Selenium exposure in subjects living in areas with high selenium
Transcription
Selenium exposure in subjects living in areas with high selenium
Environment International 40 (2012) 155–161 Contents lists available at ScienceDirect Environment International j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e n v i n t Selenium exposure in subjects living in areas with high selenium concentrated drinking water: Results of a French integrated exposure assessment survey Barron Emmanuelle a, 1, Migeot Virginie a, b,⁎, Séby Fabienne c, Ingrand Isabelle b, Potin-Gautier Martine d, Legube Bernard a, Rabouan Sylvie a, b a Université de Poitiers, Laboratoire de Chimie et Microbiologie de l'Eau, UMR CNRS 6008, 40, avenue du Recteur Pineau, 86022 Poitiers Cedex, France Université de Poitiers, Faculté de Médecine et Pharmacie, 6 rue de la Milétrie, 86034 Poitiers Cedex, France Ultra Traces Analyses Aquitaine, Hélioparc Pau Pyrénées, 2 avenue du Président Angot, 64053 Pau Cedex 9, France d Université de Pau et de l'Adour, Equipe de Chimie Analytique Bio-Inorganique et Environnement, UMR CNRS/UPPA 5254, avenue de l'Université, 64000 Pau, France b c a r t i c l e i n f o Article history: Received 23 December 2010 Accepted 10 July 2011 Available online 7 August 2011 Keywords: Selenium Drinking water Monitoring Intake Toenail France a b s t r a c t Background: Selenium is an essential element which can be toxic if ingested in excessive quantities. The main human exposure is food. In addition, intake may be boosted by consumption drinking water containing unusual high selenium concentration. Objective: We measured the individual selenium level of people exposed to selenium concentration in drinking water greater than the maximum recommended limit which is 10 μg/L. Methods: We carried out a prospective cohort study on 80 adults (40 exposed subjects i.e. living in the involved area and 40 non-exposed ones i.e. living elsewhere) in western France. We used three different approaches: (1) direct measurement of ingested selenium by the duplicate portion method, (2) dietary reconstitution with a food frequency questionnaire (FFQ) and (3) evaluation of the individual selenium status by measuring the selenium content in toenail clippings. Analyses were performed by inductively coupled plasma-mass spectrometry. The association between toenail selenium concentration and area of residence was analyzed using linear regression with repeated measurements. Results: We estimated selenium intake from FFQ at 64 ± 14 μg/day for exposed subjects as opposed to 52 ± 14 μg/day for the non-exposed ones. On the basis of 305 duplicate diet samples, average intake was estimated at 64 ± 26 μg/day for exposed subjects. Area of residence (p = 0.0030) and smoking (p = 0.0054) were independently associated with toenail selenium concentration. Conclusion: Whatever method used for estimating selenium intake, the selenium level in this studied area with high selenium concentrated drinking water is much lower than in seleniferous areas. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Essentiality and toxicity of selenium for humans are well known (Césarini 2004; Dodig and Cepelak 2004; Navarro-Alarcon and Cabrera-Vique, 2008; Papp et al., 2009; Simonoff and Simonoff 1991; Tinggi 2008). Insufficient intakes have been linked to serious health effects, such as dilated cardiomyopathy in Keshan disease (Levander and Beck 1997; Liu et al., 2002; Yang and Zhou, 1994; Zhang et al., 2010) and osteoarthritis in Kashin Beck disease (Suetens et al., 2001). These ⁎ Corresponding author at: Université de Poitiers, Faculté de Médecine et Pharmacie, 6 rue de la Milétrie, 86034 Poitiers Cedex, France. Tel.: + 33 549 454 359; fax: + 33 549 454 073. E-mail addresses: [email protected] (B. Emmanuelle), [email protected] (V. Virginie). 1 Present address: Université Victor Segalen, UFR Sciences Pharmaceutiques, FRE CNRS 3396, 146 rue Léo Saignat, 3076 Bordeaux cedex, France. 