The data review involved examining a number of our group’s data sets from which a total of 196 male endurance-trained distance runners and their corresponding, respective matched controls were identified. We delimited ourselves to examining distance runners (≥ 10 km competitive events or greater) who trained a minimum of 7 h a week (or more) and who had been running for at least 1 year or more. The design approach in these studies was observational cross-sectional (case-control) as each endurance runner was matched with a healthy, non-exercising control subject. Matching factors were as follows: age (± 1 year), BMI (± 1 kg/m2), and ethnicity. All subjects were required to have a stable body weight (no weight changes > 3% during the last 4 months) and were medically screened to determine state of health; all distance runners were asked to pursue a period of normal, steady-state training (no ramping-up to increase training load and/or tapering to reduce training loads) during the last 6 weeks and no major injury which curtailed training during the last 12 months. All subjects provided a morning (0700 to 0900 h) resting blood sample after a minimum of 8 h fasting and having no sexual activity (> 24 h before) and, for the runners, a minimum of 18 h discontinuation of any form of exercise training. Additionally, exercise training history, fatigue state, and running competitive performances were obtained through questionnaires. Blood samples were analyzed for total testosterone using standard clinical bioanalytical techniques (radioimmunoassay or enzyme-linked immunoassays) with appropriate quality control procedures for the relative time period of the study. Hormonal assays were batched and conducted so that the same analytical procedures were used for athletes and their matched controls.
An individual runner’s testosterone values were expressed as a representation of their respective matched control subject. From this latter comparison, the runner’s testosterone levels are reported as a percent change (being greater than or less than that of their control reference subject). Hormonal responses for athletes were stratified and categorized by years of training: 1 year, 2 (± 1) years, 5 (± 2) years, 10 (± 2) years, and > 15 (± 2) years. Responses for each of these year categories were statistically analyzed via an ANOVA and the Fisher LSD post hoc procedures.
Figure 1 illustrates that the longer an endurance runner is engaged in consistent and chronic endurance training, the lower their resting testosterone becomes (p < 0.01). The results suggest that the level of reductions observed plateaus at approximately − 30% (after 5 years training), which was significantly lower than the reductions observed for individuals training < 5 years (p < 0.001), while no differences existed between 5, 10, and 15 years (p > 0.05).