Testosterone in aging male twins: relationship with physical functioning, quality of life, and depression
McKenzie, Ruth Ellen
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Research has investigated testosterone and its role in biological and psychological functioning. Testosterone levels decrease as men age, and aging has been associated with declines in muscle mass and strength. Decreased functional mobility can impact quality of life. Aging has also been associated with increased vulnerability to depressive symptomatology. The purpose of this study was to investigate interrelationships among testosterone, physical functioning, quality of life, and depression in the Vietnam Era Twin Study of Aging (VETSA). The mean age of the 1,237 men in VETSA was 55.4 (+2.5). Testosterone data collection began in the third year of VETSA, yielding an available sample of 778. It was hypothesized that there would be significant associations between testosterone and physical functioning, depression, and quality of life as well as between physical functioning and depression and quality of life. Contrary to expectations, when mixed models for linear regression were used, testosterone was shown to be related only to physical functioning. As predicted, however, physical functioning was significantly related to depression and quality of life. Cholesky decompositions were conducted to address the hypothesis that there were shared genetic determinants of each phenotype. Best fitting bivariate models included additive genetic and unique environmental but not common environmental influences. Significant genetic correlations were found between physical functioning and depression, and physical functioning and the mental health component score of the Short Form Health Survey (SF-36). Contrary to expectations, while testosterone and physical functioning were significantly correlated with each other phenotypically, there was no genetic correlation between the two. Trivariate models revealed genetic influences specific to depression as well as genetic influences shared with quality of life and depression. Finally, path analysis demonstrated that testosterone had a direct impact on physical functioning. Physical functioning, but not testosterone, directly impacted depression and quality of life. As there was no genetic correlation between testosterone and physical functioning, but there was a phenotypic correlation, it may be that other factors, such as cortisol, influenced the association. In sum, in this sample, physical functioning seemed to be more important than testosterone to both depressive symptomatology and quality of life.