Distribution of toenail selenium levels in young adult Caucasians and African Americans in the United States: The CARDIA Trace Element Study

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Abstract

Background

Data on selenium (Se) levels in American young adults, especially in African Americans, are lacking.

Objective

This study presented toenail Se distributions in American young adults of both genders, including both Caucasians and African Americans; and explored potential predictors of toenail Se levels.

Data and methods

Data from the Coronary Artery Risk Development in Young Adults study among 4252 American young adults, aged 20–32 in 1987 was used to examine toenail Se levels by instrumental neutron-activation analysis. The distribution of Se levels was described and multivariable linear regression was used to examine potential modifiers of toenail Se concentration within ethnicity–gender subgroups.

Results

The geometric mean of toenail Se in this cohort was 0.844 μg/g (95% CI, 0.840–0.849 μg/g) and the median was 0.837 μg/g (95% CI, 0.833–0.844 μg/g). Median levels from lowest to highest quintile were 0.691, 0.774, 0.838, 0.913 and 1.037 μg/g. Se levels varied geographically, and were generally in accordance with its concentrations in local soil. Males, African Americans, current smokers, heavy drinkers and less educated participants were more likely to have low Se levels.

Conclusion

This study suggests that toenail Se levels vary geographically depending on soil Se concentrations. In addition to gender, ethnicity and education level, smoking status and alcohol consumption are two important indicators of Se status since they are modifiable lifestyle factors. Findings from this study might aid public health professionals in identifying people at relatively high or low Se levels, so that chronic disease prevention efforts can be directed toward these subgroups.

Research highlights

► Average of toenail Se levels in this cohort was 0.844 μg/g (95% CI, 0.840–0.849 μg/g). ► Toenail Se levels vary geographically depending on soil Se concentrations. ► Males, African Americans and less educated participants have low Se levels. ► Smoking status and alcohol consumption are two important indicators of Se status.

Introduction

Selenium (Se) presents a nutritional challenge because of its dual features as an essential trace element and a potential toxin. Severe deficiency of Se can lead to Keshan disease (a congestive cardiomyopathy) (Keshan Disease Research Group, 1979) and also contributes to Kashin–Beck disease (an endemic osteoarthropathy) (Diplock, 1987). Both were reported in areas of China and other countries until they were virtually eradicated with Se supplementation. The apparent benefits of Se may not be limited to alleviating overt deficiency. Low serum, plasma, erythrocyte and toenail Se statuses have been associated with increased risk of cardiovascular disease (CVD) and cancer in some epidemiological studies (Burguera et al., 1990, Flores-Mateo et al., 2006, Knekt et al., 1990, Knekt et al., 1998, Russo et al., 1997, van den Brandt et al., 2003, Willett et al., 1983, Yoshizawa et al., 1998, Young and Lee, 1999), illustrating Se's important role in human nutrition. Conversely, Se toxicity can occur in areas with elevated soil concentrations or from high-dose supplementation (Helzlsouer et al., 1985; Longnecker et al., 1991; MacFarquhar et al., 2010; Yang et al., 1989a, Yang et al., 1989b, Yang et al., 1983), though these scenarios are uncommon in the United States (US). Because deficient and excessive levels of Se can both be harmful to human health, the need for further study to assess individual Se intake and tissue levels is clear.

The primary dietary sources of Se are meat, poultry, fish, grains and cereals. Se levels in food are mainly determined by soil levels, which vary widely throughout the US (Shacklette and Boerngen, 1984). Accurate and adequate assessment of Se intake based on individual food consumption is exceptionally difficult because of the minute amounts and wide variations in the same foods grown in different areas (Willett, 1998). Also, estimating Se intake directly in meals is not feasible in large studies. Thus, assay of biological specimens, which reflects consumption, is preferred both as a measure of intake and as a means to validate other forms of exposure assessment (Willett, 1998). One drawback of using blood (including whole blood, serum and plasma) or urine samples to identify Se status in the human body is that they respond to and reflect only recent changes. Even Se levels in erythrocytes may not reflect long-term exposure to Se, as erythrocytes can only circulate for about 120 days in the body (Joliet, 1953). Se measured in human toenails is more reliable than other body measures because: (1) toenails can reflect a time-integrated measure of exposure and have successfully predicted Se intake (R2=0.52) (Longnecker et al., 1996); (2) they are less prone to superficial contamination than hair because of their lower surface: volume ratio; (3) they are environmentally sheltered in populations that wear shoes (Hunter et al., 1990a); (4) they are easily collected, transported, stored and cleaned (Morris et al., 1983). Toenail Se concentrations are highly correlated with Se levels in other critical organs; therefore they are very useful in large-scale epidemiological studies (Morris et al., 2004). Thus, toenail Se measurements are being increasingly used in human studies.

Previous studies have reported data on Se levels in the US and worldwide (Hunter et al., 1990a, Kotsopoulos et al., 2010, Yoshizawa et al., 2003). However, these studies were conducted among middle-aged or older populations or only in one gender group. In particular, data on African Americans are limited. Therefore, we aim to present toenail Se distributions in American young adults of both genders, including both Caucasians and African Americans; and to explore potential predictors of toenail Se levels using data from the Coronary Artery Risk Development in Young Adults (CARDIA) Trace Element Study.

Section snippets

Study population

The CARDIA cohort was established in 1985 when 5115 American young adults, 18–30 years of age, participated in a study of the psychological and other lifestyle factors that might affect the evolution of coronary artery disease risk. Details of the study design have been published elsewhere (Friedman et al., 1988). Briefly, the cohort was enrolled from four US cities including Birmingham, Alabama; Chicago, Illinois; Minneapolis, Minnesota; and Oakland, California and was roughly balanced by age

Results

Geometric means and selected percentiles with 95% CIs for toenail Se levels, overall and from subgroups defined by selected variables, are shown in Table 1. The overall geometric mean concentration of toenail Se was 0.844 μg/g (95% CI, 0.840–0.849 μg/g). Median levels from lowest to highest quintile were 0.691, 0.774, 0.838, 0.913 and 1.037 μg/g. Average toenail Se levels from Birmingham were the lowest, while those from Minneapolis were the highest. Women, Caucasians, non-current smokers and

Discussion

These results extend recently released estimates of toenail Se levels in American young adults including African Americans and Caucasians using data from the CARDIA Trace Element Study (Xun et al., 2010b). In this study, toenail Se levels were associated with gender, ethnicity, study center, smoking status and alcohol consumption. Men, African Americans, those in Birmingham, current smokers and heavy drinkers were more likely to have low Se levels. In African American women, those who were

Acknowledgments

This study was supported by Grants R01HL081572 and P30ES10126, and Contracts N01-HC-48047, N01-HC-48048, N01-HC-48049, N01-HC-48050 and N01-HC-95095 from the National Institutes of Health. The authors thank Dr. Jared Reis for his valuable comments and Wang Xin for verifying the STATA programming. The authors also thank the other investigators and staff of the Coronary Artery Risk Development in Young Adults (CARDIA) Study for their valuable contributions.

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