The Science of the Total Environment, 126 (1992) 61-74 Elsevier Science Publishers B.V., Amsterdam
Fish
as a source
o f exposure selenium
61
to
mercury
and
B.-G. Svensson a, A. Schfitz a, A. Nilsson a, I. ,~kesson a, B. ~,kesson b and S. Skerfving a aDepartment of Occupational and Environmental Medicine, University Hospital and bDepartment of Medical and Physiological Chemistry, University of Lund, Sweden (Received August 9th, 1991; accepted September 23rd, 1991)
ABSTRACT In a total of 395 subjects with varying fish consumption habits, mercury levels in whole blood (B-Hg), and selenium levels in plasma (P-Se) were studied. Also, in subcohorts, mercury levels in blood cells (Ery-Hg; n = 79), plasma (P-Hg; n = 158) and urine (U-Hg; n = 125) were analysed. There were statistically significant associations between fish intake on the one hand, and B-Hg, Ery-Hg and P-Hg, on the other, but not so with U-Hg. In subjects who never had fish, the average B-Hg was 1.8 ng/g, in subjects who had at least two fish meals each week, 6.7 ng/g. Ery-Hg, and to a less extent P-Hg, were associated with levels of marine n-3 polyunsaturated fatty acids (PUFA) in serum phosphatidylcholine. P-Hg and U-Hg were associated with numbers of teeth with amalgam fillings. P-Se also correlated with fish intake. In subjects who never had fish, P-Se averaged 80 t,g/l, in subjects who had at least two fish meals per week, 91 /zg/1. There was an association between P U F A and P-Se. Further, there were statistically significant associations between P-Se on the one hand, and B-Hg, Ery-Hg and PHg on the other. The data clearly demonstrate the importance of fish for the exposure to methylmercury and selenium in the Swedish diet, and the impact of amalgam as a source of exposure to inorganic mercury.
Key words: fish; mercury; methylmercury; selenium
INTRODUCTION
Mercury, an important pollutant when deposited in water, is methylated and accumulates in most aquatic biota, with the highest concentrations in large predatory fish (Clarkson et al., 1988). Fish in acidic lakes contain higher levels of methylmercury than those which are less acidified (Bj6rklund et al., 1984). Acid precipitation has increased during the last decades. In man the nervous system is particularly sensitive to damage from methylmercury 0048-9697/92/$05.00
© 1992 Elsevier Science Publishers B.V. All rights reserved
62
B.-G. SVENSSON ET AL.
(WHO, 1990). Prenatal exposure to methylmercury causes psychomotor retardation in children (Marsh et al., 1981). Considerable efforts have been made in Sweden to reduce the mercury pollution. In spite of this, many lakes and coastal waters in Sweden are blacklisted because the levels of methylmercury in fish exceed 1 mg/kg. Some 10 000 lakes in Sweden are estimated to have mercury levels in fish above this level. People are recommended not to eat fish from these lakes. Further, there is a general recommendation that fish such as perch, pike, pike-perch and turbot should not be consumed more than once a week, and totally avoided by pregnant women during the whole period of pregnancy. There is cause for concern about the present exposure to methylmercury in the population. Fish is assumed to be an important source of dietary selenium (WHO, 1987). In the Swedish population, which has a low intake of selenium, however, this has not been evaluated. Further, there is a metabolic interaction between selenium and mercury. Accumulation of mercury and selenium in equimolar amounts have been found in the kidneys, the pituitary, the brain and the thyroid from retired miners occupationally exposed to mercury (Kosta et al., 1975). Exposure to elemental mercury affects the excretion of selenium in urine (Alexander et al., 1983). Also, a study of their correlation is important from another point of view as some of the toxic effects of inorganic mercury (Magos and Webb, 1980) and methylmercury (Skerfving, 1978) are reduced by selenium. As thus fish is a main source of both mercury and selenium and because of the potential interactions, we studied mercury and selenium levels in subjects with varying fish intake. STUDY POPULATIONS
Random sample From three rural areas in south-east Sweden, 505 persons, aged 20-66 years, were randomly chosen from the electoral registers. All persons lived in the countryside or in small villages without any local industrial activity. Persons in the sample were invited for examination; 330 (165 males and 165 females; 66% of the original sample) responded and were examined. Data on non-responders have been reported elsewhere (Svensson et al., 1987).
High consumers offish Fifty-four persons (16 females), mostly fishermen, and aged 15-79 years, who had full fish-meals at least twice a week, were examined in the same way.
FISH AS A SOURCE OF EXPOSURE TO MERCURY AND SELENIUM
63
Low consumers offish Eleven males, aged 25-57 years, who never eat fish, in most cases due to allergy, were also included. None of these were vegetarians. These two 'extreme' consumption groups were non-random and recruited by advertising in different media. METHODS
Questionnaire Questions about lifestyle and environmental factors, such as occupation, smoking habits and present health status, were asked for in a questionnaire. Fish consumption was estimated by questions about the number of fishmeals by the week and about species of fish. Also, use of dietary supplements of selenium tablets was noted.
Dental status On a sample of 48 subjects, the oral cavity was inspected and the numbers of dental amalgam fillings were registered.
Blood sampling Venous blood for analysis of mercury and selenium was obtained from a cubital vein in metal-free Vacutainer ® tubes. Analyses of mercury in whole blood (B-Hg) and selenium in plasma (P-Se) were performed on samples from 395 and 392 persons, respectively. Analyses of mercury in plasma (PHg) erythrocytes (Ery-Hg) separately were made on samples from 158 and 79 persons, respectively. Sampling and analysis of mercury in whole blood have been performed on two occasions (at least 8 months apart) for 94 subjects from the random sample.
Urine sampling Overnight urine for mercury analysis (U-Hg) was collected in acid-washed bottles by 125 persons (93 from the random group and 32 from the low and high consumption groups).
Mercury analyses The mercury (Hg) content was determined in wet-digested samples by a
64
B.-G. SVENSSON ET AL.
