tables 1 and 2. Mean concentrations ranged from 0.229 mg g1 for red tilapia to 0.959 mg g1 for tuna. In a study of several pelagic fish near Hawaii, Kaneko and Ralston (2007) also documented that Se levels were generally highest in tuna. In a study of Taiwanese seafood, Chien et al. (2003) measured Se and like the present authors found that concentrations in fish were generally higher than in crustaceans. From their study, Chien et al. calculated seafood to be a major contributor of Se to the Taiwanese consumer. In order to compare Hg and Se values, concentrations were converted to molar concentrations and
displayed (Table 2 and Figure 2). In all species studied there was a molar excess of Se over Hg. The decreasing rank order of mean Se:Hg ratios was red tilapia (166.8)4abura (87.9)4river prawn (82.4)4whiteleg shrimp (64.2)4butterfish (44.6)4milkfish (37.0)4 tuna (15.6)4grouper (13.9)4ayu (13.4)4coral hind (13.0)4weever (11.8)4saury (9.0)4shark (7.8)4marlin (4.2). In other studies, a majority of species investigated also showed a molar excess of Se over Hg in both marine (Kaneko and Ralston 2007) and freshwater (Peterson et al. 2009) ecosystems. There are some notable marine species with molar excess of Hg over Se, including mako shark (Kaneko and Ralston 2007) and lemon shark (Nam et al. 2011). Swordfish have a near equal molar ratio of Hg and Se (Kaneko and Ralston 2007). Se is an essential nutrient with important antioxidant roles and anticancer properties
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Figure 3. Selenium health benefit value of 14 commonly consumed Taiwanese seafoods. Bars represent the mean standard deviation.
(Chen and Berry 2003). There is ample experimental evidence that Se protects against Hg toxicity (Chapman and Chan 2000). As Se may mitigate the toxicity of Hg, the molar ratio of Hg:Se has been suggested to be one essential criterion for assessing the true health risks posed by Hg (Kaneko and Ralston 2007; Ralston et al. 2007). Kaneko and Ralston (2007) proposed an index to help identify fish species with low Hg levels and high Se content. Their SHBV index is calculated by considering both the absolute and the relative concentrations of Hg and Se on a molar basis in a given sample. Here, the SHBV exceeded 200 in tuna (295.7), butterfish (214.5), shrimp (251.1), river prawn (289.4), and exceeded 500 in abura (540.0) and red tilapia (537.4) (Figure 3). The SHBV of tuna is of particular note as this is a species widely consumed worldwide and often subject to advisories. The Se content measured here is among the highest across species, and the SHBV reported here is similar to several tuna species studied by Kaneko and Ralston (2007). It is established that Asians are avid consumers of fish. However, knowledge of Hg exposure profiles in Asian countries (except for Japan) is limited when compared with North America, several European nations and susceptible populations worldwide (Mergler et al. 2007). Here we focused on characterizing Hg and Se levels in commonly consumed Taiwanese seafood. In Taiwan, 84% of people show a strong preference for consuming fish (Li et al. 2001). Taiwanese regularly consume three or more fish meals per week (Taiwan Department of Health 1999) and consume 42.4 kg of seafood per capita annually (Li et al. 2001). This contrasts with the United States where the average consumption of fish is 2.4 kg annually (Agency for Toxic Substances and Disease Registry (ATSDR) 1999). Despite high fish consumption in Taiwan relatively little is known about Hg
content in both consumed fish and in human consumers. In a cross-sectional study of 65 pregnant Taiwanese women, 89% had blood Hg levels that exceeded the USEPA’s guideline value of 5.8 mg l1 (Hsu et al. 2007). By comparison in the United States 8% of women had Hg levels that exceeded this benchmark (Mahaffey et al. 2004). Further, in a modelling study by Chien et al. (2007) about 50% of high fish consumers were found to exceed the USEPA’s Hg reference dose of 0.1 mg kg1 day1. Such findings document that Hg exposure is elevated in Taiwan, though additional work is needed to understand the role of beneficial nutrients in fish and seafood. Further, exposure values were compared with the United States and it needs to be stressed that guideline values and risk equations from one ethnic and cultural group may not accurately predict risk in others (Canuel et al. 2006). While some seafood commercially available in Taiwan has elevated levels of Hg (marlin, shark), we report on several others popular seafood that are low in Hg and high in Se (e.g., Japanese butterfish, milkfish).
