Food and Chemical Toxicology 57 (2013) 161–169
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Lead, mercury, and cadmium in blood and their relation to diet among Swedish adults Helena Bjermo a, Salomon Sand a, Cecilia Nälsén a, Thomas Lundh b, Heléne Enghardt Barbieri a, Monika Pearson a, Anna Karin Lindroos a, Bo A.G. Jönsson b, Lars Barregård c, Per Ola Darnerud a,⇑ a b c
National Food Agency, Uppsala, Sweden Division of Occupational and Environmental Medicine, Lund University, Lund, Sweden Department of Occupational and Environmental Medicine, Sahlgrenska University Hospital, Gothenburg, Sweden
a r t i c l e
i n f o
Article history: Received 18 December 2012 Accepted 14 March 2013 Available online 26 March 2013 Keywords: Heavy metals Lead Mercury Cadmium Diet Blood
a b s t r a c t The aim of the present study was to examine the body burden of lead (Pb), mercury (Hg), and cadmium (Cd) in blood among Swedish adults and the association between blood levels, diet and other lifestyle factors. The study was based on a subgroup (n = 273) of the national survey Riksmaten 2010–2011 (4-day food records and questionnaire). Lead, Hg, and Cd were measured in whole blood, and Cd additionally in urine, by mass or fluorescence spectrometry methods. The median values (5–95th percentiles) of the metals in blood were as follows; Pb: 13.4 (5.8–28.6) lg/ L, Hg: 1.13 (0.31–3.45) lg/L, and Cd: 0.19 (0.09–1.08) lg/L. All three metals increased with increasing age. Lead levels in blood were positively associated with intakes of game and alcohol, Hg was related to fish intake, and blood Cd related to smoking and low iron stores and to a low meat intake. Body burdens of the studied metals were generally below health based reference values, but several individuals had blood Pb levels above the reference point for possible nephrotoxic and developmental neurotoxic effects. As health effects cannot be excluded, individuals with high Pb exposure should aim at decreasing their body burden, both from food and from other exposure routes. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Toxic metals such as lead (Pb), mercury (Hg) and cadmium (Cd) are natural components of our earth crust. However, the environment has been enriched by industrial processes, and man-made sources such as mining, industries, motor vehicle exhaust, and batteries contribute to the environmental levels and to human exposure. Even though the contribution from the man-made sources has decreased substantially, the environmental contamination will remain for many decades. Pb and Hg, and probably also Cd have detrimental effects on the central nervous system in the developing infant (Bellinger, 2008; Mendola et al., 2002; Kippler et al., 2012). Even if neurotoxicity is the most sensitive endpoint, Pb may also affect blood pressure, kidney function, cause mutagenesis and have reproductive effects (Nordberg et al., 2007). Humans are
Abbreviations: Pb, lead; Hg, mercury; Cd, cadmium; OEMC, Occupational and Environmental Medicine Center; BMI, body mass index; B, blood; U, urine; CTQ, Centre de Toxicologie du Quebec; SD, standard deviation; SE, standard error. ⇑ Corresponding author. Address: The National Food Agency, P.O. Box 622, SE-751 26 Uppsala, Sweden. Tel.: +46 18 17 55 00; fax: +46 18 10 58 48. E-mail address: per.ola.darnerud@slv.se (P.O. Darnerud). 0278-6915/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fct.2013.03.024
exposed to Pb by a number of contributing food sources, but also by drinking water and air, whereas in case of Hg there is one major source, namely fish (Florea and Busselberg, 2006; Martorell et al., 2011). The toxic effects of Cd are mainly affecting the kidneys and skeleton. Long-term exposure can cause renal tubular dysfunction. In addition, Cd exposure may lead to osteoporosis, and the metal has been classified as a human carcinogen (Nordberg et al., 2007; Satarug and Moore, 2004). Diet is considered the main source of Cd intake among non-smokers (Skerfving et al., 1999), and especially food cultured in Cd-rich soil constitutes a major source for Cd (Satarug and Moore, 2004). In Sweden, the body burdens of Pb and Hg are decreasing with time whereas this is not evident for Cd levels (Barregard et al., 2010; Wennberg et al., 2006), for Cd levels perhaps with the exception for smoking men. Thus, the dietary Cd exposure in Sweden seems to be unchanged (Wennberg et al., 2006) and in a market basket study from 2010, similar Cd exposures from food were reported in studies from 1987, 1999 and 2010 (NFA, 2012a). This fact needs further investigation, especially since the toxic effects on kidney and bone may be observed at lower Cd concentrations than previously believed (Akesson et al., 2005; Ferraro et al., 2010; Engstrom et al., 2011). At low urinary (U-) Cd levels, associations