'cold vapour', atomic absorption technique, using automatic equipment (Einarsson et al., 1984). Samples of plasma (1.0 g) and whole blood (0.5 g) were digested with nitric and perchloric acids at 65°C overnight (Skare, 1972) and urine (0.5 ml) with potassium permanganate and sulphuric acid at room temperature (Lindstedt, 1970). The detection limit was 0.1 ng/g in plasma, 0.2 ng/g in whole blood and 0.2 ng/ml in urine. All samples were analysed in duplicate. The precision, as calculated from the duplicate analyses, was 13% (coefficient of variation) for plasma samples in the range 0.2-1.5 ng/g (mean: 1.0 ng/g; n = 192) and 7% in the range 1.6-11.6 ng/g (mean 2.3 ng/g; n = 133). For whole blood, the precision was 8% in the range 1.1-3.0 ng/g (mean 2.4 ng/g; n = 143) and 6% in the range 3.1-13.1 ng/g (mean 4.2 ng/g; n = 182). For urine the precision was 9% in the range 0.2-5.0/~g/1 (mean 2.9 #g/l; n = 164) and 7% in the range 5.1-25 #g/1 (mean 9.0/~g/1; n = 155). P-Hg and B-Hg values are given in ng/g. The figures are 3% and 6%, respectively, lower than if expressed as/~g/1. The accuracy was checked by analyses, in each series, of reference samples. For lyophilised serum ('Seronorm', Nycomed AS, Diagnostics, Oslo), the reference value was 1.1/~g/l and our results in different series averaged 1.14 /~g/l (S.D. = 0.11; range 1.0-1.4; n = 27). For lyophilised whole blood (Behring Institute, Marburg, Germany), the reference value was 9.9 #g/1 and our average was 9.9 ~g/1 (S.D. - 0.30; range 9.1-10.4; n = 54). For lyophilised urine ('Lanonorm', Behring Institute), the assigned value was 9.7 #g/l (confidence range 8.2-11.2 #g/l) and our results averaged 9.5/zg/l (S.D. = 0.48; range 8.6-11.7; n = 51).
Creatinine analyses Creatinine in urine was determined by a modified kinetic Jaff6 method (Lustgarten and Wenk, 1972, modified by P. Masson, Department of Clinical Chemistry, University Hospital, Lund). The precision was 5.9% as calculated from duplicate analyses of 93 samples in the range of 4-31 (mean 12.9) mmol/l.
Selenium analyses Selenium was determined by a fluorimetric method (La Londe et al., 1982). The detection limit was 2/zg/1. All samples were analysed in duplicate. The precision, as calculated from the duplicate analyses, was 3.2%. The accuracy was checked by including a reference sample ('Seronorm', Nycomed AS, Diagnostics, Oslo), containing 90/~g/1 (value certified by IUPAC; S.D. = 6). Our results obtained by the method used in this study averaged 90.9/~g/1 (S.D. = 2.0, range 88.3-94.4/zg/1; n = 10).
FISH AS A SOURCE OF EXPOSURE TO MERCURY AND SELENIUM
65
Fatty acids Serum samples from 47 subjects (14 from the random sample, 11 and 22 of the low and high consumers, respectively) were analysed for n-3 polyunsaturated fatty acids (PUFA) in phosphatidylcholine. Lipids were extracted from 1 ml serum by 6 ml methylene chloride/ methanol (1:1) for 1 h. After centrifugation, the insoluble residue was reextracted with 3 ml of the same solvent mixture. The combined extracts were partitioned with 4 ml NaC1 (10 g/l), and the lipid phase was washed twice with 3 ml methanol/water (1:1). After concentration, the lipid extract was applied to silica gel H thin-layer plates, which were developed in methylene chloride/methanol/ammonia (60:30:5), containing 5 mg BHT as antioxidant. The phosphatidylcholine spots were visualized by spraying with 0.2% dichlorofluoroscein in ethanol and transferred to glass tubes for preparation of fatty acid methyl esters by transesterification. Fatty acid composition was determined by capillary gas chromatography, as described elsewhere (Ekstr6m et al., 1989).
Statistics Non-parametric statistics (Kendall rank correlation: tau) was used for the measure of correlation between fish consumption and metal levels. Simple and multiple regression analysis were used for the tests of correlation between exposures and mercury levels in different media. RESULTS
Mercury Mercury in whole blood (B-Hg) averaged 1.8 ng/g in the group without any fish consumption and increased to 6.7 and 6.6 ng/g in the groups with two and three fish meals a week, respectively; in the most extreme group the level was somewhat lower (Table 1). For mercury in erythrocytes (Ery-Hg), the effect of fish consumption was even larger; the non-consumers had the lowest levels: mean 2.1 ng/g, compared to 12 ng/g in the group with two fish meals per week, in which the maximum value of 42 ng/g was found (Table 1). Again, the level decreased among high consumers. The average plasma levels (P-Hg) increased from the non-fish group to the two-times a week group, and then decreased, mean levels ranging 1.0-2.6 ng/g (Table 1). A similar pattern was noted for mercury in urine with average levels 1.3-2.8 /zg/g creatinine (Table 1). A significant positive correlation was found between the number of fish meals each week and B-Hg, Ery-Hg and P-Hg (Kendalls tau = 0.31, 0.23 and
66
B.-G. SVENSSON ET AL.
TABLE 1 Mercury in whole blood (B-Hg), plasma (P-Hg), blood cells (Ery-Hg), urine (U-Hg) and plasma levels of selenium (P-Se) in groups with different fish consumption. Means and ranges are noted. P-Se values from 15 subjects with dietary supplements of selenium have been excluded Fish meals/ week
0
< 1
1
2
3
>3
N
B-Hg (ng/g)
P-Hg (ng/g)
1.8
1.0
(1.2-2.4) (n = 13) 4.0 (1.4-20) (n = 114) 4.5 (1.1-15) (n = 172) 6.7 (1.8-26) (n = 63) 6.6 (2.9-14) (n = 20) 5.1 (1.8-12) (n = 13)
(0.5-1.8) (n = 11) 1.3 (0.5-2.5) (n = 24) 1.6 (0.4-4.2) (n = 61) 2.6 (1.1-4.9) (n = 30) 2.1 (0.9-3.8) (n = 19) 1.2 (0.4-2.6) (n = 13)
395
158
Ery-Hg (ng/g)
2.1
U-Hg (/~g/g creat.)
P-Se (/xgl)
1.8
80
8.4 (3.3-18) (n = 13) 11.9 (2.6-42) (n = 24) 11.3 (4.2-22) (n = 18) 8.3 (2.8-22) (n = 13)
(0.4-3.9) (n = 11) 2.5 (0.4-5.2) (n = 24) 2.8 (0.2-10) (n = 54) 2.7 (1.0-6.2) (n = 13) 1.7 (0.9-3.1) (n = 11) 1.3 (0.4-3.4) (n = 12)
(67-104) (n = 13) 84 (60-130) (n = 108) 86 (46-122) (n = 164) 89 (42-108) (n = 60) 88 (63-102) (n = 19) 91 (71-114) (n = 13)
79
125
377
(1.4-3.0) (n= 11) ,
*No subjects in this group have been analysed for E-Hg levels.