Conclusion There is much debate concerning the risks (from toxic chemicals, such as Hg) and benefits (from dietary proteins, omega-3 fatty acids and essential elements such as Se) of fish consumption. While experimental evidence suggests an antagonistic relationship between Se and Hg, little is known about their relative levels in the same samples of various fish. It has been shown here that, across 14 commonly consumed seafood species in Taiwan, mean THg concentrations were variable and ranged nearly 100-fold across species. Fifty per cent of the marlins sampled and 35% of the sharks exceeded the 0.3 mg g1 USEPA guideline.
Food Additives and Contaminants: Part B Methylmercury, as expected, comprised a majority of the THg in all species. In all species studied there was a molar excess of Se over Hg, suggesting a positive SHBV in relation to Hg. These data concerning levels of THg, methylmercury, and Se from seafood items of commercial and nutritional importance may assist public health practitioners gauge better the risks and benefits of fish consumption in Taiwan and neighbouring regions.
Acknowledgements The authors thank Erica Boldenow, Ashley Maiuri, Stephen Zelda, Yi-Liang Huang, Jun-Han Huang and Chia-Kuan Liu for technical assistance. Funding for this study was received from the University of Michigan School of Public Health. No conflict of interest is declared.
References Agency for Toxic Substances and Disease Registry (ATSDR). 1999. Toxicological profile for mercury. Atlanta (GA): ATSDR, US Centers for Disease Control (CDC). Canuel R, de Grosbois SB, Atikesse L, Lucotte M, Arp P, Ritchie C, Mergler D, Chan HM, Amyot M, Anderson R. 2006. New evidence on variations of human body burden of methylmercury from fish consumption. Environ Health Perspect. 114:302–306. Chapman L, Chan HM. 2000. The influence of nutrition on methylmercury intoxication. Environ Health Perspect. 108:29–56. Chen J, Berry MJ. 2003. Selenium and selenoproteins in the brain and brain diseases. J Neurochem. 86:1–12. Chien LC, Yeh CY, Huang SY, Shieh MJ, Han BC. 2003. Pharmacokinetic model of daily selenium intake from contaminated seafood in Taiwan. Sci Total Environ. 311(1–3):57–64. PubMed PMID: 12826383. Chien LC, Yeh CY, Jiang CB, Hsu CS, Han BC. 2007. Estimation of acceptable mercury intake from fish in Taiwan. Chemosphere. 67(1):29–35. Epub 2006 Dec 8. PubMed PMID: 17157893. Clarkson TW, Magos L. 2006. The toxicology of mercury and its chemical compounds. Crit Rev Toxicol. 36: 609–662. Food and Agriculture Organization of the United Nations (FAO). 1991. Codex Alimentarius guideline levels for methylmercury in fish. CAC/GL 7-1991. Geneva (Switzerland): World Health Organization (WHO). Hightower JM, O’Hare A, Hernandez GT. 2006. Blood mercury reporting in NHANES: identifying Asian, Pacific Islander, Native American, and multiracial groups. Environ Health Perspect. 114:173–175. Hsu CS, Liu PL, Chien LC, Chou SY, Han BC. 2007. Mercury concentration and fish consumption in Taiwanese pregnant women. BJOG. 114(1):81–5. Epub 2006 Nov 1. PubMed PMID: 17081179. Kaneko JJ, Ralston NVC. 2007. Selenium and mercury in pelagic fish in the central North Pacific near Hawaii. Biol Trace Elem Res. 119:242–254.
