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Seaweed rich in vitamin A, vitamin B1 and vitamin B2, is more important is its ability to help the body rid their bodies of waste and excess water.
Arsenic in seaweed—Forms, concentration and dietary exposure a,* Martin Rose , John Lewis a, Nicola Langford a, Malcolm Baxter a, Simona Origgi b, Matthew Barber b, Helen MacBain b, Kara Thomas b a Defra Central Science Laboratory, Sand Hutton, York YO41 1LZ, UK b Food Standards Agency, 125 Kingsway, London WC2 6NH, UK Abstract This study has measured the content of total and inorganic forms of arsenic in seaweed available on retail sale for consumption, to provide data for dietary exposure estimates and to support advice to consumers. A total of 31 samples covering ﬁve varieties of seaweed were collected from various retail outlets across London and the internet. All of the samples were purchased as dried product. For four of the ﬁve varieties, soaking was advised prior to consumption. The recommended method of preparation for each individual sample was followed, and total and inorganic arsenic were analysed both before and after preparation. The arsenic remaining in the water used for soaking was also measured. Arsenic was detected in all samples with total arsenic at concentrations ranging from 18 to 124 mg/kg. Inor- ganic arsenic, which can cause liver cancer, was only found in the nine samples of hijiki seaweed that were analysed, at concentrations in the range 67–96 mg/kg. Other types of seaweed were all found to contain less than 0.3 mg/kg inorganic arsenic, which was the limit of detection for the method used. Since consumption of hijiki seaweed could signiﬁcantly increase dietary exposure to inorganic arsenic, the UK Food Standards Agency (FSA) issued advice to consumers to avoid eating it. Crown Copyright Keywords: Arsenic; Seaweed; Hijiki; Dietary intake; Consumer exposure 1. Introduction trial food chain and these higher levels result in the poten- tial for elevated dietary exposure through diets rich in ﬁsh Arsenic is an environmental contaminant and can arise and other food harvested or farmed at sea. Consumption from natural sources such as rocks and sediments and also of ﬁsh and seafood is known to have beneﬁcial health as a result of anthropogenic activities such as coal burning, eﬀects and it is recommended that the general population copper smelting and the processing of mineral ores. Levels in some countries, especially where ﬁsh consumption is of arsenic are higher in the aquatic environment than in lower than average, should increase the amount of ﬁsh most areas of land as it is fairly water-soluble and may (especially ﬁsh rich in oils) in the diet (Food Standards be washed out of arsenic-bearing rocks. In particular, sea- Agency, 2006; COT, 2004a). Arsenic exists in diﬀerent weed is known to contain high concentrations of arsenic in chemical forms, or ‘species’. These are either as ‘free’ inor- comparison to terrestrial plants owing to the ability of mar- ganic arsenic species such as As(III) or As(V), or as arsenic ine plants to concentrate the arsenic they derive from sea present in organic molecules such as arsenobetaine and water (Norman et al., 1987). Higher levels of arsenic are arseno-sugars (Francesconi and Kuehnelt, 2004). Inorganic thus expected in the aquatic food chain than in the terres- arsenic has no known beneﬁcial eﬀect, and long-term expo- sure may be harmful to health. Arsenic is genotoxic and is a known human carcinogen associated especially with liver, bladder, lung and skin cancer. As(III) is generally recogni- sed as being more toxic than As(V). Human exposure to inorganic arsenic compounds should thus be kept as low as the Environment (COT) noted the evidence produced since reasonably practicable (COT, 2003). the JECFA evaluation and the IARC opinion and con- The 1997 UK Total Diet Study showed a concentration of cluded that dietary exposure to inorganic arsenic should 