0.29, respectively; all P values < 0.002). For U-Hg, a negative correlation was found (tau = -0.13; P = 0.03). There was a significant correlation between the number of teeth with amalgam fillings and P-Hg (r = 0.38, P = 0.008; Fig. 3) and U-Hg (r = 0.51, P = 0.0002), but no such correlations could be found with B-Hg or Ery-Hg. Using multiple regression analysis, it was found that P-Hg levels were affected both by dental amalgam and fish intake. B-Hg (F = 30.8, p = 0.0001) and Ery-Hg levels (F = 30.3, p = 0.0001) were significantly associated with fish intake, while U-Hg reflected the oral mercury status (F = 14.8, P = 0.0004). There were significant correlations between P-Hg levels on the one hand, and Ery-Hg and U-Hg on the other (Figs. 1 and 2). Ery-Hg did not correlate with U-Hg.
67
FISH AS A SOURCE OF EXPOSURE TO MERCURY AND SELENIUM
Ery-Hg ng/g 45
[]
40 35 30
ng/g 0
.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Fig. 1. Relationship between levels of mercury in plasma (P-Hg) and erythrocytes (Ery-Hg) in 79 subjects, y = 4.34x + 0.19; r = 0.72; P = 0.0001.
T h e results f r o m analysis o f B - H g in two samples, t a k e n 18 m o n t h s apart, f r o m 97 subjects, c o r r e l a t e d significantly (Fig. 4). H o w e v e r , the levels 1986 were lower t h a n 1984 ( P < 0.0001; W i l c o x o n m a t c h e d - p a i r s signed-ranks test). T w o p e r s o n s (a m a r r i e d couple) h a d the highest levels in 1984, close to
U-Hg I-tg/ g creat
10
o o
'
[]
4" 2'
0' 0
P-Hg ng/g .5
1
1.5
2
2.5
3
3.5
4
4.5
Fig. 2. Relationship between levels of mercury in plasma (P-Hg) and urine (U-Hg) in 125 subjects, y = 1.98x - 0.28; r = 0.72; P = 0.0001.
68
B.-G.
SVENSSON
ET
AL.
P-Hg ng/g 3. [] []
2.5.
[] []
[]
2.
[]
[]
[]
[]
[] []
1.5. []
~
D O
I
[] ~]
ooo
°B
[]
[]
[]
[]
.5
[]
_ u
[]
Teeth with
fillings O.
,
0
i
2
•
i
4
,
i
6
•
i
•
8
i
10
•
i
•
i
12
,
14
i
16
•
i
18
,
|
20
•
/
•
22
24
Fig. 3. Relationship between number of teeth with amalgam fillings and mercury levels in plasma (P-Hg) in 48 subjects.
B-Hg 1986 ng/g 16
12 10
0 °
8 6
@ B-Hg 1984 ng/g
2 0
'
0
5
'
10
i
15
i
20
i
25
30
Fig. 4. Relationship between mercury levels in blood in two samples taken 18 months apart in 97 subjects. Closed symbols denote a married couple, who at the time of the the first sampiing, consumed fish from a contaminated lake, at the second from other sources. Correlation with those two subjects excluded: r = 0.55; P = 0.0001. A line of equality is indicated.
69
FISH AS A SOURCE OF EXPOSURE TO MERCURY AND SELENIUM
the average in 1986. They had caught their fish in a contaminated lake up to 1984, after which they obtained their fish from other sources. Selenium
Average plasma level of selenium (P-Se) was 80/~g/1 in the group without any fish consumption, and 91/~g/1 in the group with the highest consumption (Table 1). P-Se (15 consumers of selenium tablets excluded) correlated significantly with fish consumption (tau = 0.18, P < 0.00005). Selenium analysis 1984 and 1986 on samples from the same 97 subjects disclosed a significant correlation (r = 0.55, P = 0.0001). There was a weak but significant correlation between P-Se levels, on the one hand, and B-Hg (r = 0.28, P = 0.0001), Ery-Hg (Fig. 5), and P-Hg (r = 0.24; P = 0.005), on the other. U-Hg levels did not correlate with P-Se. P U F A vs. selenium and mercury
The non-fish consumers had a mean S - P U F A level of 5.8 weight %, while the corresponding figure for the high consumers was 10.8 weight %. There were significant associations between S - P U F A levels and P-Se (Fig. 6), B-Hg (r = 0.78; P = 0.0001), Ery-Hg (Fig. 7) and P-Hg (r = 0.32; P = 0.03). U-Hg levels did not correlate with S-PUFA.
Eryng/gHg 45. 401
[]
351 30. 251 20. 15. 101
~P.Se
5. 0
0
20
40
60
80
11)0
gg/I li0
Fig. 5. Relationship between levels of selenium in plasma (P-Se) and mercury in erythrocytes (Ery-Hg) in 77 subjects, y = 0.29x - 17.7; r = 0.42; P = 0.0001.
70
B.-G.
S V E N S S O N
E T
A L
P-Se ~g/I 120.
[] o°
I00.
o
o
~
80.
O--
[]
60. 40' 20.
S-PUFA
weight % 0
•
_
2
0
•
_
,
4
.
o
6
.
,
8
.
,
10
.
,
12
.
,
14
_
,
16
18
20
Fig. 6. Relationship between n-3 polyunsatured fatty acids in serum (PUFA) and selenium in plasma (P-Se) in 47 subjects, y = 2.37x + 65.3; r = 0.62; P = 0.0001.
Ery-Hg ng/g 25. 22.5.
[]
20. 17.5. 15.
[]
12.5.
[]
10. 7.5.
[]
[] [] []
[]
5,
S-PUFA
2.5' 0
weight % •
0
2
.
.
4
.
.
6
.
.
8
.
o
10
-
.
12
-
.
14
_
.
16
18
20
Fig. 7. Relationship between n-3 polyunsatured fatty acids in serum (PUFA) and mercury in erythrocytes (Ery-Hg) in 47 subjects, y = 1.01x - 2.98; r = 0.77; P = 0.0001.