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Li HS, Houston JE, Wang SM, Lee HJ. 2001. Factors affecting consumer preferences for fish in Taiwan. Proceedings of the International Institute of Fisheries Economics and Trade. Mahaffey KR, Clickner RP, Bodurow CC. 2004. Blood organic mercury and dietary mercury intake: National Health and Nutrition Examination Survey, 1999 and 2000. Environ Health Perspect. 112:562–570. Mergler D, Anderson HA, Chan HM, Mahaffey KR, Murray M, Sakamoto M, Stern AH. 2007. Methylmercury exposure and health effects in humans: a worldwide concern. Ambio. 36:3–11. Mozaffarian D, Rimm EB. 2006. Fish intake, contaminants, and human health: evaluating the risks and the benefits. J Am Med Assoc. 296:1885–1899. Nam DH, Adams D, Reyier EA, Basu N. 2011. Mercury and selenium levels in lemon sharks (Negaprion brevirostris) in relation to a harmful red tide event. Environ Monit Assess. 176(1–4):549–559. Epub 2010 Jul 14. PubMed PMID: 20625820. Nam DH, Basu N. 2011. Rapid methods to detect organic mercury and total selenium in biological samples. Chem Central J. 5:3. Oken E, Kleinman KP, Berland WE, Simon SR, Rich-Edwards JW, Gillman MW. 2003. Decline in fish consumption among pregnant women after a national mercury advisory. Obstet Gynecol. 102:346–351. Peterson SA, Ralston NVC, Peck DV, Sickle JV, Robertson JD, Spate VL, Morris JS. 2009. How might selenium moderate the toxic effects of mercury in stream fish of the western U.S. Environ Sci Technol. 43:3919–3925. Ralston NVC, Blackwell JL, Raymond LJ. 2007. Importance of molar ratios in selenium-dependent protection against methylmercury toxicity. Biol Trace Elem Res. 119:255–268. Scheuhammer AM, Meyer MW, Sandheinrich MB, Murray MW. 2007. Effects of environmental methylmercury on the health of wild birds, mammals, and fish. Ambio. 36(1):12–18. Swain EB, Jakus PM, Rice G, Lupi F, Maxson PA, Pacyna JM, Penn A, Spiegel SJ, Veiga MM. 2007. Socioeconomic consequences of mercury use and pollution. Ambio. 36:45–61. Taiwan Department of Health. 1999. Nutrition and Health Survey in Taiwan I, 1993–1996 (NAHSIT I). Taipei (Taiwan): Department of Health, Ministry of Interior, Executive Yuan. US Environmental Protection Agency (USEPA). 1997. Mercury study report to Congress, Vol. VII: Characterization of human health and wildlife risks from mercury exposure in the United States. Washington (DC): Office of Research and Development. US Environmental Protection Agency (USEPA). 2001. Water quality criteria for the protection of human health: methylmercury. EPA-823-R-01-001. Washington (DC): USEPA. US Environmental Protection Agency (USEPA). 2009. 2008 National listing of fish advisories: technical fact sheet; [cited 2011 Jan 16]. Available from: http://water. Epa.Gov/scitech/swguidance/fishshellfish/fishadvisories/ tech2008.Cfm/
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Mercury and selenium content of Taiwanese seafood G.C. Fanga, D.H. Namb and N. Basub* a
Department of Safety, Health and Environmental Engineering, Hungkuang University, Sha-Lu, Taichung 433, Taiwan; Department of Environmental Health Sciences, University of Michigan, Ann Arbor, MI 48109, USA
b
(Received 22 March 2011; final version received 22 June 2011) Fish consumption is avid in Taiwan (and other Asian nations), but little is known about the mercury and selenium content in local seafood. This paper reports on total mercury, methylmercury and selenium levels from 14 commonly consumed seafood items obtained from Taichung, Taiwan. Mean total mercury concentrations varied nearly 100-fold across species. Fifty per cent of the marlins sampled and 35% of the sharks exceeded the 0.3 mg g1 US Environmental Protection Agency (USEPA) guideline. Methylmercury comprised a majority of the total mercury in all species. In all species studied there was a molar excess of selenium over mercury. The rank order of mean selenium–mercury molar ratios was red tilapia (166.8)4abura (87.9)4river prawn (82.4)4whiteleg shrimp (64.2)4butterfish (44.6)4milkfish (37.0)4tuna (15.6)4grouper (13.9)4ayu (13.4)4 coral hind (13.0)4weever (11.8)4saury (9.0)4shark (7.8)4marlin (4.2). Keywords: fish; mercury; selenium; risk–benefit; selenium health benefit value