4.4 mg/kg of total arsenic in the ﬁsh group, which accounts be as low as reasonably practicable (ALARP). JECFA for 94% of the average population exposure to arsenic (Ysart are scheduled to review the PTWI for arsenic. et al., 2000). Most of this arsenic was present in the less The aim of this study was to assess dietary exposure of toxic organic forms such as arsenobetaine, with inorganic inorganic and organic forms of arsenic arising due to the arsenic only accounting for ca. 1–3% of the total arsenic. consumption of seaweed in order to form the basis of Seaweed was not included in these total diet samples. advice to consumers. The reason that the fraction of inorganic arsenic is so low in ﬁsh is because most aquatic organisms have the 2. Materials and methods ability to metabolise the toxic inorganic forms of arsenic to the less toxic forms, such as dimethylarsinate (DMA) 2.1. Samples and arsenobetaine. Certain seaweeds do not use this meta- A total of 31 samples comprising ﬁve diﬀerent varieties of seaweed on bolic conversion process, and deposit the arsenic in a range retail sale within the UK were collected from London and the surrounding of chemical forms, including signiﬁcant proportions as the area between October and December 2003. Samples were collected at toxic inorganic forms, As(III) and As(V). There are several random from a variety of retail outlets ranging from specialist retailers to suggestions for the reasons for this, including (i) lack of the major supermarket chains, according to availability. It was ensured that genetic capability, (ii) it is energetically too expensive to all major imported varieties were included in the sampling exercise. Because all of the purchased samples were imported, there is no reason to perform the metabolic transformation, and (iii) the stores believe that there would be any regional variation in quality within the of inorganic arsenic within the cellular structure act as a UK. The instructions for preparation and cooking were translated from form of protection against predation (Castlehouse et al., the product package labels. All samples were analysed for total arsenic 2003; Devalla and Feldmann, 2003). and inorganic arsenic as sold. Where package instructions directed further Seaweed is rich in some nutrients such as iodine (Lin preparation prior to consumption, the directions were followed. This applied for 24 of the samples (four out of the ﬁve diﬀerent varieties) and et al., 2003) and Eastern varieties are increasingly consumed for these products the samples were analysed after preparation as well as in Western countries including the United Kingdom as the water that was used for soaking the product (see below). oriental dishes become more popular. Following a report that the Canadian Food Inspection Agency was advising 2.2. Sub-sampling and preparation of samples consumers to avoid the consumption of hijiki seaweed owing to its high inorganic arsenic content (CFIA, 2001), Nori was the only seaweed type purchased as ready-to-eat. The other the UK Food Standards Agency (FSA) conducted a survey types required either soaking in cold or boiling water prior to consump- tion. In order to assess the impact of cooking/preparation on the levels of of seaweed available on the UK market with the Central total and inorganic arsenic in the seaweed, these samples were analysed Science Laboratory (CSL) performing the analyses. both ‘as bought’ (without preparation) and ‘as consumed’ (following There are no Europe-wide regulations for arsenic in preparation). The water remaining after preparation was also analysed, food. In the UK, the Arsenic in Food Regulations (SI because it was claimed by some producers that the preparation steps 1959 no. 831), as amended, lay down a general limit of signiﬁcantly reduced the levels of inorganic arsenic in their products due to the greater water solubility. All stages of sample preparation were there- 1 mg/kg for total arsenic in