FISH AS A SOURCE OF EXPOSURE TO MERCURY AND SELENIUM
71
DISCUSSION A population without any occupational exposure to mercury, such as the one under study, is exposed to both inorganic and organic (methyl-) mercury from the general environment (air, water), food (mainly fish) and amalgam dental fillings (Clarkson et al., 1988; WHO, 1990; WHO 1991). The total exposure is reflected in the blood levels of B-Hg. However, the distribution of mercury between erythrocytes and plasma varies for different chemical forms. Metallic and inorganic mercury distributes about equally between erythrocytes and plasma, while exposure to methylmercury causes much higher levels in erythrocytes compared to plasma (Skerfving, 1974). Consequently, Ery-Hg and P-Hg can be used as an index of methylmercury and inorganic mercury, respectively. Absorbed methyl mercury is mainly excreted via faeces, while inorganic mercury to a much higher extent is excreted in urine (WHO, 1991). Mercury levels in urine thus reflects the latter type of exposure. The present population has a slightly higher average B-Hg than those reported from other populations in Sweden and elsewhere (Skerfving, 1972; Kr6ncke et al., 1980; Grasmick and Huel, 1985; Langworth et al., 1988). This could, to a certain extent, be explained by differences in analytical techniques, but also, a true difference should be considered. About 25% of the population under study had fish from local lakes and coastal waters, many of them polluted by mercury. This causes higher levels of methylmercury in locally caught fish, thus giving a higher mercury exposure to a substantial part of the population. The well-known role of fish as a dietary source of methylmercury (Swedish Expert Comm., 1971; Clarkson et al. 1988; WHO, 1990) is confirmed in our findings by the great influence of fish consumption on the B-Hg levels. It is worth noticing, that the average Ery-Hg level in the group with two fish meals per week is more than five-times that of the group without any fish consumption. The group with the highest fish consumption (more than three fish meals per week) have unexpectedly low levels of mercury in blood and erythrocytes. However, the 13 members of this group all consumed fish caught off shore (salmon and herring) which has low levels of mercury compared to the fish from local lakes and coastal areas consumed by substantial parts of the groups with two and three fish meals per week. According to the regression correlation in Fig. 1, there is a ratio of about four between mercury levels in erythrocytes and in plasma. This indicates that methylmercury (from fish) is a major part of the total mercury exposure for the studied population. The observed ratio is similar to earlier reports from Swedes consuming contaminated fish (Skerfving, 1974). The impor-
72
B.-G. S V E N S S O N E T A L .
tance of fish as a major source of mercury exposure is further stressed by the strong association between the S-PUFAs, with their main dietary origin from fish and B-Hg or Ery-Hg. Several studies have demonstrated that amalgam tooth fillings releases some mercury vapour, that can be absorbed, thus causing an exposure to inorganic mercury (Clarkson et al., 1988; WHO 1991). This could be expected to affect U-Hg and P-Hg, and accordingly associations between the number of amalgam-filled teeth and mercury levels in these media were present in our study. A relationship between mercury levels in urine o and amalgam have been reported in some studies (Langworth et al., 1988; Akesson et al., 1991) while others (Kr6ncke et al., 1980) could not find a similar relation. There was no association between amalgam and Ery-Hg, where it is obscured by methylmercury from fish. The exposure to inorganic mercury, as reflected in the observed levels in plasma and urine is far below the levels where toxic effects could be expected (Clarkson et al., 1988; WHO 1991). Also, methylmercury exposure, as indicated by concentrations in whole blood and erythrocytes, are without danger of toxic effects for the major part of the population. Levels well above the present ones have been found earlier in adult Swedes without symptoms or signs of toxic effects (Skerfving, 1974). However, as the developing human embryo/fetus has a particular sensitivity to methylmercury, some of the noted mercury levels do not permit a sufficient safety margin for pregnant women, although this was not studied here. The levels of selenium in plasma are low (WHO,o 1987), but similar to those in other reports from Swedish populations (Akesson and Steen, 1987; Gustafson et al., 1987; Ohlsson et al., 1988). Different diets span a wide range of daily intakes of selenium. For geochemical reasons, the selenium contents may vary considerably in foods such as cereals, meat and vegetables (Magos and Berg, 1988). In Swedish cereals, the selenium content is low. However, fish is an important source and contributes more than a third of the dietary selenium intake in many countries (WHO, 1987). The present study shows that selenium is absorbed from fish, as the P-Se values correlated positively to the number of fish meals per week. Moreover, there was a strong association between P-Se and n-3 fatty acids, peculiar to marine foods. The effect of fish on P-Se is somewhat greater than that observed in volunteers, who had 150-200 g fish/day for 6-11 weeks (Thorngren and ]kkesson, 1987). The biological effect of the selenium in fish is still unknown. The observed correlation between P-Se and Ery-Hg is most probably explained by the fact that fish is a major source of both selenium and methylmercury and intakes of these compounds thus occur simultaneously. Indeed, this combined exposure might be of value because of the possible interaction between selenium and methylmercury in the body.
FISH AS A SOURCE OF EXPOSURE TO MERCURY AND SELENIUM
73
ACKNOWLEDGEMENTS T h i s s t u d y w a s s u p p o r t e d b y g r a n t s f r o m the N a t i o n a l S w e d i s h E n vironmental Protection Board and the Swedish Work Environment Fund. REFERENCES ,~,kesson, B. and B. Steen, 1987. Plasma selenium and glutathione peroxidase in relation to cancer, angina pectoris and short-term mortality in 68-year-old men. Compr. Gerontol. 1: 61-64. .~kesson, I., A. Schiitz, R. Attewell, S. Skerfving and P-O. Glantz, 1991. Status of mercury and selenium in dental personell - - impact of amalgam work and own fillings. Arch. Environ. Health, 46: 102-109. Alexander, J., Y. Thomassen and J. Aasteh, 1983. Increased urinary excretion of selenium among workers exposed to elemental mercury vapor. J. Appl. Toxicol., 3: 143-145. Bj6rklund, I., H. Borg and K. Johansson, 1984. Mercury in Swedish lakes - - Its regional distribution and causes. Ambio, 13:118-121. Clarkson, T., J.B. Hursh, P.R. Sager and T.L.M. Syversen 1988. Mercury. In: T.W. Clarkson, L. Friberg, G.F. Nordberg and P.R. Sager (Eds), Biological Monitoring of Toxic Metals. Rochester Series on Environmental Toxicity. Plenum Press, New York. Einarsson, 0., G. Lindstedt and T.A. Bergstrfm, 1984. A computerised automatic apparatus for determination of mercury in biological samples. J. Autom. Chem., 6: 74-79. Ekstr6m, B., A. Nilsson and B. ,~kesson, 1989. Lipolysis of polyenoic fatty acid esters of human chylomicrons by lipoprotein lipase. Eur. J. Clin. Invest., 19: 259-264. Grasmick, C. and G. Huel, 1985. Interindividual variations of blood total mercury levels according to sex age and area of residence. Sci. Total. Environ., 44: 101-109. Gustafson, /~., A. Schiitz, P. Andersson and S. Skervfing, 1987. Small effect on plasma selenium level by occupational lead exposure. Sci. Total. Environ., 66: 39-43. Kosta, L., A.R. Vyrne and V. Zelenko, 1975. Correlation between selenium and mercury in man following exposure to inorganic mercury. Nature, 254: 238-239. Kr6ncke, A., K. Ott, A. Petschelt, K.-H. Schaller, M. Szecsi and and H. Valentin, 1980. Uber die Quecksilberkonzentrationen in Blut und Urn yon Personen mit und ohne Amalgamfiillungen. Dtsch. zahn/irztl., 35:803-808 (in German). La Londe, L., Y. Jean, K.D. Roberts, A. Chapdelaine and G. Bleau, 1982. Fluorometry of selenium in serum or urine. Clin. Chem., 28: 172-174. Langworth, S., C.G. Elinder and A. Akesson, 1988. Mercury exposure from dental fillings. I. Mercury concentrations in blood and urine. Swed. Dentl J., 12: 69-70. Lindstedt, G., 1970. A rapid method for the determination of mercury in urine. Analyst, 95: 264-271. Lustgarten J. and R. Wenk, 1972. Simple, rapid kinetic method for serum creatinine measurement. Clin. Chem., 18: 1419-1422. Magos, L. and M. Webb, 1980. The interactions of selenium with cadmium and mercury. CRC Crit. Rev. Toxicol., 4: 1-42. Magos, L. and G.G. Berg, 1988. Selenium. In: T.W. Clarkson, L. Friberg, G.F. Nordberg and P.R. Sager (Eds), Biological Monitoring of Toxic Metals, Rochester Series on Environmental Toxicity. Plenum Press, New York. Marsh, D.O., G.J. Meyers, T.W. Clarkson, L. Amin-Zaki, S. Tikriti, M.A. Majeed and A.R. Dabbagh, 1981. Dose-response relationship for human fetal exposure to methylmercury. Clin. Toxicol., 18: 1311-1318.