Introduction Seafood (fish and shellfish) are a major source of dietary proteins, essential elements and omega-3 fatty acids (Chapman and Chan 2000; Mozaffarian and Rimm 2006). Harvesting seafood is of immense recreational, economical and cultural importance to millions of people worldwide. However, the presence of toxic chemicals in seafood has spurred many people to avoid consuming seafood (Oken et al. 2003). The avoidance of seafood consumption may pose a greater threat to public health than consuming contaminated products (Mozaffarian and Rimm 2006). Accordingly, greater scientific understanding is required to improve our ability to communicate the risks and benefits associated with seafood consumption. Most seafood consumption advisories are driven by the presence of mercury (Hg) (US Environmental Protection Agency (USEPA) 2009). Though a naturally occurring element, Hg is mainly mobilized into the environment by human activities such as fossil fuel combustion (Swain et al. 2007). Upon deposition into aquatic ecosystems, inorganic Hg may be methylated by microorganisms into methyl-Hg (MeHg). As a methylated chemical, MeHg can effectively cross biological membranes and accumulate in organisms, biomagnify through aquatic food chains, and build up in the tissues of predatory animals. The concentrations of MeHg in tissues of predatory animals (such as
*Corresponding author. Email: [email protected] ISSN 1939–3210 print/ISSN 1939–3229 online ß 2011 Taylor & Francis http://dx.doi.org/10.1080/19393210.2011.605526 http://www.tandfonline.com
humans) may be 10 million times greater than ambient levels in the environment (USEPA 1997). As a result, dietary consumption of fish represents the major route of Hg exposure for human (Mergler et al. 2007) and wildlife (Scheuhammer et al. 2007) populations. Once inside the body, MeHg readily crosses the blood–brain barrier via the methionine uptake system (Clarkson and Magos 2006) and accumulate in the central nervous system. Owing to its highly reactive nature and strong affinity for sulfhydryl groups, MeHg can impair multiple neurological pathways and cause neurotoxicity. There exists a strong correlation between Hg exposure and fish consumption, especially the consumption of predatory, long-lived species such as sharks, marlin and swordfish. Consumption can vary according to ethnicity. In the United States, for example, participants in the NHANES survey that identified themselves as Asian had higher blood Hg levels (via fish consumption) that other ethnic groups (Hightower et al. 2006). Though Asians are among the highest seafood consumers worldwide, when compared with North America, several European nations and certain susceptible groups such as Indigenous Peoples (Mergler et al. 2007), much less is known about Hg exposure profiles in Asian countries (except perhaps for Japan) both in terms of Hg levels in fish as well as Hg exposures (via sampling of hair or blood) in people.
Food Additives and Contaminants: Part B The purpose of this study was to report on Hg (both total Hg and MeHg) levels in 14 commonly consumed seafood products from Taiwan. Selenium (Se) values are also reported given the strong belief that this essential nutritional element affords protection against Hg toxicity (Kaneko and Ralston 2007; Ralston et al. 2007). The Hg data are related to guideline values, and the Hg and Se data are compared with each other on a molar basis and indexed through a Se health benefit value (SHBV) equation. By documenting Hg and Se values from same samples and from commonly consumed seafood, public health practitioners may better gauge the risks and benefits.
Methods Sample collection and preparation Twenty edible samples of 14 different seafood species were obtained from local restaurants and markets in the town of Taichung, Taiwan. Samples are considered to be commonly consumed representative items. Common names and scientific names are provided (Table 1). Samples were kept frozen at 20 C until processed at Hungkuang University. Samples were oven dried at 60 C for 3 days and then pulverized into a powder. Moisture content was calculated for each sample (Table 1). All concentrations in this paper are reported on a wet weight basis.
Analysis of total mercury (THg) and methylmercury (MeHg) Concentration of THg in each sample (n ¼ 20 per species) was measured in a Direct Mercury Analyser 80 (DMA-80, Milestone Inc., Shelton, CT, USA) according to USEPA Method 7473 as previously described by Nam and Basu (2011). About 10–30 mg of dried
213
sample were weighted in a nickel sampling boat and placed into the DMA-80. Following decomposition of the sample at 800 C, liberated Hg was next trapped using gold and then subsequently desorbed, carried to an absorbance cell and quantified spectrophotometrically. For MeHg, eight to ten samples for each species were analysed according to methods outlined by Nam and Basu (2011). Briefly, approximately 50 mg of dried samples were homogenized in 50 mM Tris-HCl buffer (pH 8.5) which contained protease. The homogenate was next incubated at 50 C for 1 h, and then sequentially mixed with 40% NaOH, 1% cysteine, 25 mM CuSO4, acidic NaBr and toluene. The mixture was centrifuged at 13 000g for 5 min, and the top toluene layer was transferred to a test tube and mixed twice with of Na2S2O3 (5 mM) to permit back-extraction of MeHg into an aqueous phase. The aqueous layer was placed into another test tube for analysis of MeHg by direct analysis in the DMA-80.