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between U-Cd and proteinuria may reflect renal physiology rather than toxicity of Cd (Chaumont et al., 2012; Akerstrom et al., 2013), but this problem does not occur for effects found on bone. Due to the irreversible health effects of Pb, Hg, and Cd, primary prevention is essential. It is therefore necessary to examine the exposure and body burden of these metals in the general population using biomarkers, and to obtain knowledge about the major sources of the metals. The aim of the present study was to examine the biomarkers of Pb, Hg, and Cd among Swedish adults. The dietary associations, based on a 4-day food record and a food frequency questionnaire, as well as associations with other lifestyle factors for the metal concentrations in blood were also investigated. Measured levels of these metals were subsequently compared to internationally agreed reference health values. 2. Material and methods 2.1. Study design and population The study was based on a subgroup of Riksmaten 2010–2011, a Swedish national survey investigating dietary habits among adults (18–80 years) conducted between May 2010 and July 2011 (Amcoff et al., 2012). The subgroup consisted of participants who in addition to dietary registration and questionnaire also donated blood and urine samples. The sampling was performed at Occupational and Environmental Medicine Centers (OEMCs). Therefore Sweden was divided into seven regions according to affiliation to Swedens seven OEMCs (Fig. 1). Each region included the region capital (Linköping, Lund, Stockholm, Umeå, Uppsala, Gothenburg, Örebro) and two additional counties that were randomly selected. Data were collected at four occasions; May/June 2010, August/September 2010, January/February 2011, and April/May 2011. Within each region, an equal number of individuals were asked to participate independently of population size (12 individuals per county and occasion). Of the 1008 randomly selected individuals, 300 (30%) chose to participate in the blood and urine sampling. Blood metal concentrations were measured in 297 participants. Of these, 22 individuals lacked food records and/or questionnaire data and were excluded. Moreover, since the investigated metals are excreted via urine and feces, two individuals with known kidney disease were omitted. Thus, 273 participants with blood analyses were included in the present study. Urine levels of Cd were measured in 289 individuals. Since urinary flow can affect Cd concentrations, we corrected assessed urinary cadmium concentrations for creatinine levels. Also, we excluded individuals with creatinine concentrations 61 mmol/L (n = 3) since very dilute urine samples are considered not to provide good estimates of the urinary excretion of biomarkers, even after adjustment (Aitio et al., 2007; Soharan et al., 2008). After further exclusion of those lacking food records or questionnaire and the two individuals with kidney disease, 262 individuals were included in the statistical analyses with U-Cd levels. The sample selection was performed by Statistics Sweden (SCB). The study was approved by the regional ethical committee in Uppsala. All participants gave oral informed consent before entering the study.
2.2. Assessment of diet and lifestyle In the national dietary survey Riksmaten 2010–2011, a representative sample of 5000 individuals between 18–80 years and living in Sweden were invited to participate (Amcoff et al., 2012). The data collection took place between May 2010 and July 2011. The participants, all together 1797 women and men, reported everything they ate and drank during four consecutive days. The reporting was done in a webbased food diary. To cover all days of the week, starting day was randomly selected (Tuesday, Wednesday, Saturday or Sunday). A questionnaire with about 50 questions was additionally used to collect data about less frequently consumed food items (e.g. consumption frequency of different classes of fishes and meat), education, smoking, and breast-feeding. Education was divided into elementary school, high-school, and higher education. Smoking status was classified according to never smoker, former smoker, occasional smoker, and daily smoker. Self-reported weight and height were assessed and body mass index (BMI) was calculated (weight [kg] divided by height [m] squared). Associations between metal concentrations and the following food groups were tested: dairy products, eggs, poultry, vegetables, fruits, potatoes, cereals, fish, meat, sausage, offal, alcohol, and discretionary food (defined as sweets, snacks, ice-cream, pastries, jam, and soft drinks).