food. However, this limit does fore performed gravimetrically, allowing a mass balance calculation to be not apply to ﬁsh and edible seaweed where it is naturally performed. present. There is a lack of standard validated methods Homogenisation of the ‘as-bought’ samples was carried out using for the determination of arsenic species. There are no fully sharp stainless steel knives, on plastic chopping boards. For the kelp-like characterised certiﬁed reference materials (CRMs) for seaweeds, which are sold as dry sheets of material, the samples were split vertically to give two equivalent halves. One half was homogenised, ready arsenic species, although there are some materials which for analysis ‘as-bought’. The other half was prepared, homogenised using are partially characterised e.g., DORM 2 dogﬁsh muscle a Buchi Mixer B-400 and taken for immediate analysis. A representative ¨ with a value for total As of 18 mg/kg dry weight and arsen- sub-sample of the water remaining after preparation of each seaweed was obetaine at 16.4 mg/kg and BCR 627 tuna ﬁsh with an transferred to an acid-washed Nunc vial and stored at 4 °C until required arsenobetaine content of 52 lmol/kg, dimethyl arsinic acid for analysis. For those samples of seaweed supplied in other forms (typically (DMAA) at 2 lmol/kg and total As at 4.8 mg/kg (NRCC, resembling grass), every eﬀort was made to mix the individual strands so 1993; EC BCR, 1997). as produce a representative sub-sample of each packet. When sub-sam- In 1989, JECFA established a provisional tolerable pling these varieties, the samples were cut horizontally so as to give a weekly intake (PTWI) for arsenic of 15 lg/kg bodyweight. representative sub-sample of the many individual plants making up each In 2002, the International Agency for Cancer Research sample. (IARC) concluded that arsenic in drinking water (primarily inorganic arsenic as arsenate and to a lesser extent arsenite) 2.3. Analytical method (i) total arsenic was carcinogenic to humans on the basis of suﬃcient evi- Samples (0.5 g dry weight or equivalent wet weight, typically 4 g) were dence for an increased risk for cancer of the urinary blad- quantitatively digested in quartz high pressure closed vessels using con- der, lung and skin. In May 2003, the UK Committee on centrated nitric acid (5 ml) and a microwave-assisted digestion system Toxicity of Chemicals in Food, Consumer Products and (Multiwave, Perkin–Elmer Ltd, Beaconsﬁeld, Buckinghamshire, UK). The digest liquor was quantitatively transferred to a graduated test tube and to calculate that, even using the upper bound value of made up to volume (10 ml) with water. An aliquot (1 ml) of the resulting 0.3 mg/kg inorganic arsenic, the percentage of the speci- solution was quantitatively transferred to a test tube, and an aliquot (4 ml) of diluent (an aqueous solution containing rhodium, as internal standard) ated form could not have been greater than 2%. Levels that was added. Total arsenic was measured using inductively coupled plasma- were not detected were at the limit of detection. This was mass spectrometry (ICP-MS). The method used was that published pre- indicated in the table as <2 and agrees with other values viously (Ysart et al., 2000). reported, i.e., about 3% (Devalla and Feldmann, 2003). For those seaweeds where the levels of inorganic arsenic 2.4. Analytical method (ii) inorganic arsenic are above the LOD, i.e., the hijiki samples, values for the percentage of inorganic arsenic have been calculated and The non-chromatographic methodology used was based on that developed by Munoz et al. (2000). Following dissolution of samples in are quoted in Table 1. concentrated hydrochloric acid, and conversion of inorganic arsenic into In hijiki, the mean percentage of inorganic arsenic is As(III), the As(III) was converted into a covalent halide and extracted into highest in seaweed as sold (73%), less in prepared seaweed chloroform. The arsenic was back extracted into