Selenium analysis For Se, eight to ten samples for each species were analysed according to methods outlined previously (Nam et al. 2011; Nam and Basu 2011). Concentrated HNO3 (0.9 ml, 70% trace element grade) was added to dried samples in a Teflon tube, capped and allowed to digest overnight in a fume hood. After gentle shaking, tubes were heated for 3–4 h at 100 C until digestion was complete. After samples were cooled, the tube was rinsed with MilliQ water and the digest was transferred into clean test tubes. Se concentrations were determined by graphite furnace AAS (Varian AAS 220FS) equipped with a deuterium corrector to minimize background signals. Into a graphite tube, 20 ml of standard solution, tissue samples or blanks were combined with 20 ml of nickel nitrate modifier
Table 1. Mercury (total and MeHg) and selenium concentrations in 14 commonly consumed Taiwanese seafoods. Values represent mean standard deviation. Common name Marlin Shark Tuna Grouper Weever Pacific saury Coral hind Ayu sweetfish Japanese butterfish Milkfish Abura Penaeid Shrimp River Prawn Red Tilapia
Scientific name
Total Hg (mg g1)
% MeHg
Se (mg g1)
Mokaira mazara Carcharhinus sp. Thunnus albacares Epinephelus hexagonatus Lateolabrax japonicus Cololabis saira Cephalopholis miniata Plecoglossus altivelis altivelis Psenopsis anomala Chanos chanos Johnius distinctus Penaeus vannamei boone Macrobrachium rosenbergii Oreochromis hybrids
0.329 0.267 0.305 0.256 0.158 0.083 0.091 0.028 0.083 0.051 0.079 0.048 0.077 0.042 0.047 0.007 0.029 0.048 0.016 0.004 0.014 0.009 0.013 0.006 0.010 0.011 0.004 0.001
80.4 31.2 82.8 33.1 89.3 15.0 93.2 7.9 97.0 6.4 81.3 11.8 92.1 7.8 97.8 7.0 83.9 7.1 71.8 14.1 92.4 11.0 102 2.9 80.8 21.6 99.7 0.8
0.424 0.285 0.457 0.340 0.959 0.913 0.500 0.245 0.241 0.059 0.265 0.104 0.331 0.189 0.248 0.090 0.330 0.141 0.230 0.111 0.402 0.103 0.256 0.167 0.261 0.052 0.229 0.075
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(6 mg ml1). Samples were then dried at 85–120 C, heated slowly to 1000 C and atomized at 2600 C. Standard curves and detection limits were determined using a series of diluted Se standards.
equation: SHBV ¼ ½ðm mol Se kg1 Þ ðSe=HgÞ ½ðm mol Hg kg1 Þ ðHg=SeÞ
Quality control Analytical accuracy and precision were determined through the use of Standard Reference Materials (DOLT-3, DOLT-4 and TORT-2 from the National Research Council of Canada) and intermittent analysis of duplicate samples. Average recovery rates of SRMs for THg, MeHg and Se were 95.7% 7.6%, 94.2% 8.4%, and 90.1% 7.9%, respectively, and deemed to be excellent. For analyses of THg, MeHg and Se, the relative standard deviation (% RSD) was lower than 9% for all replicate measures. The detection limit for the direct Hg analyser was 0.021 ng and ranged from 0.011 to 0.028 ng. For Se, the limit of measurement by graphite furnace AAS was 0.001 mg g1 of Se. All concentrations are expressed as mg g1 (ppm) wet weight.