2.3. Sampling of blood and urine Non-fasting blood and single spot urine were sampled at the OEMC in each region or by district health care centers. Blood was drawn from an antecubital vein. Plasma for the ferritin analyses was separated by centrifugation before storage. The samples were stored at 20 °C until analysis.
2.4. Chemical analyses The concentrations of Cd in urine (U-Cd) corrected for molybdenum oxide interference and Cd and Pb in whole blood (B-Pb) were determined by inductively coupled plasma mass spectrometry (ICP-MS; Thermo X7, Thermo Elemental, Winsford, UK) (Barany et al., 1997). The limits of detection (LOD) for U-Cd, B-Cd and B-Pb were 0.02, 0.04 and 0.11 lg/L, respectively. Mercury in whole blood (B-Hg) was determined in acid-digested samples by cold vapor atomic fluorescence spectrophotometry (Sandborgh-Englund et al., 1998). The detection limit was 0.09 lg/L. To ensure the accuracy of the analytical methods and results, quality control (QC) samples were analysed along with the collected samples (Table 1). All analysed samples were prepared in duplicate and the method imprecisions (calculated as the coefficients of variation in measurements of duplicate preparations) were 6.3%, 4.0%, 1.6%, and 8.2%, for U-Cd, B-Cd, B-Pb and B-Hg, respectively. Plasma ferritin concentration was assessed by chemiluminescent microparticle immunoassay (ARCHITECTÒ, Abbott, US). Creatinine was measured in urine by an enzymatic method as previously described (Mazzachi et al., 2000). 2.5. Statistical analyses The statistical analyses were carried out by STATA version 12 (StataCorp LP, US). Variables are presented as mean ± SD or median (percentiles 5–95th). Non-normal variables were logarithmically transformed and if not attaining normality, nonparametric tests were used. The relationships between metal concentrations and explanatory variables were examined by stepwise forward regression (significance level: p < 0.05). Comparisons between groups were performed by ANOVA or Kruskal–Wallis test and adjusted for potential confounders by ANCOVA or residual method. Adjusted means were calculated by linear model. Correlations were investigated by Pearson correlation and, for non-normal variables, Spearmans rank correlation. To avoid bias caused by outliers, the analyses were also performed after excluding outliers (3rd quartile + 1.5 interquartile range). Limits and number of outliers were as follows; B-Pb: 32.33 lg/L (n = 6), B-Hg: 3.98 lg/L (n = 11), B-Cd: 0.58 lg/L (n = 23), U-Cd: 0.57 lg/g creatinine (n = 17). P < 0.05 was considered statistically significant.
3. Results 3.1. Population characteristics Population characteristics and metal concentrations in whole blood and urine by gender are presented in Table 2. None of the participants had B-Pb and B-Cd levels below the detection limit (<0.11 and <0.04 lg/L, respectively), four individuals had BHg 6 0.09 lg/L, and eight individuals had U-Cd 6 0.02 lg/L. These results were included even if measured concentrations were below the detection limit. The median values (5–95th percentiles) of the metals in blood among all participants were as follows; B-Pb: 13 (5.8–29) lg/L, B-Hg: 1.1 (0.31–3.5) lg/L, and B-Cd: 0.19 (0.09– 1.1) lg/L. The median (5th–95th percentiles) of U-Cd was 0.16 (0.04–0.63) lg/g creatinine. The distributions of the metal concentrations are shown in Fig. 2. Among fertile women (18–45 years, n = 64), blood concentrations were as follows; B-Pb: 9.8 (4.7–18) lg/L, B-Hg: 0.70 (0.12–2.5) lg/L, and B-Cd: 0.19 (0.08–0.94) lg/L. Urinary Cd concentrations among fertile women (n = 60) were 0.11 (0.04–0.34) lg/g creatinine. 3.2. Blood concentrations of lead B-Pb levels were positively associated with age (mean% increase per year [standard error, SE]: 1.0 [0.2]), male gender (mean% difference [SE]: 22.5 [6.4]), and smoking (mean% change from non-smoker to daily smoker [SE]: 22.3 [10.7]), but not with education, BMI, reported energy intake or plasma ferritin in multivariable analysis. There were no statistically significant differences in B-Pb concentrations between Swedish regions (i.e. areas of Linköping, Lund, Stockholm, Umeå, Uppsala, Gothenburg, Örebro; see map, Fig 1). In stepwise regression analysis, B-Pb was associated with intakes of alcohol and potatoes (mean% changes per g/d [SE]: 0.04 [0.01] and 0.08 [0.03], respectively), but not vegetables, fruits, cereals, fish, meat, sausage, offal, or discretionary food. Alcohol but not potatoes remained related after adjustments for age,
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Fig. 1. Map over Sweden showing the seven OEMCs and their respective reception regions. (OEMCs = Occupational and Environmental Medicine Centers).