0160-4120/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.envint.2011.07.007 disorders have been observed in areas with poor selenium soil (said to be seleniprive). Minimal requirement is today estimated at 20 μg/day (Thomson 2004). But, selenium can also be toxic if ingested in excessive quantities in the long run (i.e. in seleniferous area). The most frequent symptoms described are skin and scalp lesions, nails and hair abnormalities including loss (Hira et al., 2004; Srivastava et al., 1995; Thérond et al., 1997; Yang and Zhou, 1994).Yang and others have monitored selenium exposure and clinical symptoms of subjects living in seleniferous area. They proposed, for safety, a maximum safe daily dietary intake at 400 μg/day (Yang and Zhou 1994). Health effects of intermediate levels of intake are less certain therefore conflicts rise up about the optimal intake (Combs 2005; Dodig and Cepelak 2004; Fairweather-Tait et al., 2010a; NavarroAlarcon and Cabrera-Vique 2008; Rayman et al., 2008). The World Health Organization (WHO) has set the No Observed Adverse Effect Level (NOAEL) at 240 μg/day (4 μg/kgBW/day) and has recommended consequently a maximum upper limit of 10 μg/L for 156 B. Emmanuelle et al. / Environment International 40 (2012) 155–161 selenium in drinking water (which corresponds to about 10% of the NOAEL) (WHO 1996). This limit of 10 μg/L has been adopted in the European regulations (Directive 98/83/CE — 03.11.98) and translated into French law (Ministère de la santé et des solidarités 2007). The major source of selenium is food but intake from beverage is highly neglected (Rayman, 2008). Most of drinking water concentrates selenium less than 1 μg/L (Conde and Sanz Alaejos 1997). However when water gets down through particular geological formations (i.e. rich in selenium), selenium content can be increased. La Vienne, western France, is the case where eight drinking water stations, supplying 40,000 to 55,000 inhabitants contain up to 40 μg/L of selenium (Barron et al., 2009). We carried out a study to quantify selenium intake of Vienne inhabitants using three different approaches: (1) direct measurement of ingested selenium by the duplicate portion technique, dietary reconstitution based on a food frequency questionnaire (FFQ) and the use of a biological marker. 2. Material and methods 2.1. Study design We carried out a prospective cohort study over 9 months (October 2005 to July 2006). 2.2. Study population We recruited 80 subjects: 40 “exposed” adults, i.e. living in the studied area for at least 2 years, and 40 “non-exposed” subjects, i.e. living outside the studied area for at least 2 years. The exposed group was representative of the residents in terms of age, sex and socioprofessional status. The subjects in both groups were matched for the same criteria. All participants gave written informed consent. Exclusion criteria were taking a selenium supplement and intending to leave the area within the coming year. Participants completed a pre-inclusion questionnaire, four food frequency questionnaires (FFQ) (one per season) and a health questionnaire. They provided two samples of toenail clippings (Slotnick and Nriagu 2006). The exposed group also provided a duplicate of what they had eaten and drunk over 2 days per season. The study was approved by the relevant ethics committees, which advocate that all data remain confidential and anonymous. 2.3. Data collection All data collected and the sampling strategy were evaluated in a pre-test involving three subjects over two consecutive days. 2.3.1. Self-administered questionnaires The pre-inclusion questionnaire was administered to collect sociodemographic data (age, sex, occupation, income level, educational level, place and length of residence, intention of moving), anthropometric measurements (weight, height) and selenium supplement ingestion. The FFQ (24-hour recall), adapted from a questionnaire validated in France, enabled us to estimate both total and fractionated (i.e. food vs. drinking water) selenium intake (Guilbert and Perrin-Escalon 2002). A health questionnaire was developed from two questionnaires validated in France (Guilbert et al., 2000; CREDES Centre de Recherche d'Etude et de Documentation en Economie de la Santé 2004). The main categories of data collected were current and previous medical pathology, lifestyle habits such as smoking status, alcohol consumption and physical activity, and Duke health profile (Guillemin et al., 1997). 