Data analyses Statistical operations were performed using SPSS (v11.5, Chicago, IL, USA). Data analysis included the tabulation of descriptive statistics for all measurements. To relate Hg values to suggested fish consumption guidelines by the World Health Organization (WHO) (1 mg g1 in fish; Food and Agriculture Organization of the United Nations (FAO) 1991) and USEPA (0.3 mg g1 in fish; USEPA 2001), the percentage of samples exceeding values was calculated. An SHBV for each sample was calculated according to Kaneko and Ralston (2007) based on the following
Results and discussion Hg (total, MeHg) was detected in each sample. The mean THg concentration (mass as microgram per gram, molar as micromole per kilogram) for each of the 14 species studied is presented in Tables 1 and 2. Mean concentrations across seafood varied nearly 100fold and ranged from 0.004 mg g1 for red tilapia to 0.329 mg g1 for marlin (Figure 1). The WHO uses a THg guidance value of 1 mg g1 wet weight in fish (FAO 1991). Mean THg concentrations were below 1 mg g1 for all species, and only one shark sample exceeded 1 mg g1 (¼1.046). The USEPA uses a THg guidance level of 0.3 mg g1 wet weight to protect public health (USEPA 2001). Here, mean THg values in marlin and shark both exceeded this 0.3 mg g1. Fifty per cent of the marlins sampled and 35% of the sharks exceeded 0.3 mg g1. The THg values reported here in shark are similar to those reported previously on Taiwanese sharks (0.73 0.54 mg g1) (Chien et al. 2007). Further, THg in tuna and milkfish reported here are similar to values reported previously from Taiwan, though tilapia concentrations reported here are lower than values reported by Chien et al. MeHg comprised a majority of the THg in all species, but the proportion varied (Table 1). In milkfish, MeHg comprised 71.8% of the THg, whereas MeHg comprised495% of the total in weever, ayu, whiteleg shrimp and red tilapia (Figure 1). Understanding and reporting MeHg levels in fish is
Table 2. Molar concentrations of mercury and selenium, as well as molar ratios and calculated selenium health benefit value. Values represent mean standard deviation.
Species
Hg (mmol kg1)
Se (mmol kg1)
Hg/Se molar ratio
Se/Hg molar ratio
Se HBV
Marlin Shark Tuna Grouper Weever Pacific saury Coral hind Ayu sweetfish Japanese butterfish Milkfish Abura Penaeid shrimp River prawn Red tilapia
1.64 1.33 1.53 1.28 0.79 0.41 0.46 0.14 0.42 0.26 0.40 0.24 0.39 0.21 0.23 0.03 0.14 0.24 0.08 0.02 0.07 0.04 0.06 0.03 0.05 0.05 0.02 0.01
5.36 3.61 6.21 4.40 12.14 11.56 6.32 3.09 3.06 0.74 3.36 1.31 4.19 2.39 3.14 1.14 4.17 1.78 2.92 1.41 5.09 2.44 3.24 2.12 3.30 0.66 2.89 0.95
0.46 0.27 0.24 0.12 0.13 0.13 0.10 0.10 0.06 0.03 0.01 0.02 0.02 0.01
4.19 7.83 15.57 13.86 11.77 9.01 12.95 13.40 44.58 36.98 87.90 64.21 82.44 166.77
26.74 54.64 295.68 104.93 37.19 33.72 67.41 47.89 214.51 134.54 540.00 251.14 289.40 537.43
Food Additives and Contaminants: Part B
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Figure 1. Total mercury concentrations (mg g1) in 14 commonly consumed Taiwanese seafoods. Bars represent the mean standard deviation.
Figure 2. Molar ratios of mercury:selenium in 14 commonly consumed Taiwanese seafoods. All means are below a 1 : 1 molar ratio. Bars represent the mean standard deviation.
relevant given that this form of Hg is readily absorbed into the body and crosses protective barriers for the brain and placenta (Clarkson and Magos 2006). The mean total Se concentration (mass as microgram per gram, molar as micromole per kilogram) for each of the 14 species studied is presented in Tables 1 and 2. Mean concentrations ranged from 0.229 mg g1 for red tilapia to 0.959 mg g1 for tuna. In a study of several pelagic fish near Hawaii, Kaneko and Ralston (2007) also documented that Se levels were generally highest in tuna. In a study of Taiwanese seafood, Chien et al. (2003) measured Se and like the present authors found that concentrations in fish were generally higher than in crustaceans. From their study, Chien et al. calculated seafood to be a major contributor of Se to the Taiwanese consumer. In order to compare Hg and Se values, concentrations were converted to molar concentrations and
displayed (Table 2 and Figure 2). In all species studied there was a molar excess of Se over Hg. The decreasing rank order of mean Se:Hg ratios was red tilapia (166.8)4abura (87.9)4river prawn (82.4)4whiteleg shrimp (64.2)4butterfish (44.6)4milkfish (37.0)4 tuna (15.6)4grouper (13.9)4ayu (13.4)4coral hind (13.0)4weever (11.8)4saury (9.0)4shark (7.8)4marlin (4.2). In other studies, a majority of species investigated also showed a molar excess of Se over Hg in both marine (Kaneko and Ralston 2007) and freshwater (Peterson et al. 2009) ecosystems. There are some notable marine species with molar excess of Hg over Se, including mako shark (Kaneko and Ralston 2007) and lemon shark (Nam et al. 2011). Swordfish have a near equal molar ratio of Hg and Se (Kaneko and Ralston 2007). Se is an essential nutrient with important antioxidant roles and anticancer properties
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Figure 3. Selenium health benefit value of 14 commonly consumed Taiwanese seafoods. Bars represent the mean standard deviation.