dilute hydrochloric acid (67%) and is further reduced when it is soaked (55%). In and was measured using ICP-MS operating at a resolution of 10 000. The the other two varieties of seaweed the percentage of inor- back extraction step also extracts the toxic mono-methylated forms of arsenic (MMA) but the procedure will not extract the relatively non-toxic ganic arsenic was higher in the prepared seaweed than in compounds such as arsenobetaine and arseno-sugars. the seaweed as sold. All of the hijiki samples, both before and after pre- 2.5. Analytical quality assurance paration, had signiﬁcantly high levels of inorganic arsenic (68–73% and 61–73%, respectively). These data seem to CRMs for total arsenic, reagent blanks and reagent blanks spiked with contradict the claims made by some producers, that the a known amount of analyte (for recovery purposes) were analysed preparation steps can signiﬁcantly reduce the fraction of alongside samples. The method used for total arsenic is accredited to the ISO 17025 (UKAS) standard and the method for inorganic arsenic was water soluble inorganic arsenic. This variety of seaweed carried out with similar rigorous quality control criteria i.e., in an envi- also had signiﬁcantly higher levels of total arsenic than ronment with controlled record keeping and using in-house reference the four other varieties tested. materials where CRMs were not available. Proﬁciency testing exercises The levels of both total and inorganic arsenic were mea- carried out around the time of this study for arsenic in food, gave satis- sured in the preparation water and were also found to be factory results for both total and inorganic arsenic. There are no CRMs for inorganic arsenic. QC was monitored by recovery from fortiﬁed tissue. higher in the samples from hijiki than from any of the other The criteria used for the survey were those routinely applied in the varieties. laboratory and were always met: 4. Risk assessment • Results for duplicate analysis were acceptable if the relative standard deviation was less than 25%, or if the diﬀerence was no more than In order to conduct an eﬀective risk assessment and two times the limit of detection, whichever was greater. establish a safe level of seaweed consumption, it is neces- • Procedural blanks and fortiﬁed procedural blanks analysed to estimate recovery were accepted if recovery was 80–120%. sary to know the typical seaweed portion size. No compre- • Analysis of the CRM gave results within the certiﬁed range, or within hensive data are available for seaweed consumption in the 25% of the quoted value, whichever was greater. UK: seaweed is not a regular part of the UK diet and con- sumption patterns have not been captured during routine dietary surveys (Henderson et al., 2002). The packaging 3. Results and discussion labels on the samples purchased did not state a recom- mended quantity of seaweed consumption per person, but Results for the survey are shown in Table 1. in some instances did include recipe suggestions. In order All data are corrected for reagent blank and spike recov- to calculate an average portion size for each of the ﬁve dif- ery. The limit of detection (LOD) was calculated as three ferent types of seaweed tested in this study, a range of rec- times the standard deviation of reagent blank values ipes were obtained and an average amount of each type of adjusted for dilution and sample weight. Table 1 presents seaweed used calculated (see Table 2). both the totals and the inorganic arsenic data obtained for Hijiki seaweed was the only type of seaweed for which the seaweed samples ‘as sold’ and ‘after preparation’, and levels of inorganic arsenic were above the LOD. Consump- also the water remaining after preparation of the samples. tion of the maximum suggested amount of hijiki seaweed Where possible, the percentage of inorganic arsenic was (25 g) with the maximum concentration of inorganic calculated for all three sample types, i.e., ‘as sold’, ‘pre- arsenic (22.7 mg/kg) would lead to an inorganic arsenic pared’ and ‘water’. As nori seaweed is eaten without further intake of 0.57 mg. Results from the survey of total and preparation, there were no ‘water’ data to accompany the inorganic arsenic in the 1999 Total Diet Study (FSA, seven samples of this species. 