Whitfield et al. dentists, Martin and Naleway (2004) observed lower excretion and, by inference, either lower absorption or greater retention among the drinkers. For Se, there are conflicting reports. Snook (1991) found that both plasma and whole-blood Se were significantly higher in light drinkers than in nondrinkers, whereas Ringstad et al. (1993) reported lower plasma Se in a group of men averaging 32 g alcohol/day than in abstainers. Lloyd et al. (1983) explic- itly reported erythrocyte Se, which showed no detectable effect of drinking in apparently healthy volunteers. Results of the present study, covering a range of alcohol use from none to more than four drinks per day, suggest that alcohol is associated with slight but significant increases in Se and Hg and stronger increases in Pb and As; however, alcohol consumption appears to be associated with Cd only because of the association between alcohol use and smoking. We also found some associations between measures of socioeconomic status and element concentrations. Increased time (years) of edu- cation was associated with decreasing As, Hg, and Pb concentrations. This association might be due to less occupational or residential expo- sure, but we observed no detectable effect of self-reported social class. It is likely that meas- ured As and Hg concentrations are affected by seafood consumption (Balshaw et al. 2007; Borak and Hosgood 2007) and that much of the As or Hg is in the form of organic com- pounds, but we did not differentiate between organic and inorganic forms, nor did we have information on the diets of our subjects. Another characteristic that might affect element concentrations is the use of mineral supplements. This could occur directly, for example, from Se or Zn in the supplements or indirectly from interactions between essential and toxic elements. Information from partici- pants in another study conducted at about the same time, but restricted to people > 50 years of age, suggests that about 10% of these older participants were taking mineral supplements (Whitfield JB, Martin NG, unpublished data). However, we found no significant effect of mineral supplement use for any of the ele- ments measured (data not shown). There were insufficient data on mineral supplement use to determine whether MZ twin pairs were more likely to be concordant than DZ twin pairs. Genetic influences. Twin-pair correlations for MZ and DZ twin pairs, after adjustment of the data for analytical variation, age, sex, smoking, alcohol use, and socioeconomic variables are shown in Table 2. For all the elements except Cd, the 95% CIs for MZ and DZ correlations do not overlap. This suggests the existence of genetic variation as a con- tributor to individual differences. We there- fore proceeded to estimate the contributions of additive genetic effects (A), environmental 780 effects shared within families (C), and non- shared environmental effects (E) on variation. There was evidence of significant genetic effects on all of these elements except Cd (Table 2); even for Cd the 95% CI for addi- tive genetic effects was 0–30%. These results are compatible with either genetic influences on behaviors that affect exposure [potentially including genetic effects on choice of diet (Hasselbalch et al. 2008), although there is no evidence for such effects on mineral content of the diet] or with genetic effects contribut- ing to variation in uptake, retention, or dis- tribution of these elements. Effects of shared environment (C) could not be demonstrated. Models in which shared environmental effects were excluded did not show a significant dete- rioration in fit to the data for any element, although effects in the range of 10–25% could not be excluded. Nonshared environmental effects (E) accounted for 60–80% of varia- tion, including the effects of measurement error, day-to-day variation, and exposure to different environments and diets not shared by co-twins. However, and most important, the 95% CI for nonshared environmental effects excluded 100% for all elements measured; therefore, a purely environmental explanation for variation is not consistent with our data. We next addressed the question of whether each element is subject to its own unique effects, or whether some genetic or environ- mental factors have common effects across several elements. This was examined in two stages: first by estimating the overall (pheno- typic) correlation matrices for men and for women, and second, by estimating the genetic and environmental components of variation and covariation. We observed highly signifi- cant phenotypic correlations between con- centrations of pairs or groups of elements, as shown in Table 1. In particular, results for Cu, Se, and Zn were strongly correlated, as were those for As and Hg. The correlations were almost identical in men and women. Such correlations could potentially be due to common effects on the analytical process, exposure to multiple elements present in con- taminated environments, or to variation in metabolic processes affecting more than one element. Insight into these possible causes can be gained using the twin study method to dis- tinguish genetic from environmental causes of variation. Results of a multivariate analysis in which unique and common genetic and environ- mental effects were separately estimated are presented in Supplemental Material, Table 3 (doi:10.1289/ehp.0901541). Most genetic variation was unique to each element, shown by the largest effects being on the diagonal of the additive genetic matrix, except for the As–Hg element pair, which shows a substan- tial common (off-diagonal) effect. We found no evidence for shared environmental effects impacting more than one element. Nonshared environmental effects, which reflect both the aspects of the environment unique to each person (such as differing exposure) and ana- lytical error, are element-specific, apart from some common effects between Cu, Zn, and Se. These common effects could be due to differences between people in their residential location or choice of diet, or to some people taking vitamin/mineral supplements contain- ing all three elements, but we are unable to test for or distinguish between these possibilities. The main point of interest in the multi- variate analysis is the common genetic effect on As and Hg. These elements are not close chem- ically, but biologically each is found in organic compounds that may be absorbed as a result of eating fish or shellfish. However, we have no relevant data on diet for our subjects, still less on genetic influences on diet, and we cannot yet identify the reason for the genetic correla- tion between As and Hg concentrations. This may have to wait for identification of genetic loci or polymorphisms that affect both of them. More generally, there will be interactions between concentrations and toxic effects of the essential and toxic elements (Peraza et al. 1998) and between the elements reported here and iron status, but again, these will be easier to clarify once the main causes of variation in each single element are understood. Localization of genes by linkage analy‑ sis. The strongest linkage result for any of the seven elements was for Pb on chromosome 3, as previously reported (Whitfield et al. 2007). Availability of microsatellite marker data for extra pairs of DZ twins allowed a reanalysis, and the evidence for linkage increased from the previous LOD score of 2.63 to a highly significant 4.21 (Figure 1). The gene discussed in our previous article (Whitfield et al. 2007), SLC4A7 (solute carrier family 4, sodium bicarbonate cotransporter, member 7), which codes for a transporter affecting Pb influx into erythrocytes, remains a strong candidate for harboring variation affecting erythrocyte Pb. Two chromosomes showed linkage sig- nals, although not at genome-wide significant levels, affecting more than one element [see Figure 2 and Supplemental Material, Table 4 (doi:10.1289/ehp.0901541)]. Chromosome 2 contained a region around 220 cM with a sug- gestive peak (LOD = 2.73) for Zn and minor peaks (each LOD ≈ 1.25) for Se and Cd. Chromosome 4 contained a region around 130 cM with a suggestive peak (LOD = 2.46) for Se and minor peaks for Cu (LOD = 1.28) and Cd (LOD = 0.56). Other regions contain- ing peaks with LOD ≥ 1.6, the usual sugges- tive threshold for likely occurrence once in each genome-wide linkage analysis, were at 120 and 245 cM on chromosome 2 for Cd, 175 cM on chromosome 5 for Hg, 25 cM on ovlume 118 | number 6 | June 2010 • Environmental Health Perspectives