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Ohlsson, K., A. Schiitz, R. Attewell and S. Skerfving, 1988. Selenium status in females with occupational cervico-brachial complaints. Int. Arch. Occup. Environ. Health. 61: 167-169. Skare, I., 1972. Microdetermination of mercury in biological samples. Part III. Automated determination of mercury in urine, fish and blood samples. Analyst, 97" 148-155. Skerfving, S. (1972). Normal concentrations of mercury in human tissue and urine. In: L. Friberg and J. Vostal (Eds), Mercury in the Environment. An Epidemiological and Toxicological Appraisal. CRC Press, Cleveland, Ohio, pp. 109-112. Skerfving, S., 1974. Methylmercury exposure, mercury levels in blood and hair, and health status in Swedes consuming contaminated fish. Toxicology, 2: 3-23. Skerfving, S., 1978. Interaction between selenium and methyl mercury. Environ. Health Perspect. 25: 57-65. Svensson, B-G., ,~. Bj6rnham, A. Schfitz, U. Lettevall, A. Nilsson and S. Skerfving, 1987. Acidic deposition and humane exposure to toxic metals. Sci. Total Environ., 67, 101-115. Swedish Expert Committee. (1971). Methyl mercury in fish. A toxicological epidemiologic evaluation of risks. Report from an expert group. Nord. Hyg. Tidskr., Suppl. 4. Thorngren, M. and B. A~kesson, 1987. Effect of dietary fish on plasma selenium and its relation to haemostatic changes in healthy adults. Int. J. Vit. Nutr. Res., 57: 429-435. World Health Organization, 1987. Environmental Health Criteria 58. Selenium. Geneva, Switzerland, pp. 1-306. World Health Organization, 1990. Environmental Health Criteria 101. Methylmercury. Geneva, Switzerland, pp. 1-144. World Health Organization, 1991. Environmental Health Criteria 118. Mercury. Geneva, Switzerland, pp. l - 168.
Fish
as a source
o f exposure selenium
61
to
mercury
and
B.-G. Svensson a, A. Schfitz a, A. Nilsson a, I. ,~kesson a, B. ~,kesson b and S. Skerfving a aDepartment of Occupational and Environmental Medicine, University Hospital and bDepartment of Medical and Physiological Chemistry, University of Lund, Sweden (Received August 9th, 1991; accepted September 23rd, 1991)
ABSTRACT In a total of 395 subjects with varying fish consumption habits, mercury levels in whole blood (B-Hg), and selenium levels in plasma (P-Se) were studied. Also, in subcohorts, mercury levels in blood cells (Ery-Hg; n = 79), plasma (P-Hg; n = 158) and urine (U-Hg; n = 125) were analysed. There were statistically significant associations between fish intake on the one hand, and B-Hg, Ery-Hg and P-Hg, on the other, but not so with U-Hg. In subjects who never had fish, the average B-Hg was 1.8 ng/g, in subjects who had at least two fish meals each week, 6.7 ng/g. Ery-Hg, and to a less extent P-Hg, were associated with levels of marine n-3 polyunsaturated fatty acids (PUFA) in serum phosphatidylcholine. P-Hg and U-Hg were associated with numbers of teeth with amalgam fillings. P-Se also correlated with fish intake. In subjects who never had fish, P-Se averaged 80 t,g/l, in subjects who had at least two fish meals per week, 91 /zg/1. There was an association between P U F A and P-Se. Further, there were statistically significant associations between P-Se on the one hand, and B-Hg, Ery-Hg and PHg on the other. The data clearly demonstrate the importance of fish for the exposure to methylmercury and selenium in the Swedish diet, and the impact of amalgam as a source of exposure to inorganic mercury.
Key words: fish; mercury; methylmercury; selenium
INTRODUCTION
Mercury, an important pollutant when deposited in water, is methylated and accumulates in most aquatic biota, with the highest concentrations in large predatory fish (Clarkson et al., 1988). Fish in acidic lakes contain higher levels of methylmercury than those which are less acidified (Bj6rklund et al., 1984). Acid precipitation has increased during the last decades. In man the nervous system is particularly sensitive to damage from methylmercury 0048-9697/92/$05.00
© 1992 Elsevier Science Publishers B.V. All rights reserved
62
B.-G. SVENSSON ET AL.
(WHO, 1990). Prenatal exposure to methylmercury causes psychomotor retardation in children (Marsh et al., 1981). Considerable efforts have been made in Sweden to reduce the mercury pollution. In spite of this, many lakes and coastal waters in Sweden are blacklisted because the levels of methylmercury in fish exceed 1 mg/kg. Some 10 000 lakes in Sweden are estimated to have mercury levels in fish above this level. People are recommended not to eat fish from these lakes. Further, there is a general recommendation that fish such as perch, pike, pike-perch and turbot should not be consumed more than once a week, and totally avoided by pregnant women during the whole period of pregnancy. There is cause for concern about the present exposure to methylmercury in the population. Fish is assumed to be an important source of dietary selenium (WHO, 1987). In the Swedish population, which has a low intake of selenium, however, this has not been evaluated. Further, there is a metabolic interaction between selenium and mercury. Accumulation of mercury and selenium in equimolar amounts have been found in the kidneys, the pituitary, the brain and the thyroid from retired miners occupationally exposed to mercury (Kosta et al., 1975). Exposure to elemental mercury affects the excretion of selenium in urine (Alexander et al., 1983). Also, a study of their correlation is important from another point of view as some of the toxic effects of inorganic mercury (Magos and Webb, 1980) and methylmercury (Skerfving, 1978) are reduced by selenium. As thus fish is a main source of both mercury and selenium and because of the potential interactions, we studied mercury and selenium levels in subjects with varying fish intake. STUDY POPULATIONS
Random sample From three rural areas in south-east Sweden, 505 persons, aged 20-66 years, were randomly chosen from the electoral registers. All persons lived in the countryside or in small villages without any local industrial activity. Persons in the sample were invited for examination; 330 (165 males and 165 females; 66% of the original sample) responded and were examined. Data on non-responders have been reported elsewhere (Svensson et al., 1987).
