Methyl mercury exposure in Swedish women with high fish consumption

https://doi.org/10.1016/j.scitotenv.2004.09.033Get rights and content

Abstract

We studied the exposure to methyl mercury (MeHg) in 127 Swedish women of childbearing age with high consumption of various types of fish, using total mercury (T-Hg) in hair and MeHg in blood as biomarkers. Fish consumption was assessed using a food frequency questionnaire (FFQ), including detailed information about consumption of different fish species, reflecting average intake during 1 year. We also determined inorganic mercury (I-Hg) in blood, and selenium (Se) in serum. The average total fish consumption, as reported in the food frequency questionnaire, was approximately 4 times/week (range 1.6–19 times/week). Fish species potentially high in MeHg, included in the Swedish dietary advisories, was consumed by 79% of the women. About 10% consumed such species more than once a week, i.e., more than what is recommended. Other fish species potentially high in MeHg, not included in the Swedish dietary advisories, was consumed by 54% of the women. Eleven percent never consumed fish species potentially high in MeHg.

T-Hg in hair (median 0.70 mg/kg; range 0.08–6.6 mg/kg) was associated with MeHg in blood (median 1.7 μg/L; range 0.30–14 μg/L; rs=0.78; p<0.001). Hair T-Hg, blood MeHg and serum Se (median 70 μg/L; range 46–154 μg/L) increased with increasing total fish consumption (rs=0.32; p<0.001, rs=0.37; p<0.001 and rs=0.35; p=0.002, respectively). I-Hg in blood (median 0.24 μg/L; range 0.01–1.6 μg/L) increased with increasing number of dental amalgam fillings. We found no statistical significant associations between the various mercury species measured and the Se concentration in serum. Hair mercury levels exceeded the levels corresponding to the EPA reference dose (RfD) of 0.1 μg MeHg/kg b.w. per day in 20% of the women. Thus, there seems to be no margin of safety for neurodevelopmental effects in fetus, for women with high fish consumption unless they decrease their intake of certain fish species.

Introduction

In Sweden, the use of mercury (Hg) in industry and for dentistry has decreased substantially, but lakes and coastal areas will remain contaminated for many decades. Hg has accumulated over many years in soil and it is continuously released to lakes and coastal waters. In addition, there is a global transport of Hg emissions leading to a significant contribution to the environmental mercury pool (Johansson et al., 2001). Hg biomagnifies in the aquatic food web as methyl mercury (MeHg); hence, large predatory species have the highest concentrations (UNEP, 2002).

MeHg is a well-documented neurotoxicant, and in particular, the developing brain is vulnerable (NRC, 2000). Therefore, the Swedish National Food Administration recommends pregnant and lactating women to refrain from eating the following predatory species: pike, perch, pike–perch, burbot, eel and halibut (SLV, 2003). We have recently shown that the compliance to follow the advisories is good among pregnant women and that exposure of MeHg in general is low in this group (Björnberg et al., 2003). Other groups, including women of childbearing age, are recommended to avoid eating these species more than once a week. However, the compliance is not known. Furthermore, since the biological half-life of MeHg in blood is nearly 2 months (NRC, 2000), consumption of MeHg contaminated fish before the onset of pregnancy will lead to early fetal exposure. The main purpose of this study was to assess the exposure to MeHg in Swedish women of childbearing age with high consumption of various types of fish, and to evaluate the risk of health effects, especially fetal effects, if the women were to become pregnant.

Selenium (Se) is an essential, antioxidative trace element that has been shown to interact with both MeHg and inorganic Hg (I-Hg) in experimental studies, and thereby be protective against Hg toxicity; however, the effect is not well documented in humans (NRC, 2000, Watanabe, 2002). In Sweden, Se is naturally low in rocks, soils and plants, and the intake among Swedish women is low (32 μg/day on average; Alexander and Meltzer, 1995, Becker and Pearson, 2002). Fish is considered to be an important source of Se intake. Therefore, we assessed the Se status and possible associations between Se and different Hg species in Swedish women with high fish consumption.

Section snippets

Study participants and sampling

From June to August 2001, we recruited 127 Swedish women with high fish consumption (=eat fish several times per month), 19–45 years of age (median 38 years), by advertising in magazines and local press. The women were recruited from parts of Sweden where the levels of MeHg in fish are known to be high (Johansson et al., 2001). The women donated hair, blood and serum. Full-length hair samples were tied and cut close to the scalp end and put into plastic bags. For most women, 12 cm of hair from

Results

The median total fish consumption was 4.3 times/week (range 1.6–19 times/week; n=127) according to the FFQ and 2 times/week (range 0.5–7 times/week or more; n=125) according to the cross-check question on the average total fish consumption. The correlation coefficient (rs) between the two estimates of total fish consumption was 0.55 (p<0.001). The FFQ data was used in the following analyses.

The concentrations of Hg in hair and blood, and Se in serum are given in Table 2. As expected, hair T-Hg

Discussion

The total fish consumption (approximately 4 times/week) as reported in the food frequency questionnaire (FFQ) was twice as high as reported in the cross-check question. It has been shown that intake estimates often are overestimated when the number of questions is increased (Krebs-Smith et al., 1995) and underestimated when several items are summarized in one question (Serdula et al., 1992). We used the fish consumption data reported in the FFQ when analyzing the data to separate between

Acknowledgements

We express our gratitude to the participating women. Brita Palm and Birger Lind are acknowledged for technical assistance. The study was performed through a financial support from the Swedish Environmental Protection Agency, Stockholm, Sweden.

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