(Chen and Berry 2003). There is ample experimental evidence that Se protects against Hg toxicity (Chapman and Chan 2000). As Se may mitigate the toxicity of Hg, the molar ratio of Hg:Se has been suggested to be one essential criterion for assessing the true health risks posed by Hg (Kaneko and Ralston 2007; Ralston et al. 2007). Kaneko and Ralston (2007) proposed an index to help identify fish species with low Hg levels and high Se content. Their SHBV index is calculated by considering both the absolute and the relative concentrations of Hg and Se on a molar basis in a given sample. Here, the SHBV exceeded 200 in tuna (295.7), butterfish (214.5), shrimp (251.1), river prawn (289.4), and exceeded 500 in abura (540.0) and red tilapia (537.4) (Figure 3). The SHBV of tuna is of particular note as this is a species widely consumed worldwide and often subject to advisories. The Se content measured here is among the highest across species, and the SHBV reported here is similar to several tuna species studied by Kaneko and Ralston (2007). It is established that Asians are avid consumers of fish. However, knowledge of Hg exposure profiles in Asian countries (except for Japan) is limited when compared with North America, several European nations and susceptible populations worldwide (Mergler et al. 2007). Here we focused on characterizing Hg and Se levels in commonly consumed Taiwanese seafood. In Taiwan, 84% of people show a strong preference for consuming fish (Li et al. 2001). Taiwanese regularly consume three or more fish meals per week (Taiwan Department of Health 1999) and consume 42.4 kg of seafood per capita annually (Li et al. 2001). This contrasts with the United States where the average consumption of fish is 2.4 kg annually (Agency for Toxic Substances and Disease Registry (ATSDR) 1999). Despite high fish consumption in Taiwan relatively little is known about Hg
content in both consumed fish and in human consumers. In a cross-sectional study of 65 pregnant Taiwanese women, 89% had blood Hg levels that exceeded the USEPA’s guideline value of 5.8 mg l1 (Hsu et al. 2007). By comparison in the United States 8% of women had Hg levels that exceeded this benchmark (Mahaffey et al. 2004). Further, in a modelling study by Chien et al. (2007) about 50% of high fish consumers were found to exceed the USEPA’s Hg reference dose of 0.1 mg kg1 day1. Such findings document that Hg exposure is elevated in Taiwan, though additional work is needed to understand the role of beneficial nutrients in fish and seafood. Further, exposure values were compared with the United States and it needs to be stressed that guideline values and risk equations from one ethnic and cultural group may not accurately predict risk in others (Canuel et al. 2006). While some seafood commercially available in Taiwan has elevated levels of Hg (marlin, shark), we report on several others popular seafood that are low in Hg and high in Se (e.g., Japanese butterfish, milkfish).
Conclusion There is much debate concerning the risks (from toxic chemicals, such as Hg) and benefits (from dietary proteins, omega-3 fatty acids and essential elements such as Se) of fish consumption. While experimental evidence suggests an antagonistic relationship between Se and Hg, little is known about their relative levels in the same samples of various fish. It has been shown here that, across 14 commonly consumed seafood species in Taiwan, mean THg concentrations were variable and ranged nearly 100-fold across species. Fifty per cent of the marlins sampled and 35% of the sharks exceeded the 0.3 mg g1 USEPA guideline.
Food Additives and Contaminants: Part B Methylmercury, as expected, comprised a majority of the THg in all species. In all species studied there was a molar excess of Se over Hg, suggesting a positive SHBV in relation to Hg. These data concerning levels of THg, methylmercury, and Se from seafood items of commercial and nutritional importance may assist public health practitioners gauge better the risks and benefits of fish consumption in Taiwan and neighbouring regions.
Acknowledgements The authors thank Erica Boldenow, Ashley Maiuri, Stephen Zelda, Yi-Liang Huang, Jun-Han Huang and Chia-Kuan Liu for technical assistance. Funding for this study was received from the University of Michigan School of Public Health. No conflict of interest is declared.
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