gender, education, smoking, and plasma ferritin (Fig. 3). Meat intake assessed by food records was not related to B-Pb. However, when examining the impact of type of meat on B-Pb assessed by
food frequency questionnaire, frequency of game intake, but not reindeer, sheep, horse, bovine, pig, bird, or ďŹ sh, was associated with B-Pb. This was valid also after adjusting for age, gender,
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Table 1 Results from analytical quality control (QC) of cadmium in urine (U-Cd), cadmium in blood (B-Cd), lead in blood (B-Pb) and total mercury in blood (B-Hg). Quality control material Seronorm QTCb
a
Seronorma QTCb
Seronorma a b
Batch
Analyte
Reference value mean ± SD (lg/L)
Obtained value mean ± SD (lg/L)
n
OK0436 D0514 D0905 1003191 1003191 C0616 L0909 L0807 1003191 0512627
U-Cd U-Cd U-Cd B-Cd B-Pb B-Cd B-Pb B-Pb B-Hg B-Hg
0.26–0.36 1.01 ± 0.09 5.1 ± 0.26 0.59–0.75 14–16 1.1 ± 0.14 23 ± 1.1 114 ± 9.1 1.8–2.2 16–20
0.28 ± 0.01 0.94 ± 0.03 4.9 ± 0.09 0.65 ± 0.07 16 ± 0.57 0.98 ± 0.06 23 ± 1.1 116 ± 7.3 1.9 ± 0.10 15 ± 0.79
9 9 9 11 11 11 11 11 31 31
SERO AB, Billingstad, Norway. Centre de Toxicologie du Quebec International Comparison program, Quebec Canada.
Table 2 Population characteristics and metal concentrations among women and men. Women (n = 145)
Men (n = 128)
Age (years) BMI (kg/m2) Education Elementary school High-school Higher education Smoking status Daily Occasional Ceased Never
48.2 ± 16.5 24.1 (19.8–32.9)
52.5 ± 17.0 25.2 (21.8–32.5)
18 (12%) 65 (45%) 62 (43%)
23 (18%) 53 (41%) 52 (41%)
13 10 41 78
5 (4%) 12 (9%) 43 (34%) 68 (53%)
B-Pb (lg/L) B-Hg (lg/L) B-Cd (lg/L) U-Cd (lg/g creatinine)b P-Ferritin (lg/L)
12 (5.3–25) 0.97 (0.17–2.9) 0.22 (0.09–1.2) 0.20 (0.05–0.75) 51 (8–186)
p
a
0.03 0.10 0.44
categories were related. However, after adjusting for age, gender, education, smoking and plasma ferritin, the association with fresh-water fish was no longer significant whereas it remained significant for shellfish and salt-water fish. Drinking water from water well or municipal water supply system was not associated with BHg concentrations.