2.3.2. Toenail clippings We collected toenail clippings twice during the study to measure selenium level (van den Brandt et al., 1993a). The first sampling was performed three months into the study and the second at the end of the study (i.e. 6 months after the first collection). 2.3.3. Duplicate diet portions Exposed subjects duplicated everything they ingested over two entire days (Gay 2000) in each season. A meeting was organized to explain the context, objective and schedule of the study (Kazutoshi et al., 2003; Kazutoshi et al., 2004). Each participant received compensation (15 euros) for each duplicate day (Lightowler and Davies 2002). 2.4. Data analysis 2.4.1. Food frequency questionnaire (24-hour recall) Based on this questionnaire, the daily selenium intake was reconstituted by adding together the amount contained in each item of food (or in beverage). The selenium content of foods was calculated from the French table of selenium concentration of foods (Leblanc et al., 2005). The selenium content of water was fixed at 1 μg/L for subjects in the non-exposed group (Simonoff and Simonoff 1991) and at the median selenium concentration of the municipal water supply system for the exposed group (Barron et al., 2009). The weight of each portion of food was defined from nutritional recommendations (GPEM/DA 1999). The liquid quantity was defined as 150 mL/glass. 2.4.2. Duplicate diet portions and toenail clipping samples 2.4.2.1. Sample preparation. All solutions were prepared with ultrapure water (18.2 MΩ cm) obtained from a Millipore Milli-Q water purification system (Bedford, MA, USA). After collection, toenail clippings were cleaned to minimize external contamination. Each set of clippings was sonicated first in a detergent [triton ×100, 1%, Sigma-Aldrich (SaintLouis, MO, USA)] and then in ultrapure water. Between these two steps, the clippings were rinsed three times in ultrapure water. They were then air-dried for several days and placed in a dry place for 24 h to ensure uniform humidity between samples. Duplicate diet portions were collected in polythene containers cleaned in 10% nitric acid and rinsed in ultra pure water before use. These samples were kept in a cool place until their preparation for analysis (household refrigerator, transport to laboratory in cool boxes and subsequent storage in a cold room [5± 2 °C]). At the laboratory, inedible parts were first discarded (e.g. bones, fruit stones) and then food and drinks were weighed. Food and drink corresponding to the same day were mixed together using a grinder (Bermixer B2000, 4400 W Electrolux). Subsamples (about 30 g) were collected, weighed and stored at −20 °C before lyophilisation (Freeze-Dryer Christ Alpha). 2.4.2.2. Sample digestion. Duplicate diet and toenail samples were digested using a graphite heating block system (DigiPREP system with temperature control [SCP Science, Quebec, Canada]). For duplicate diet portions, 1 g of sample was accurately weighed in a 50 mL polypropylene tube. Four milliliters of HNO3 (Baker-INSTRA, 70%) were then added and this mixture was left overnight before the addition of 2 mL of H2O2 (Baker analyzed, 30%), obtained from Baker (Phillipsburg, NJ, USA). The tubes were then tightly sealed with a polypropylene screw cap and heated from room temperature to 45 °C within 20 min. This temperature was maintained for 40 min. A further temperature rise to 80 °C was then effected, again within 20 min, and this temperature was maintained for 220 min. After cooling, digests were diluted to 50 mL with ultrapure water before total selenium analysis. For duplicate diet portion samples, all digestions were performed in duplicate (independent aliquots). For nail samples, 2 mL of HNO3 were added to the collected sample (about 100–200 mg) and this mixture was left overnight before the addition of 0.5 mL of hydrogen peroxide. Digestion was performed using the same temperature program as described B. Emmanuelle et al. / Environment International 40 (2012) 155–161 above for food samples, except that the step at 