2004a) showed upper-bound daily intakes ranged from Where calculation of the inorganic arsenic used results 0.1 lg/kg bw for a normal-consuming adult to 0.3 lg/kg below the LOD for the method, this was represented in bw for a high consuming toddler (1.5–4.5 years). The addi- the table by ‘–’ (not measured). For those samples where tional exposure from a 25 g portion of hijiki seaweed would the levels of total arsenic were very high, it was possible represent a 30–50-fold increase in these arsenic intakes. In Table 1 Total and inorganic arsenic in seaweeds Total arsenic – as Seaweed (as sold) Prepared seaweed Soaking water sold (mg/kg) Inorganic arsenic – as Percentage Total arsenic (mg/kg) Inorganic arsenic Percentage Total arsenic Inorganic arsenic Percentage sold (mg/kg) inorganic arsenic (wet weight) (mg/kg) (wet weight) inorganic arsenic (mg/kg) (mg/kg) inorganic arsenic Hijiki 107 73 69% 19 13 71% 14.0 7.8 55% 112 80 72% 7.9 5.1 64% 1.5 0.9 60% 116 83 71% 12 7.9 69% 4.6 2.9 64% 100 69 69% 16 10 66% 3.4 2.1 64% 95 67 71% 14 8.3 60% 3.5 2.3 64% 110 81 73% 11 7.9 70% 6.0 3.6 59% 112 76 68% 31 23 73% 3.0 0.4 15% 102 72 71% 8.9 5.5 61% 4.7 4.0 86% 124 96 72% 26 19 71% 8.2 – Mean: 109 77 71% 16 11 67% 5.4 3 55% Arame 32 <0.3 <0.9% 2.6 <0.3 <11% 1.0 <0.01 31 <0.3 <1.0% 2.7 <0.3 <11% 1.1 – 28 <0.3 <1.1% 3.4 <0.3 <8.8% – – Mean: 30 2.9 1.1 Wakame 35 <0.3 <0.9% 5.2 <0.3 <5.8% 0.2 <0.01 42 <0.3 <0.7% 6.1 <0.3 <4.9% – – 34 <0.3 <0.9% 4.6 <0.3 <6.5% 0.1 <0.01 29 <0.3 <1.0% 2.6 <0.3 <12% 0.09 <0.00 36 <0.3 <0.8% 3.4 <0.3 <8.9% 1.3 <0.01 Mean: 35 4.4 0.4 Kombu 51 <0.3 <0.6% 2.3 <0.3 <13% 0.28 <0.01 32 <0.3 <0.9% 0.9 <0.3 <34% 0.08 <0.01 69 <0.3 <0.4% 6.5 <0.3 <4.6% – – 75 <0.3 <0.4% 5.2 <0.3 <5.7% 0.64 <0.01 75 <0.3 <0.4% 5.8 <0.3 <5% 0.69 <0.01 19 [0.3] [1.6%] 1.4 <0.3 <21% 0.07 <0.01 28 <0.3 <1.1% 2.3 <0.3 <13% 0.05 <0.01 Mean: 50 3.5 0.3 Nori 23 <0.3 <1.3% 22 <0.3 <1.4% 18 <0.3 <1.6% 26 <0.3 <1.1% 32 <0.3 <0.9% LOD for total arsenic = 0.02 mg/kg for solids and 0.003 mg/kg for water 18 <0.3 <1.6% LOD for inorganic arsenic = 0.3 mg/kg for solids and 0.01 mg/kg for water 29 <0.3 <1.0% Mean: 24 Square brackets indicate concentration above the limit of detection but below the limit of quantiﬁcation and these values may therefore have greater uncertainty. – Indicates not measured. Table 2 Castlehouse, H., Smith, C., Raab, A., Meharg, A.A., Feldmann, J., 2003. Estimated average portion sizes for diﬀerent seaweed varietiesa Biotransformation of arsenic in soil amended with seaweed. Environ. Sci. Technol. 37, 951–957. Type Amount No. of Quantity Average portion size servings per portion (g) per person (g) Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment (COT), 2003. Statement on arsenic in food: results of the Arame 1 cup 4 12.5 14 1999 total diet survey, <http://www.food.gov.uk/multimedia/pdfs/ 0.34 cup 4 4.25 ArsenicStatement.PDF>. 0.5 cup 1 25 Committee On Toxicity Of Chemicals In Food, Consumer Products And Hijiki 1.5 oz 5 8.4 18 The Environment (COT), 2004a. Advice on ﬁsh consumption: beneﬁts 0.5 cup 2 12.5 & risks. The Stationary Oﬃce, Norwich. ISBN 0 11 243083 X. 0.5 cup 1 25 Available from: <http://www.food.gov.uk/multimedia/pdfs/ 1.4 oz 2 19.6 ﬁshreport2004full.pdf>. 0.5 cup 1 25 Committee On Toxicity Of Chemicals In Food, Consumer Products And The Environment (COT), 2004b. Tox/2004/35. Urgent COT Opinion Kombu 2 strips 2 n/a 7 On Arsenic In Seaweed, <http://www.food.gov.uk/multimedia/pdfs/ 1 oz 4 7 TOx-2004-35.PDF>. Nori 2.8 g 1 2.8 3 Devalla, S., Feldmann, J., 2003. Determination of lipid-soluble arsenic species in seaweed-eating sheep from Orkney. Appl. Organmet. Chem. Wakame 30 g 2 15 10 17, 906–912. 