The association of cardiorespiratory fitness, body mass index, and age with testosterone levels at screening of healthy men undergoing preventive medical examinations: The Cooper Center Longitudinal Study.

Background: Currently, exogenous hormone replacement is used in many men with hypogonadism without clear organic cause. This study examines the contribution of modifiable health behaviors, i.e., physical activity and weight control, to the maintenance of testosterone levels with aging.
Methods: In a cross-sectional study of 2994 healthy men aged 50-79 years examined at a preventive medicine clinic from January 2012 to March 2016, screening morning total testosterone levels were measured and categorized as low (<250 ng/dL), low normal (250-399 ng/dL), and normal (>400 ng/dL). Cardiorespiratory fitness (fitness) was estimated from a maximal exercise treadmill test. Multiple logistic regression models were used to test the associations between low testosterone levels and age, body mass index (BMI), and fitness.
Findings: Mean testosterone levels were in the normal range for each age group (50-59, 60-69, and 70-79). There was a similar prevalence of low testosterone in each age group (11·3%, 10%, and 10·5%, respectively). The prevalence of low testosterone was positively associated with BMI and negatively associated with fitness but was not associated with age.
Interpretation: This study found no evidence that low testosterone is an inevitable consequence of aging. Maintenance of healthy weight and fitness may help maintain normal testosterone levels.
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