0.29 (9%) (7%) (29%) (55%)
15 (7.0–29) 1.3 (0.39–4.4) 0.17 (0.08–0.85) 0.12 (0.04–0.48) 143 (36–449)
3.4. Blood and urine concentrations of cadmium <0.001 <0.001 <0.001 <0.001 <0.001
Age is presented as mean ± SD. The other numerical variables are presented as median (5–95th percentiles). a p Denotes differences between genders. b n = 262 (137 women, 125 men).
education, smoking, and plasma ferritin (Fig. 3). B-Pb concentrations did not differ if the participants received their drinking water from water well or municipal water supply system. 3.3. Blood concentrations of mercury Higher age (mean% increase per year [SE]: 1.9 [0.3]), male gender (mean% difference [SE]: 34.1 [13.1]), and higher education (mean% change from lowest to highest education [SE]: 51.1 [21.8]) were associated with higher B-Hg concentrations whereas there was no relationship with BMI, energy intake, smoking or plasma ferritin in multivariable analysis. There were no statistically significant differences between Swedish regions with regard to B-Hg concentrations. Intakes of fish and vegetables assessed by food records were positively related to B-Hg in stepwise regression (mean% changes per g/d [SE]: 0.88 [0.12] and 0.13 [0.04], respectively), whereas there were no associations for potatoes, fruits, cereals, meat, sausage, offal, alcohol, and discretionary food. These associations remained after adjusting for age, gender, education, smoking and plasma ferritin. Fish intake assessed by food frequency questionnaire was also associated with B-Hg (Fig. 3). Other types of meat (i.e. game, reindeer, sheep, horse, bovine, pig, and bird) were not related to B-Hg taking age, gender, education, smoking, and plasma ferritin into account, and excluding outliers. When associating BHg with type of fish assessed by food frequency questionnaire (i.e. shellfish, salt-water fish, and fresh-water fish), all three
Cd levels in blood and urine (creatinine adjusted) were correlated (r = 0.65, p < 0.001) (Fig. 4). B-Cd concentrations were higher among smokers (mean% change from non-smoker to daily smoker [SE]: 367 [49]), higher among women (mean% difference [SE]: 20.3 [5.5]), positively associated with age (mean% increase per year [SE]: 1.3 [0.2]), and inversely related to plasma ferritin levels (mean% change [SE]: 0.10 [0.03]), whereas no relations were observed for education, BMI, or energy intake in multivariable analysis. When measured in urine, Cd was associated with smoking, gender, and age but not the other variables. No statistically significant differences in blood or urinary Cd concentrations were observed between different Swedish regions. Intakes of meat and discretionary food assessed by food records were negatively associated with B-Cd in stepwise regression (mean% changes per g/d [SE]: 0.34 [0.09] and 0.07 [0.02], respectively) (Fig. 3). The association with meat remained after adjusting for age, gender, education, smoking, and plasma ferritin. Intake of sausage was additionally negatively associated with B-Cd in the analysis when outliers were excluded. The other food items included in the analysis (i.e. potatoes, vegetables, fruits, cereals, fish, offal, and alcohol) were not associated. Even though total meat intake was related to B-Cd, no association with type of meat assessed by food frequency questionnaire was observed. When measured in urine, Cd concentrations were inversely related to intakes of meat, discretionary food, and cereals, and positively related to intake of fruits and berries. All associations but meat intake disappeared after adjustments for age, gender, education, smoking, and plasma ferritin. Cd levels in blood or urine were not dependent on whether the participants received their drinking water from water well or municipal water supply system. Since smoking were strongly related to Cd levels, the associations between Cd and personal characteristics, diet and other lifestyle factors were additionally investigated among never-smokers only. In these analyses, the results were similar as in the primary analyses when smokers were included. However, among neversmokers only, high consumptions of pig and game assessed by food frequency questionnaire were associated with lower concentrations of B-Cd.
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Fig. 2. Distribution of metal concentrations in the study population. Blood analyses: n = 273, urine analysis: n = 262.
3.5. Exclusion of outliers To avoid extreme values from affecting the results, all analyses were additionally performed after excluding outliers. This did however not appreciably change the results or the interpretation (data not shown).