80 °C lasted for 40 min instead of 220 min, because this was easily enough time to dissolve nail clippings. Finally, samples were diluted to 10 mL with ultrapure water before total selenium analysis. For nail samples, only one digestion per sample was performed because of the small quantity. For each digestion run, two blanks and two Certified Reference Materials (CRMs) with a matrix close to that of the samples under study were processed at the same time as the real samples: bovine muscle BCR 184 (0.3 g) (IRMM [Geel, Belgium]) and dogfish muscle DORM-2 (0.2 g) (NRCC [Ottawa, Canada]) for digestion of food-based samples and 2 replicates of hair sample BCR 397 (0.1 g) (IRMM [Geel, Belgium]) for nail samples. Since there was no CRM for nail clippings, a keratin-based hair sample with a very similar matrix was used. 2.4.2.3. Sample analysis. Total selenium was analyzed in all the digested extracts by inductively coupled plasma-mass spectrometry (ICP-MS) (Agilent model 7500ce, Tokyo, Japan) using a spectrometer equipped with an octopole reaction cell. For sample introduction, a microconcentric nebulizer (Glass Expansion, Romainmotier, Switzerland) was fitted with a double-pass Scott spray chamber (2 °C). ICP-MS measurement conditions (nebulizer gas flow, RF power and lens voltage) were optimized daily using a multi-element solution (Li, Y and Tl). Selenium detection was achieved after removing the Ar2+ interferences using H2 reaction gas (octopole bias −18 V; quadrupole bias −17 V) in the octopole reaction cell. Appropriate Internal Quality Control (IQC) parameters were used to evaluate the validity of the analytical process and to check that no outliers occurred during routine analysis sequences. Quantification of total selenium was performed using the standard additions method to avoid any matrix effect and this procedure was regularly performed during analysis (every 10 samples); CRMs were also regularly analyzed throughout the analytical sequence. Their use enabled us to validate and control the method on a routine basis (Shewhart control charts). Detection limits were estimated at 1.5 μg kg−1 and 2.2 μg kg−1 (dw) for food and toenail samples, respectively, based on the International Union of Pure and Applied Chemistry's harmonized definitions. Quantification limits were estimated at 4.5 μg kg−1 and 7.0 μg kg−1 (dw) for food and toenail samples, respectively. Repeatability, as estimated from analysis of 6 different digested aliquots and expressed as % relative standard deviation, ranged from 1 to 2%. The amount of selenium in a duplicated day was calculated by taking the mean of the content obtained from two different mineralizations of the same duplicate portion. Intake was standardized by body weight as recommended by Kroes et al. (2002). However, this standardization is not systematically performed in the literature, because studies from public establishments such as hospitals, canteens and army barracks do not collect the weight of subjects providing duplicate portions. For the purpose of making comparisons, our results were also expressed in μg/day. 2.5. Statistical analysis We estimated that to demonstrate a difference in toenail selenium concentration corresponding to a difference in selenium intake of 32 μg/day between the exposed and non-exposed populations (Longnecker et al., 1993), it would be necessary to include 40 subjects in each group (allowing for a significance of 5%, a power of 80% and a loss to follow-up of 20%). Results are expressed as mean ± standard deviation. Normal distribution of data was verified using the Kolmogorov– Smirnov test. Thus, we carried out log transformations for selenium intake and selenium toenail concentration and calculated geometric means and corresponding 95% confidence intervals (CI). We used Student's t-test for matching data for quantitative variables and McNemar's test for qualitative variables. Differences were considered significant at the level of p ≤ 0.05. We checked the correlation 157 between the two methods of intake estimation (FFQ and duplicate diet portions) using Pearson's correlation coefficient. The dependent variable was toenail selenium