High consumers offish Fifty-four persons (16 females), mostly fishermen, and aged 15-79 years, who had full fish-meals at least twice a week, were examined in the same way.
FISH AS A SOURCE OF EXPOSURE TO MERCURY AND SELENIUM
63
Low consumers offish Eleven males, aged 25-57 years, who never eat fish, in most cases due to allergy, were also included. None of these were vegetarians. These two 'extreme' consumption groups were non-random and recruited by advertising in different media. METHODS
Questionnaire Questions about lifestyle and environmental factors, such as occupation, smoking habits and present health status, were asked for in a questionnaire. Fish consumption was estimated by questions about the number of fishmeals by the week and about species of fish. Also, use of dietary supplements of selenium tablets was noted.
Dental status On a sample of 48 subjects, the oral cavity was inspected and the numbers of dental amalgam fillings were registered.
Blood sampling Venous blood for analysis of mercury and selenium was obtained from a cubital vein in metal-free Vacutainer ® tubes. Analyses of mercury in whole blood (B-Hg) and selenium in plasma (P-Se) were performed on samples from 395 and 392 persons, respectively. Analyses of mercury in plasma (PHg) erythrocytes (Ery-Hg) separately were made on samples from 158 and 79 persons, respectively. Sampling and analysis of mercury in whole blood have been performed on two occasions (at least 8 months apart) for 94 subjects from the random sample.
Urine sampling Overnight urine for mercury analysis (U-Hg) was collected in acid-washed bottles by 125 persons (93 from the random group and 32 from the low and high consumption groups).
Mercury analyses The mercury (Hg) content was determined in wet-digested samples by a
64
B.-G. SVENSSON ET AL.
'cold vapour', atomic absorption technique, using automatic equipment (Einarsson et al., 1984). Samples of plasma (1.0 g) and whole blood (0.5 g) were digested with nitric and perchloric acids at 65°C overnight (Skare, 1972) and urine (0.5 ml) with potassium permanganate and sulphuric acid at room temperature (Lindstedt, 1970). The detection limit was 0.1 ng/g in plasma, 0.2 ng/g in whole blood and 0.2 ng/ml in urine. All samples were analysed in duplicate. The precision, as calculated from the duplicate analyses, was 13% (coefficient of variation) for plasma samples in the range 0.2-1.5 ng/g (mean: 1.0 ng/g; n = 192) and 7% in the range 1.6-11.6 ng/g (mean 2.3 ng/g; n = 133). For whole blood, the precision was 8% in the range 1.1-3.0 ng/g (mean 2.4 ng/g; n = 143) and 6% in the range 3.1-13.1 ng/g (mean 4.2 ng/g; n = 182). For urine the precision was 9% in the range 0.2-5.0/~g/1 (mean 2.9 #g/l; n = 164) and 7% in the range 5.1-25 #g/1 (mean 9.0/~g/1; n = 155). P-Hg and B-Hg values are given in ng/g. The figures are 3% and 6%, respectively, lower than if expressed as/~g/1. The accuracy was checked by analyses, in each series, of reference samples. For lyophilised serum ('Seronorm', Nycomed AS, Diagnostics, Oslo), the reference value was 1.1/~g/l and our results in different series averaged 1.14 /~g/l (S.D. = 0.11; range 1.0-1.4; n = 27). For lyophilised whole blood (Behring Institute, Marburg, Germany), the reference value was 9.9 #g/1 and our average was 9.9 ~g/1 (S.D. - 0.30; range 9.1-10.4; n = 54). For lyophilised urine ('Lanonorm', Behring Institute), the assigned value was 9.7 #g/l (confidence range 8.2-11.2 #g/l) and our results averaged 9.5/zg/l (S.D. = 0.48; range 8.6-11.7; n = 51).
Creatinine analyses Creatinine in urine was determined by a modified kinetic Jaff6 method (Lustgarten and Wenk, 1972, modified by P. Masson, Department of Clinical Chemistry, University Hospital, Lund). The precision was 5.9% as calculated from duplicate analyses of 93 samples in the range of 4-31 (mean 12.9) mmol/l.
Selenium analyses Selenium was determined by a fluorimetric method (La Londe et al., 1982). The detection limit was 2/zg/1. All samples were analysed in duplicate. The precision, as calculated from the duplicate analyses, was 3.2%. The accuracy was checked by including a reference sample ('Seronorm', Nycomed AS, Diagnostics, Oslo), containing 90/~g/1 (value certified by IUPAC; S.D. = 6). Our results obtained by the method used in this study averaged 90.9/~g/1 (S.D. = 2.0, range 88.3-94.4/zg/1; n = 10).
FISH AS A SOURCE OF EXPOSURE TO MERCURY AND SELENIUM
65
Fatty acids Serum samples from 47 subjects (14 from the random sample, 11 and 22 of the low and high consumers, respectively) were analysed for n-3 polyunsaturated fatty acids (PUFA) in phosphatidylcholine. Lipids were extracted from 1 ml serum by 6 ml methylene chloride/ methanol (1:1) for 1 h. After centrifugation, the insoluble residue was reextracted with 3 ml of the same solvent mixture. The combined extracts were partitioned with 4 ml NaC1 (10 g/l), and the lipid phase was washed twice with 3 ml methanol/water (1:1). After concentration, the lipid extract was applied to silica gel H thin-layer plates, which were developed in methylene chloride/methanol/ammonia (60:30:5), containing 5 mg BHT as antioxidant. The phosphatidylcholine spots were visualized by spraying with 0.2% dichlorofluoroscein in ethanol and transferred to glass tubes for preparation of fatty acid methyl esters by transesterification. Fatty acid composition was determined by capillary gas chromatography, as described elsewhere (Ekstr6m et al., 1989).
Statistics Non-parametric statistics (Kendall rank correlation: tau) was used for the measure of correlation between fish consumption and metal levels. Simple and multiple regression analysis were used for the tests of correlation between exposures and mercury levels in different media. RESULTS
Mercury Mercury in whole blood (B-Hg) averaged 1.8 ng/g in the group without any fish consumption and increased to 6.7 and 6.6 ng/g in the groups with two and three fish meals a week, respectively; in the most extreme group the level was somewhat lower (Table 1). For mercury in erythrocytes (Ery-Hg), the effect of fish consumption was even larger; the non-consumers had the lowest levels: mean 2.1 ng/g, compared to 12 ng/g in the group with two fish meals per week, in which the maximum value of 42 ng/g was found (Table 1). Again, the level decreased among high consumers. The average plasma levels (P-Hg) increased from the non-fish group to the two-times a week group, and then decreased, mean levels ranging 1.0-2.6 ng/g (Table 1). A similar pattern was noted for mercury in urine with average levels 1.3-2.8 /zg/g creatinine (Table 1). A significant positive correlation was found between the number of fish meals each week and B-Hg, Ery-Hg and P-Hg (Kendalls tau = 0.31, 0.23 and
66
B.-G. SVENSSON ET AL.
