1 Department of Anthropology, Harvard University, Cambridge, MA, 2 Department of Anthropology, Yale University, New Haven, CT, USA, 3 Department of Anthropology, University College London, London, UK, 4 Department of Anthropology, Boston University, Boston, MA, USA, 5 Department of Anthropology, Durham University, Durham, UK and 6 Department of Anthropology, University of New Mexico, Albuquerque, NM, USA
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Abstract |
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Key words: ageing/population variation/salivary testosterone/testosterone
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Introduction |
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Despite evidence of its clinical importance, relatively little is known about population variation in age patterns of testosterone. This contrasts strikingly with the increasing awareness of population variation in female ovarian steroid profiles and its relationship to disease risk (Ellison et al., 1993; O'Rourke and Ellison, 1993
; Ellison, 1999
; Jasienska et al., 2000
; Jasienska and Thune, 2001
). There are scattered reports of male testosterone levels from non-Western, non-clinical populations, but the comparability of these data is limited by differences in sample collection, handling, and assay procedures (Guerra-Garcia et al., 1969
; Smith et al., 1975
; Christiansen, 1991a
,b
; Beall et al., 1992
; Campbell, 1994
).
This report presents data on age variation in male salivary testosterone values from four populations spanning broad genetic, ecological, and life style differences. All the samples were collected using the same protocols and were assayed in the same laboratory using consistent methods. The data are used for a preliminary test of the hypothesis that significant population variation exists in the pattern of age-related decline in male testosterone. By analogy with findings for women (Ellison et al., 1993), we expect that non-Western males will have lower levels of testosterone in adulthood and slower rates of decline in testosterone with increasing age.
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Materials and methods |
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In all cases participants provided between one and five morning saliva samples collected within 2 h of waking from 15 days according to established collection protocols which minimize the risk of contamination with serum or gingival fluid (Lipson and Ellison, 1989). The samples were preserved with sodium azide until they could be transferred to the laboratory and frozen at 20°C. Testosterone levels have been found to be stable under these collection and handling procedures for up to 6 months (Lipson and Ellison, 1989
). Testosterone values were determined for each sample by a tritium-based radioimmunoassay according to published protocols (Ellison et al., 1989
). The sample values for individual participants were averaged to provide mean values for each subject.
The age pattern of testosterone within each population was analysed using simple linear regression. Sample sizes were too small to justify higher order curve fitting. Population variation in average testosterone values is assessed by analysis of variance and by multiple linear regression with age, population, and age multiplied by population interaction as independent variables. For the purposes of the multiple regression population was re-coded as a binary variable, USA and non-USA.
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Results |
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Discussion |
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The testosterone levels of older men (>45 years) tend to converge for all the populations studied while the levels of younger men (<30 years) vary significantly. This suggests that variability in the pattern of testosterone decline with age, results from population variation in the reproductive physiology of young males rather than in population differences in the rate of reproductive senescence among older males. If so, research into the mechanisms underlying age patterns of male testosterone should focus on the determinants of young adult levels as well as the causes of age-related decline.
Because population differences in age patterns of testosterone have not previously been appreciated, the implications of such variation for patterns of health risk have not been considered. If, for example, set-points for muscle anabolism and bone mineral density are established relative to testosterone levels in young adulthood, a steeper decline in testosterone from higher young-adult levels might result in more rapid age-related changes in male body composition, bone mineral density, and related health risks. Higher testosterone exposure throughout adulthood in populations with high young-adult levels may also lead to greater prostate cancer risk. These possibilities provide additional motivation for increased efforts to study global variation in male gonadal function to complement studies of Western populations.
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Notes |
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References |
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Submitted on March 7, 2002; resubmitted on June 11, 2002; accepted on July 31, 2002.