concentration (from toenail clippings collected at the end of the study). The variable of interest was the subject's place of residence. The independent variables were educational level (elementary, college, secondary, university or equivalent), household size (≤2 persons, N2), smoking status (smoker, non-smoker), alcohol use (at least twice a week, maximum once a week), physical activity (at least once during the two previous weeks, less), body mass index (b20, 20–24, 25–30, N30), Duke score (Guillemin et al., 1997), annual frequency of homeproduced and locally produced food consumption (N5; [4–5]; [2–3]; b2) corresponding to the sum of the frequency of consumption recorded for each season (i.e. 0 = maximum once a week; 1 = maximum 3 times a week; 2 = maximum every day), and total selenium intake (average of the results from the four seasons as estimated by FFQ). To perform our analysis, we used SAS's PROC MIXED procedure for repeated measures to take into account the seasonal effect. Independent variables were included in the initial regression model if they were associated with selenium toenail concentration at a pvalue b 0.20. From the initial regression model, variables were selected using a stepwise descending process. We tested the first-order interactions in the final model. We captured data derived from questionnaires with EPI-Info version 3.3.2. The controls and statistical analysis were performed using SAS for Windows, version 8.0 (SAS Institute Inc., Cary, NC, USA). 3. Results 3.1. Study population Eighty volunteers were selected to participate in the study. One non-exposed subject left the study. Data from his matched exposed subject were excluded from the analysis. Analyses were thus performed on 78 subjects. The distribution of age, sex and occupational group within the exposed group and the local residents was similar. Mean age was the only independent variable to differ significantly between the two study groups (Table 1). 3.2. Daily selenium intake 3.2.1. Estimation from FFQ Selenium intakes from food observed in exposed and non-exposed subjects over the four seasons are shown in Fig. 1. We observed no significant difference between the two groups (0.70 ± 0.18 μg/kgBW/day vs. 0.74 ± 0.25 μg/kgBW/day respectively, p = 0.53) (equivalent to 48 ± 12 μg/day vs. 50 ± 14 μg/day) (Table 2). We observed no seasonal effect (p = 0.07). We observed a significant difference in selenium intake from drinking water between the two groups as shown in Fig. 2 (0.23 ± 0.13 μg/kgBW/day vs. 0.024 ± 0.010 μg/kgBW/day respectively, p b 0.0001). Thus, selenium intake from water represented on average 24% (0–57) of total intake for exposed subjects as opposed to only 3% (1–8) for non-exposed subjects. Finally, we estimated total selenium intake (from food and beverage) from the FFQ at 0.94 ± 0.22 μg/kgBW/day for the exposed group (64 ± 14 μg/day) and 0.77 ± 0.25 μg/kgBW/day (equivalent to 52 ± 14 μg/day) for the non-exposed group. 3.2.2. Estimation from duplicate diet portions We collected 305 duplicate diet portions. Samples provided by one subject were excluded because of systematic and obvious disregard of quantities. Average intake was 0.94 ± 0.37 μg/kgBW/day (0.25–2.73) (64 ± 26 μg/day). The correlation coefficient between intake estimated by FFQ and by duplicate diet portions was calculated at 0.36 (p b 0.0001). 3.3. Toenail selenium concentration Toenail selenium concentration according to the characteristics of the 78 participants is shown in Table 3. The multivariate analysis showed that area of residence (p = 0.0030) and smoking (p = 0.0054) were independently associated with toenail selenium concentration (Table 4) without interaction (p = 0.50). We estimated the difference in toenail selenium concentration between exposed and non-exposed subjects at 70 μg/kg (95% CI [27; 110]) and between smokers and non-smokers at − 111 μg/kg (95%CI [− 201; −33]) (Table 4). 