TABLE 1 Mercury in whole blood (B-Hg), plasma (P-Hg), blood cells (Ery-Hg), urine (U-Hg) and plasma levels of selenium (P-Se) in groups with different fish consumption. Means and ranges are noted. P-Se values from 15 subjects with dietary supplements of selenium have been excluded Fish meals/ week
0
< 1
1
2
3
>3
N
B-Hg (ng/g)
P-Hg (ng/g)
1.8
1.0
(1.2-2.4) (n = 13) 4.0 (1.4-20) (n = 114) 4.5 (1.1-15) (n = 172) 6.7 (1.8-26) (n = 63) 6.6 (2.9-14) (n = 20) 5.1 (1.8-12) (n = 13)
(0.5-1.8) (n = 11) 1.3 (0.5-2.5) (n = 24) 1.6 (0.4-4.2) (n = 61) 2.6 (1.1-4.9) (n = 30) 2.1 (0.9-3.8) (n = 19) 1.2 (0.4-2.6) (n = 13)
395
158
Ery-Hg (ng/g)
2.1
U-Hg (/~g/g creat.)
P-Se (/xgl)
1.8
80
8.4 (3.3-18) (n = 13) 11.9 (2.6-42) (n = 24) 11.3 (4.2-22) (n = 18) 8.3 (2.8-22) (n = 13)
(0.4-3.9) (n = 11) 2.5 (0.4-5.2) (n = 24) 2.8 (0.2-10) (n = 54) 2.7 (1.0-6.2) (n = 13) 1.7 (0.9-3.1) (n = 11) 1.3 (0.4-3.4) (n = 12)
(67-104) (n = 13) 84 (60-130) (n = 108) 86 (46-122) (n = 164) 89 (42-108) (n = 60) 88 (63-102) (n = 19) 91 (71-114) (n = 13)
79
125
377
(1.4-3.0) (n= 11) ,
*No subjects in this group have been analysed for E-Hg levels.
0.29, respectively; all P values < 0.002). For U-Hg, a negative correlation was found (tau = -0.13; P = 0.03). There was a significant correlation between the number of teeth with amalgam fillings and P-Hg (r = 0.38, P = 0.008; Fig. 3) and U-Hg (r = 0.51, P = 0.0002), but no such correlations could be found with B-Hg or Ery-Hg. Using multiple regression analysis, it was found that P-Hg levels were affected both by dental amalgam and fish intake. B-Hg (F = 30.8, p = 0.0001) and Ery-Hg levels (F = 30.3, p = 0.0001) were significantly associated with fish intake, while U-Hg reflected the oral mercury status (F = 14.8, P = 0.0004). There were significant correlations between P-Hg levels on the one hand, and Ery-Hg and U-Hg on the other (Figs. 1 and 2). Ery-Hg did not correlate with U-Hg.
67
FISH AS A SOURCE OF EXPOSURE TO MERCURY AND SELENIUM
Ery-Hg ng/g 45
[]
40 35 30
ng/g 0
.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Fig. 1. Relationship between levels of mercury in plasma (P-Hg) and erythrocytes (Ery-Hg) in 79 subjects, y = 4.34x + 0.19; r = 0.72; P = 0.0001.
T h e results f r o m analysis o f B - H g in two samples, t a k e n 18 m o n t h s apart, f r o m 97 subjects, c o r r e l a t e d significantly (Fig. 4). H o w e v e r , the levels 1986 were lower t h a n 1984 ( P < 0.0001; W i l c o x o n m a t c h e d - p a i r s signed-ranks test). T w o p e r s o n s (a m a r r i e d couple) h a d the highest levels in 1984, close to
U-Hg I-tg/ g creat
10
o o
'
[]
4" 2'
0' 0
P-Hg ng/g .5
1
1.5
2
2.5
3
3.5
4
4.5
Fig. 2. Relationship between levels of mercury in plasma (P-Hg) and urine (U-Hg) in 125 subjects, y = 1.98x - 0.28; r = 0.72; P = 0.0001.
68
B.-G.
SVENSSON
ET
AL.
P-Hg ng/g 3. [] []
2.5.
[] []
[]
2.
[]
[]
[]
[]
[] []
1.5. []
~
D O
I
[] ~]
ooo
°B
[]
[]
[]
[]
.5
[]
_ u
[]
Teeth with
fillings O.
,
0
i
2
•
i
4
,
i
6
•
i
•
8
i
10
•
i
•
i
12
,
14
i
16
•
i
18
,
|
20
•
/
•
22
24
Fig. 3. Relationship between number of teeth with amalgam fillings and mercury levels in plasma (P-Hg) in 48 subjects.
B-Hg 1986 ng/g 16
12 10
0 °
8 6
@ B-Hg 1984 ng/g
2 0
'
0
5
'
10
i
15
i
20
i
25
30
Fig. 4. Relationship between mercury levels in blood in two samples taken 18 months apart in 97 subjects. Closed symbols denote a married couple, who at the time of the the first sampiing, consumed fish from a contaminated lake, at the second from other sources. Correlation with those two subjects excluded: r = 0.55; P = 0.0001. A line of equality is indicated.
69
FISH AS A SOURCE OF EXPOSURE TO MERCURY AND SELENIUM
the average in 1986. They had caught their fish in a contaminated lake up to 1984, after which they obtained their fish from other sources. Selenium
Average plasma level of selenium (P-Se) was 80/~g/1 in the group without any fish consumption, and 91/~g/1 in the group with the highest consumption (Table 1). P-Se (15 consumers of selenium tablets excluded) correlated significantly with fish consumption (tau = 0.18, P < 0.00005). Selenium analysis 1984 and 1986 on samples from the same 97 subjects disclosed a significant correlation (r = 0.55, P = 0.0001). There was a weak but significant correlation between P-Se levels, on the one hand, and B-Hg (r = 0.28, P = 0.0001), Ery-Hg (Fig. 5), and P-Hg (r = 0.24; P = 0.005), on the other. U-Hg levels did not correlate with P-Se. P U F A vs. selenium and mercury
The non-fish consumers had a mean S - P U F A level of 5.8 weight %, while the corresponding figure for the high consumers was 10.8 weight %. There were significant associations between S - P U F A levels and P-Se (Fig. 6), B-Hg (r = 0.78; P = 0.0001), Ery-Hg (Fig. 7) and P-Hg (r = 0.32; P = 0.03). U-Hg levels did not correlate with S-PUFA.