4. Discussion 4.1. Lead In this general Swedish adult population, Pb concentrations in blood were higher among men than women, a difference also observed by others (Barany et al., 2002; Clark et al., 2007; Nordberg et al., 2000; Wennberg et al., 2006). This may partly be due to that B-Pb mainly is present in erythrocytes and that men have a higher fraction of erythrocytes in whole blood than women (Skerfving et al., 1999). Men are also more likely to have occupational exposure. The positive association between B-Pb levels and age among adults observed here and by others (Clark et al., 2007) may be an age cohort effect, reflecting a higher exposure early in life among the older individuals, and that the higher blood levels partly are derived from endogenous storages (Skerfving et al., 1999). For example, the reduction of lead in gasoline has caused a major exposure reduction also in B-Pb in Sweden (Stromberg et al., 2008). The higher B-Pb concentrations among smokers are in accordance with other studies (Skerfving et al., 1999; Wennberg et al., 2006). In a Swedish food basket case study, fruit was estimated to contribute the most to the dietary exposure of Pb (21%), followed by potatoes (14%), and sugar and sweets (14%) (NFA, 2012a). We did
observe an association with potatoes, which did not remain after adjusting for potential confounders. Potato consumption has been related to Pb levels in blood (Clark et al., 2007) and in breast milk (Garcia-Esquinas et al., 2011). Regarding the positive relationship between B-Pb and alcohol consumption found in the present study, an association with wine has been reported previously (Wennberg et al., 2006; Vahter et al., 1991). Even though total meat intake assessed by food records was not related to B-Pb, we observed higher levels among those consuming more game according to the food frequency questionnaire. Interestingly, Pb from lead bullets used in game hunting may be a source of Pb (Hunt et al., 2009) and the Swedish National Food Agency has recently introduced advice regarding Pb risks and consumption of game meat (NFA, 2012b).
4.2. Mercury Since total-Hg was measured and no data on dental amalgam were assessed, the contribution of inorganic Hg from amalgam (Bjornberg et al., 2005) to the observed circulating Hg levels is unknown. Most elderly people in Sweden have dental amalgam fillings, while today this is rare among those below 30 years of age. Usually 40–60% of Hg in blood is methyl mercury (SandborghEnglund et al., 1998; Vahter et al., 2000). Men had higher B-Hg levels than women. Also in a Canadian study, men had higher B-Hg than women (Clark et al., 2007), even though the gender differences do not seem as clear as for B-Pb and B-Cd (Skerfving et al., 1999). It is probable that fertile women consume less fresh-water fish due to the recommendations to limit intake in this target group and therefore lower blood levels are observed among women. B-Hg increased with age in line with previous results (Wennberg et al., 2006). One explanation may be that elderly
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Fig. 3. Blood metal concentrations (adjusteda means and 95% CI), dietary intake and smoking habits. (a) Blood concentrations were adjusted for age, gender, education, smoking, and plasma ferritin, but also intake of other food components as described in the result section, (b) alcohol, vegetable, and meat intakes were assessed by food records and divided into tertiles (n = 268, alcohol; <25 g/d, <175 g/d, 61879 g/d, vegetables; <145.3 g/d, <214.6 g/d, 6731.2 g/d, meat; <43.9 g/d, <76, 5 g/d, 6256.0 g/d), (c) game and fish consumptions were assessed by food frequency questionnaire (game; n = 51 [never], n = 148 [<1/mo], n = 49 [P1/mo], and fish; n = 30 [61/mo], n = 156 [>1/ mo], n = 62 [>1/wk]) and (d) smoking; n = 146 [never], n = 83 [ceased], n = 21 [occasional], n = 18 [daily].