158 B. Emmanuelle et al. / Environment International 40 (2012) 155–161 Table 1 Socio-demographic characteristics, anthropometric characteristics and lifestyle habits of the 78 participants by area of residence. Age (years) Length of residence (years) Exposed (n = 39) Mean ± SD Mean ± SD p 50.5 ± 13.0 27.7 ± 19.2 52.7 ± 15.0 27.7 ± 18.5 0.0011 0.33 n (%) n (%) p 1.00 19 (48.7) 19 (48.7) 19 (48.7) 19 (48.7) 1 (2.6) 1 (2.6) 4 (10.3) 2 (5.1) 8 (20.5) 5 (12.8) 3 (7.7) 15 (38.5) 1 (2.6) 2 (5.1) 5 (12.8) 1 (2.6) 8 (20.5) 4 (10.3) 3 (7.7) 15 (38.5) 2 (7.4) 7 (25.9) 9 (33.3) 9 (33.3) 12 3 (8.6) 13 (37.1) 12 (34.3) 7 (20.0) 4 6 (17.7) 13 (38.2) 8 (23.5) 7 (20.6) 5 15 (38.5) 10 (25.6) 9 (23.1) 5 (12.8) 0 22 (56.4) 17 (43.6) 22(56.4) 17 (43.6) 2 (5.1) 20 (51.3) 14 (35.9) 3 (7.7) 1 (2.6) 20 (51.3) 17 (43.6) 1 (2.6) 10 (25.6) 29 (74.4) 0 3 (8.1) 34 (91.9) 2 21 (56.8) 16 (43.2) 2 18 (48.7) 19 (51.4) 2 6 (16.2) 31 (83.8) 2 9 (24.3) 28 (75.7) 2 1.00 0.42 0.51 1.00 0.63 0.092 0.63 0.55 1 .5 0 selenium intake from food (µg/day/kg body weight) 1.5 Sex Male Female Socio-professional category Farmer Shopkeeper Executive Professional White-collar worker Blue-collar worker Other Retired Income level 450 b income ≤ 1000 euros 1000 b income ≤ 2000 euros 2000 b income ≤ 3000 euros N 3000 euros Missing data Educational level Elementary College Secondary University or equivalent Missing data Household size 1–2 people ≥3 people BMI (kg/m−2) b 20 20 ≤ BMI b 25 25 ≤ BMI b 30 ≥30 Tobacco Smoker Non-smoker Missing data Alcohol consumption Maximum once a week At least twice a week Missing data Physical activity At least one activity No activity Missing data Non-exposed (n = 39) spring summer exposed autumn winter unexposed Fig. 1. Distribution of selenium intake from food as estimated from the food frequency questionnaire stratified by season and exposure. 4. Discussion This survey studied selenium status among residents exposed to high selenium concentrated drinking water. Whatever method used for estimating selenium intake, the selenium level in this studied area is much lower than in seleniferous areas (Hira et al., 2004; Longnecker et al., 1991). This study is the first one to combine three different methods of measuring selenium exposure in a French population. Estimation of daily selenium intake provides valuable insight into the amount of selenium ingested by the subjects (Longnecker et al., 1991). The FFQ did not show any difference between food intakes in the two groups. This data may indicate that subjects had no particular eating patterns explaining a particular selenium level (for example, high consumption of fish or meat). As well as other studies, we found no seasonal variation (Swanson et al., 1990; Taylor et al., 1987). On the contrary, intakes from drinking water were significantly different between the two groups. In the non-exposed subjects, these intakes were negligible as in the general population (Barclay et al., 1995; Santé, 1986; WHO 1996). Estimation of total intake was similar to French (about 45 μg/day) and English populations (between 38 and 57 μg/day) (AFSSA., Agence Française de Sécurité Sanitaire des Aliments, 2004). In the exposed subjects, drinking water consumption explained increase in total selenium intake. Nevertheless, this intake remained lower than in an American population (between 74 and 126 μg/day) with the same method (AFSSA., Agence Française de Sécurité Sanitaire des Aliments, 2004). It is also lower than in a selenium-rich area of India where the population had selenium toxicity signs (on average 475 μg/day for men and 632 μg/day for women) (Hira et al., 2004). Duplicate portion sampling is the right method because it reflects actual selenium intake (Kroes et al., 2002; Lightowler and Davies 2002). The average selenium intake in the exposed subjects was lower than the recommended level of 1 μg/kgBW/day (Martin 2000) and lower than the toxicity threshold fixed by WHO (4 μg/kgBW/day) (NOAEL) (WHO 1996). These values were comparable with those determined in France (66 μg/day) (Noel et al., 2003) and in other European studies (Murphy et al., 2002; Robberecht et al., 1994). The correlation between the two methods of intake estimation was acceptable. The FFQ does provide a good measure of average selenium intake. However, it resulted in a lower standard deviation than the duplicate portions method, neglecting the extreme values. The duplicate