28.3 g 6 4.7 European Commission. Community Bureau of Reference (EC BCR), a Sourced from various internet recipes and instructions on seaweed 1997. Certiﬁed Reference Material BCR 627, <http://www.irmm.jrc. packaging. be/html/reference_materials_catalogue/catalogue/certiﬁcates_and_ reports/BCR-627_cert.pdf>. Food Standards Agency (FSA), 2004a. Food Survey Information Sheet 51/ other words, the amount of inorganic arsenic in a single 04. Total and Inorganic arsenic in the 1999 total diet study. <http:// www.food.gov.uk/science/surveillance/fsis2004branch/fsis5104arsenic>. portion of hijiki seaweed would be equal to the amount Food Standards Agency (FSA), 2004b, Food Survey Information Sheet of inorganic arsenic to which a consumer would normally 61/04: Arsenic in seaweed, <http://www.food.gov.uk/science/surveil- be exposed in their diet over a period of 1–2 months. lance/fsis2004branch/fsis6104>. In 2003, the UK independent Committee on Toxicity of Food Standards Agency (FSA), 2006. Healthy diet: Fish and Shellﬁsh, Chemicals in Food, Consumer Products and the Environ- <http://www.eatwell.gov.uk/healthydiet/nutritionessentials/ﬁshand- shellﬁsh/>. ment (COT) concluded that it would be more appropriate Francesconi, K.A., Kuehnelt, D., 2004. Determination of arsenic species: not to set a PTWI value because arsenic is a genotoxic A critical review of methods and applications, 2000–2003. The Analyst carcinogen, and instead, that exposure should be ‘as low 129, 373–395. as reasonably practicable (ALARP)’. Based on the results Henderson, L., Gregory, J., Swan, G., 2002. The National Diet and from the 1999 UK Total Diet Study (TDS), the high level Nutrition Survey: Adults Aged 19 to 64. The Stationary Oﬃce, London. (97.5&) consumer was estimated to be exposed to 0.05– Lin, F.-M., Wu, H.-L., Kou, H.-S., Lin, S.-J., 2003. Highly sensitive 0.1 lg/kg bodyweight/day (the range represents lower– analysis of iodine anion in seaweed as pentaﬂuorophenoxyethyl upper bound exposure where lower-bound assumes derivative by capillary gas chromatography. Journal of Agricultural non-detects are at zero concentration and upper-bound and Food Chemistry 51 (4), 867–870. assumes that arsenic is present at the LOD for the method). Munoz, O., Devesa, V., Suner, M.A., Velez, D., Montoro, R., Urieta, I., Macho, M.L., Jalon, M., 2000. Total and inorganic arsenic in fresh Consumption of a 25 g portion of hijiki would therefore and processed ﬁsh products. Journal of Agricultural and Food increase intake of inorganic arsenic by around 30 times. Chemistry 48 (9), 4369–4376. The ALARP approach indicated a need for risk manage- National Research Council, Canada (NRCC), 1993 DORM 2 dogﬁsh ment action to reduce this source of exposure and resulted muscle Certiﬁed reference material for trace metals, <http://inms- in the COT and Food Standards Agency issuing advice to ienm.nrc-cnrc.gc.ca/en/calserv/crm_ﬁles_e/DORM-2_certiﬁcate.pdf>. Norman, J.A., Pickford, C.J., Sanders, T.W., Waller, M., 1987. Human consumers not to eat hijiki seaweed (FSA, 2004b; COT, intake of arsenic and iodine from seaweed-based food supplements and 2004b). health foods available in the UK. Food Additives and Contaminants 5 (1), 103–109. Acknowledgement SI 1959 no. 831, Arsenic in Food Regulations <http://www.opsi.gov.uk/ si/si1992/Uksi_19921971_en_1.htm>. Ysart, G., Miller, P., Croasdale, M., Crews, H., Robb, P., Baxter, M., De This work was funded by the UK Food Standards L’Argy, C., Harrison, N., 2000. 1997 UK Total diet study – dietary Agency. exposures to aluminium, arsenic, cadmium, chromium, copper, lead, mercury, nickel, selenium, tin and zinc. Food Additives and Contam- References inants 17, 775–786. Canadian Food Inspection Agency (CFIA), Consumer Advisory (Ottawa, October 2, 2001) Inorganic arsenic and hijiki seaweed consumption, <http://www.inspection.gc.ca/english/corpaﬀr/foodfacts/arsenice.pdf>.
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