Fig. 4. Relation between B-Cd and U-Cd from 262 matching blood-urine samples obtained in the Riksmaten 2010–2011 study, showing a significant correlation (p < 0.001).
consume more fish, especially fresh-water fish (Johnsson et al., 2004; Amcoff et al., 2012). Moreover, lower exposure of inorganic Hg is a result of decreased use of dental amalgam; already in 2003 less than 2% of the new fillings were of amalgam type (Swedish National Board of Health and Welfare). Fish is the main contributor to Hg exposure in Sweden (NFA, 2012a) and consumption has been associated with B-Hg (Johnsson et al., 2005; Bjornberg et al., 2005; Wennberg et al., 2006). Therefore the observed relation between fish intake and B-Hg was expected. However, when dividing the fish consumption into categories, rather surprisingly, fresh-water fish did not remain associated after adjustments for confounders. Possibly the consumption was too low in relation to other fish groups including salmon and lean seawater fish. Moreover, adherence to the dietary recommendations issued by the Swedish National Food Agency may have reduced the intake of fish containing high amounts of Hg. The observed positive association between B-Hg levels and intake of vegetables may be due to that individuals consuming high amounts of fish adhere to an overall healthy diet and thus also have a higher intake of vegetables (the correlation between intakes of fish and vegetables was significant [r = 0.18, p = 0.002]).
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4.3. Cadmium Cd levels increased with age, which is primarily an effect of the very long half time (Nordberg et al., 2007; Olsson et al., 2002; Wennberg et al., 2006). As expected (Berglund et al., 1994; Andersen et al., 2004; Barany et al., 2005; Olsson et al., 2002; Wennberg et al., 2006), plasma ferritin levels were inversely related with BCd. This is probably due to up-regulation of transporters caused by low iron stores influencing Cd absorption (Vahter et al., 2007). Lower iron stores among women than men therefore provide a probable explanation for the observed higher B-Cd levels in women in the present study and elsewhere (Olsson et al., 2002; Wennberg et al., 2006). Lower iron status due to less meat intake may also explain the negative correlation between meat intake and B-Cd. No strong dietary factors were observed with regard to Cd in blood or urine. It is possible that the dietary habits within the population were too homogenous to detect any differences, or that the intake of key food items, such as cereals (NFA, 2012a), was difficult to estimate since it is present in a diversity of food products and therefore also complicated to assess accurately. Cereals, bread, potatoes, and vegetables have been described as the major dietary Cd sources (NFA, 2012a; Olsson et al., 2002). None of these associations were observed in the present study. However, higher B-Cd was observed among those consuming less discretionary food. It is possible that individuals consuming less discretionary food have a healthier lifestyle and thus a higher fiber intake. In another observational study, a high fiber intake was associated with a higher Cd exposure (Berglund et al., 1994). However, fiber intake has also been associated with lower B-Cd among smoking but not non-smoking men (Kim et al., 2010) and diets high in refined carbohydrates and low in fiber seem to increase the intestinal Cd uptake in animals (Andersen et al., 2004). Intake of meat was inversely related to B-Cd. Meat has previously been estimated to contribute to the dietary Cd burden with only approximately 4% (NFA, 2012a). Therefore, as discussed above, iron may explain the observed association. For U-Cd, an inverse association with meat intake could be caused by increased excretion of creatinine, used to adjust U-Cd levels. Smoking is a major source for Cd exposure (Ferraro et al., 2010; Wennberg et al., 2006), something we also observed in the present study. 4.4. Geographic differences within Sweden The classification of Sweden into seven regions in the present study may have been too broad to observe any geographical differences in metal concentrations. Instead smaller regions defined by Cd-soil content, as used elsewhere (Olsson et al., 2002), may be necessary to detect any geographic differences. On the other hand, a centralized food market may obstruct any potential geographic effects. Contribution of Cd from locally produced food has been estimated to 17% of total Cd intake in a Swedish study (Olsson et al., 2002). Even though there was a wide range of this contribution to body Cd levels, the impact for most individuals seemed limited. 4.5. Comparison with other populations and risk estimations The blood concentrations of Cd, Pb, and Hg are in accordance with Swedish data (Barany et al., 2002; Gerhardsson and Lundh, 2010; Olsson et al., 2002) as well as data from other countries (Table 3). However, the considerably high levels of metals in some regions suggest problems that may be related to specific exposure conditions. Among fertile women in the present study, 30% had BPb levels above the reference point of 12 lg/L for developmental neurotoxicity established by EFSA (EFSA, 2010). This reference