Eryng/gHg 45. 401
[]
351 30. 251 20. 15. 101
~P.Se
5. 0
0
20
40
60
80
11)0
gg/I li0
Fig. 5. Relationship between levels of selenium in plasma (P-Se) and mercury in erythrocytes (Ery-Hg) in 77 subjects, y = 0.29x - 17.7; r = 0.42; P = 0.0001.
70
B.-G.
S V E N S S O N
E T
A L
P-Se ~g/I 120.
[] o°
I00.
o
o
~
80.
O--
[]
60. 40' 20.
S-PUFA
weight % 0
•
_
2
0
•
_
,
4
.
o
6
.
,
8
.
,
10
.
,
12
.
,
14
_
,
16
18
20
Fig. 6. Relationship between n-3 polyunsatured fatty acids in serum (PUFA) and selenium in plasma (P-Se) in 47 subjects, y = 2.37x + 65.3; r = 0.62; P = 0.0001.
Ery-Hg ng/g 25. 22.5.
[]
20. 17.5. 15.
[]
12.5.
[]
10. 7.5.
[]
[] [] []
[]
5,
S-PUFA
2.5' 0
weight % •
0
2
.
.
4
.
.
6
.
.
8
.
o
10
-
.
12
-
.
14
_
.
16
18
20
Fig. 7. Relationship between n-3 polyunsatured fatty acids in serum (PUFA) and mercury in erythrocytes (Ery-Hg) in 47 subjects, y = 1.01x - 2.98; r = 0.77; P = 0.0001.
FISH AS A SOURCE OF EXPOSURE TO MERCURY AND SELENIUM
71
DISCUSSION A population without any occupational exposure to mercury, such as the one under study, is exposed to both inorganic and organic (methyl-) mercury from the general environment (air, water), food (mainly fish) and amalgam dental fillings (Clarkson et al., 1988; WHO, 1990; WHO 1991). The total exposure is reflected in the blood levels of B-Hg. However, the distribution of mercury between erythrocytes and plasma varies for different chemical forms. Metallic and inorganic mercury distributes about equally between erythrocytes and plasma, while exposure to methylmercury causes much higher levels in erythrocytes compared to plasma (Skerfving, 1974). Consequently, Ery-Hg and P-Hg can be used as an index of methylmercury and inorganic mercury, respectively. Absorbed methyl mercury is mainly excreted via faeces, while inorganic mercury to a much higher extent is excreted in urine (WHO, 1991). Mercury levels in urine thus reflects the latter type of exposure. The present population has a slightly higher average B-Hg than those reported from other populations in Sweden and elsewhere (Skerfving, 1972; Kr6ncke et al., 1980; Grasmick and Huel, 1985; Langworth et al., 1988). This could, to a certain extent, be explained by differences in analytical techniques, but also, a true difference should be considered. About 25% of the population under study had fish from local lakes and coastal waters, many of them polluted by mercury. This causes higher levels of methylmercury in locally caught fish, thus giving a higher mercury exposure to a substantial part of the population. The well-known role of fish as a dietary source of methylmercury (Swedish Expert Comm., 1971; Clarkson et al. 1988; WHO, 1990) is confirmed in our findings by the great influence of fish consumption on the B-Hg levels. It is worth noticing, that the average Ery-Hg level in the group with two fish meals per week is more than five-times that of the group without any fish consumption. The group with the highest fish consumption (more than three fish meals per week) have unexpectedly low levels of mercury in blood and erythrocytes. However, the 13 members of this group all consumed fish caught off shore (salmon and herring) which has low levels of mercury compared to the fish from local lakes and coastal areas consumed by substantial parts of the groups with two and three fish meals per week. According to the regression correlation in Fig. 1, there is a ratio of about four between mercury levels in erythrocytes and in plasma. This indicates that methylmercury (from fish) is a major part of the total mercury exposure for the studied population. The observed ratio is similar to earlier reports from Swedes consuming contaminated fish (Skerfving, 1974). The impor-
72
B.-G. S V E N S S O N E T A L .
tance of fish as a major source of mercury exposure is further stressed by the strong association between the S-PUFAs, with their main dietary origin from fish and B-Hg or Ery-Hg. Several studies have demonstrated that amalgam tooth fillings releases some mercury vapour, that can be absorbed, thus causing an exposure to inorganic mercury (Clarkson et al., 1988; WHO 1991). This could be expected to affect U-Hg and P-Hg, and accordingly associations between the number of amalgam-filled teeth and mercury levels in these media were present in our study. A relationship between mercury levels in urine o and amalgam have been reported in some studies (Langworth et al., 1988; Akesson et al., 1991) while others (Kr6ncke et al., 1980) could not find a similar relation. There was no association between amalgam and Ery-Hg, where it is obscured by methylmercury from fish. The exposure to inorganic mercury, as reflected in the observed levels in plasma and urine is far below the levels where toxic effects could be expected (Clarkson et al., 1988; WHO 1991). Also, methylmercury exposure, as indicated by concentrations in whole blood and erythrocytes, are without danger of toxic effects for the major part of the population. Levels well above the present ones have been found earlier in adult Swedes without symptoms or signs of toxic effects (Skerfving, 1974). However, as the developing human embryo/fetus has a particular sensitivity to methylmercury, some of the noted mercury levels do not permit a sufficient safety margin for pregnant women, although this was not studied here. The levels of selenium in plasma are low (WHO,o 1987), but similar to those in other reports from Swedish populations (Akesson and Steen, 1987; Gustafson et al., 1987; Ohlsson et al., 1988). Different diets span a wide range of daily intakes of selenium. For geochemical reasons, the selenium contents may vary considerably in foods such as cereals, meat and vegetables (Magos and Berg, 1988). In Swedish cereals, the selenium content is low. However, fish is an important source and contributes more than a third of the dietary selenium intake in many countries (WHO, 1987). The present study shows that selenium is absorbed from fish, as the P-Se values correlated positively to the number of fish meals per week. Moreover, there was a strong association between P-Se and n-3 fatty acids, peculiar to marine foods. The effect of fish on P-Se is somewhat greater than that observed in volunteers, who had 150-200 g fish/day for 6-11 weeks (Thorngren and ]kkesson, 1987). The biological effect of the selenium in fish is still unknown. The observed correlation between P-Se and Ery-Hg is most probably explained by the fact that fish is a major source of both selenium and methylmercury and intakes of these compounds thus occur simultaneously. Indeed, this combined exposure might be of value because of the possible interaction between selenium and methylmercury in the body.
FISH AS A SOURCE OF EXPOSURE TO MERCURY AND SELENIUM
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