portion sampling remains the reference method but requires time and effort. It requires also an accurate and reliable method of measurement, which is difficult to achieve in the case of selenium. In the SU.VI.MAX study (Hercberg et al., 2004), the use of models or visual images (photographs) of food eaten led to the publication of the first manual of photos for the estimation of quantities. The use of such a manual might have improved the accuracy of the estimation of weights of portions and hence of selenium intake, but would have made the study more cumbersome. In this study, we also chose to measure the selenium concentration in toenail clippings, which was significantly higher in exposed than non-exposed subjects. However, whatever group, the values were comparable to French (median [5th–95th percentile]: 620 μg/kg [440–910]) (Goulle et al., 2009) and European subjects (between 500 and 700 μg/kg) (Bergomi et al., 2002; Krogh et al., 2003; van den Brandt et al., 1993a). They were lower than in Canada, in USA (mean [5th–95th percentile]: 844 μg/kg [654–1110]) (Satia et al., 2006; Xun et al., 2011) and in the seleniferous area of South Dakota whose population however presented no signs of selenium toxicity (between 806 and 3824 μg/kg) (Longnecker et al., 1991). Thus, statistically significant differences were not clinical significant. Other biological markers such as blood selenium concentration, could have been used (Hambidge 2003; Laclaustra et al., 2009; Mayne 2003). But nails are biological tissues easy to collect and keep; they are widely used in B. Emmanuelle et al. / Environment International 40 (2012) 155–161 159 Table 2 Distribution of selenium intake as estimated from food frequency questionnaire. Non-exposed (n = 39) −1 −1 Food (μg/kgBW /day ) Beverage (μg/kgBW−1/day−1) Exposed (n = 39) Mean ± SD Adjusted mean⁎ ± SD Mean ± SD Adjusted mean⁎ ± SD Difference mean 95% CI p 0.74 ± 0.25 0.028 ± 0.011 − 0.1551 ± 0.0164 − 1.5825 ± 0.0380 0.70 ± 0.18 0.24 ± 0.13 − 0.1696 ± 0.0164 − 0.7170 ± 0.0380 − 0.0145 0.8655 − 0.0615; 0.0324 0.7565; 0.9744 0.53 b 0.0001 ⁎ Adjusted on season .6 .4 .2 0 selenium intake from beverage (µg/day/kg body weight) .8 epidemiological studies (Cardoso et al., 2010; Longnecker et al., 1991; Steevens et al., 2010; Swanson et al., 1990) and correlation with intake has been demonstrated in volunteers taking selenium supplements (Longnecker et al., 1993). Nevertheless, the usefulness of a biological marker depends upon the existence of an accurate and reliable method for its chemical analysis. It also requires an understanding of the confounding factors that need to be taken into account in the analysis of the results. Hunter and others observed that selenium concentration in toenails decreased with age (Hunter et al., 1990). This could be responsible of an underestimation of selenium concentration in exposed group which is older. But, the age difference between the two groups was less than five years. With the exception of Hunter's study, most of the studies show there is no link between age and toenail selenium concentration (Krogh et al., 2003; Morris et al., 2006; Satia et al., 2006; Swanson et al., 1990; van den Brandt et al., 1993a). We found that smoking led to a diminution of selenium concentration in toenail clippings, this finding being consistent with other studies (Hunter et al., 1990; Krogh et al., 2003; Morris et al., 2006; Swanson et al., 1990; van den Brandt et al., 1993b; Xun et al., 2011). Some authors have demonstrated a relationship between the diminution of selenium concentration and the number of cigarettes smoked (Hunter et al., 1990). Several hypotheses have been put forward to explain this finding: lower levels of intake in the diet, higher levels of antioxidant metabolism provoked by an increased production of free radicals, an effect of tobacco consumption on the incorporation of selenium into the nail matrix or increased excretion of selenium caused principally by its forming a complex with cadmium (Kayan et al., 2009; Krogh et al., 2003; van den Brandt et al., 1993a). However, whatever method for estimating selenium intake is used, it clearly cannot