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point equals the lower confidence limit on a benchmark dose corresponding to an extra risk of 1% (BMDL01), which implies a decrease in one IQ point. In the entire study population, most individuals were below the reference point (BMDL01) for potential effects on blood pressure set at 36 lg/L, but a large part of the study population seemed to be above the reference point (BMDL10) for possible nephrotoxic effects at 15 lg/L. With regard to B-Hg, concentrations below 4.8 and 8.8 lg/L are judged by the American National Research Council and the Joint FAO/WHO Expert Committee on Food Additives and Contaminants, respectively, having no risk of fetal neurotoxic damages (NRC, 2000; WHO, 2004; NFA, 2007). The highest concentration among fertile women in the present study was 3.8 lg/L and thus below the mentioned reference points. Regarding U-Cd, the critical concentration of 1 lg/g creatinine set by EFSA (EFSA, 2009) relates to a level below which no increased risk for renal adverse effects should be expected. In our study, the 95th percentile for U-Cd was 0.6 lg/g creatinine and thus there seems to be a margin to the critical U-Cd concentration for the general adult Swedish population. However, four individuals (age 40– 70 years) had U-Cd above 1 lg/g creatinine (maximum level was 3.7 lg/g creatinine) and may have increased risk for adverse effects on kidney and/or bone. 4.6. Strengths and limitations Strengths of this study are the nation-wide perspective including both gender, a broad age span of the Swedish adult population (18–80 years), and several geographic regions. The study suffers from a low participation rate (30%), but there were no statistical significant differences between participants and 377 non-participants for which register data was obtained with regard to age, gender, income, or geography, whereas education was higher among participants (data not shown). Another strength with the study is the use of both 4-days food records and food frequency questionnaire. Thus, a better characterisation of the participants´ dietary habits is probably achieved than when using only one method. Moreover, Cd levels were measured in both blood and urine, strengthening the results. One limitation is that exposure of the metals and dietary habits earlier in life may have affected the results. Also, the assessment of dietary habits and metal concentrations in blood and urine were not performed at the same time. In average blood sampling was performed 15 days after the start of food recording (p5–p95: 2–57 days). However, the registered diet is believed to sufficiently reflect habitual intake. Furthermore, an advantage is the precise and accurate analytical methods used with extensive use of reference materials and participation in inter-laboratory control programs. 5. Conclusions In this Swedish study, a nation-wide approach was used to survey dietary habits, lifestyle factors and metal concentrations in blood and urine. Among adults of 18–80 years of age, blood concentrations of Pb, Hg, and Cd were associated with age, gender and dietary factors. The major contributor to Hg exposure seemed to be fish whereas intakes of game and alcohol were related to higher Pb levels. Important factors for Cd levels in blood appeared to be smoking and iron deficiency. In case of Hg and Cd, measured blood and urinary levels were, with some minor exceptions, below health based reference points. For Pb, on the other hand, a large part of the study population seemed to be above the reference point for possible nephrotoxic effects, and 30% of fertile women had levels above the reference point for developmental neurotoxicity established by EFSA (2010). Thus, as health effects cannot be excluded, individuals with high Pb exposure should aim at
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Table 3 Mean levels of lead, mercury and cadmium in whole blood samples from various populations, in comparison to the present study (geom. means, in lg/L). Country, region
Population
Pb
Hg
Cd
References
Sweden, present study Germany, north Czech Republic Canada Canada, west coast USA, NHANES 2007–2008 Korea, South
Adults 18–74 yr, pop. based study Adult volunteers, n = 130 Adults, 18–58 yr Country representative, 6–79 yr, n = 2576–2743 Non-smoking, oyster-growers, 30–65 yr, n = 61 Adults (n = 4409) Adults (>20 yr, n = 5924)
13.4 19 65a 13.4 21 14.3 22.9
1.08 0.9 2.8a 0.69 2.9 0.96 0.97
0.22 0.38 1.0a 0.35 n.d. 0.38 4.30
Heitland and Köster (2006) Cerna et al. (2012) Haines and Murray (2012) Clark et al. (2007) Chen et al. (2013) Kim and Lee (2012)
n.d. = Not determined. a ‘‘Reference value’’ 2005–2009.
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