take into account the fraction assimilated. In fact, only a part of ingested selenium is assimilated by the organism. This amount can vary widely as a function of several factors including selenium speciation, nature of the food and meal composition (Cabanero et al., 2004; Dumont et al., 2006; Fairweather-Tait et al., 2010b). It is considered that only 50% of total selenium intake spring summer exposed autumn winter unexposed Fig. 2. Distribution of selenium intake from beverages as estimated from the food frequency questionnaire, stratified by season and exposure. is assimilated (Mahapatra et al., 2001). This fact explains why epidemiological studies rely increasingly on biologic markers of selenium status (van den Brandt et al., 1993a). This type of data can take into account multiple origin exposure, reflect previous exposure and seems altogether to be a better measure than the others (questionnaire or environmental samples) (Kroes et al., 2002). It is worth noting that this study did not take into account the selenium speciation. We know that selenium is mostly present in organic form in food and inorganic form in drinking water but the toxicity assigned to each form is not clear (Rayman et al., 2008). Vinceti et al. (2010a, 2010b) hypothesize that intake of selenium through drinking water (b8 μg/L) might be associated with a risk factor for amyotrophic lateral sclerosis (ALS). They suggest the need to further investigate this issue. 5. Conclusions This integrated exposure assessment survey showed that high selenium concentrated drinking water increased selenium intakes and toenail selenium concentrations. We also showed that selenium toenail concentration could be used as a biomarker to reveal negligible difference of selenium level. Whatever method used for estimating selenium intake, the selenium level in this studied area Table 3 Toenail selenium concentration (μg/kg) according to the characteristics of the 78 participants. Toenail selenium concentration Mean ± SD (n) Non-exposed Area of residence 532 ± 100 (36) Educational level Elementary 495 ± 117 (5) College 550 ± 130 (12) Secondary 526 ± 89 (8) University or equivalent 564 ± 46 (6) Household size 1 to 2 people 539 ± 107 (19) ≥3 people 523 ± 95 (17) Tobacco Non-smoker 558 ± 96 (27) Smoker 453 ± 66 (9) Alcohol consumption At least twice a week 546 ± 115 (15) Maximum once a week 631 ± 134 (18) Physical activity No activity 511 ± 69 (6) At least one activity 596 ± 91 (9) BMI (kg/m−2) b20 594 ± 94 (2) [20–24] 525 ± 82 (19) [25–29] 518 ± 128 (13) N29 627 ± 60 (4) Consumption of locally produced food⁎ b2 532 ± 115 (9) [2–3] 535 ± 91 (9) [4–5] 511 ± 78 (6) N5 539 ± 115 (12) Exposed p 613 ± 117 (36) 0.0030 625 ± 140 (12) 623 ± 95 (10) 568 ± 78 (9) 644 ± 166 (5) 0.0055 599 ± 120 (20) 631 ± 115 (16) 0.0036 626 ± 114 (31) 560 ± 146 (3) 0.0030 521 ± 90 (21) 596 ± 99 (18) 0.0039 536 ± 106 (30) 619 ± 126 (27) 0.0048 524 (1) 609 ± 131 (19) 619 ± 105 (15) 695 (1) 0.0035 567 ± 135 (4) 622 ± 126 (5) 577 ± 68 (9) 647 ± 130 (17) 0.0044 ⁎ Corresponding to the sum of the frequency of consumption recorded for each season (i.e. 0 = maximum once a week; 1 = maximum 3 times a week; 2 = maximum every day). 160 B. Emmanuelle et al. / Environment International 40 (2012) 155–161 Table 4 Determinants of transformed toenail selenium concentration (μg/kg). Adjusted geometric mean ± SD Area of residence Exposed Non exposed Tobacco Smoker Non smoker β 95% CI p 70 27; 110 0.0030 −111 − 201; −33 0.0054 2.753 ± 0.017 2.696 ± 0.014 2.679 ± 0.027 2.770 ± 0.010 * adjusted on educational level, household size, alcohol consumption, physical activity, Body mass index, consumption of locally produced food, Duke score and Total selenium intake estimated from food frequency questionnaire. with high selenium concentrated drinking water is much lower than in seleniferous areas. Therefore, these results could contribute to discuss the maximum recommended limit (Barron et al., 2009). Acknowledgements We thank Pascale Pierre-Eugène for technical assistance, the Direction Régionale des Affaires Sanitaires et Sociales de PoitouCharentes (J.C. 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