Dose-Response Effects of Exercise and Testosterone Replacement Therapy on Body Composition, Lean Mass, and Heart Rate Responses: A Case Report Using Wearable Technology

The case report explores the effects of testosterone replacement therapy (TRT) on body composition, lean muscle mass, and fat mass, based on the dosage of TRT and exercise intensity in a 40-year-old male. The purpose of this case study was to evaluate the dose-response relationship of TRT and exercise on muscle hypertrophy and fat loss over an eight-month period, using a validated wrist-worn wearable fitness tracker to measure daily physical activity and heart rate (HR)-based exercise intensity. The patient, a trained male with 25 years of consistent exercise experience, reported notable declines in strength and increases in body fat despite maintaining a regular workout routine. TRT was prescribed by a physician and self-administered by the patient three times per week, starting at 150 mg per week for the first three weeks and then increasing to 180 mg per week at week four. Daily step counts and strength and aerobic exercise data were tracked utilizing a wrist-worn device (Polar Ignite 2) and an HR chest strap (Polar H10) to monitor time accumulated in five different HR zones (HR Zone 1-5). The study was divided into three phases: pre-TRT (two months of no TRT, just exercise), Phase 1 TRT (three months of TRT and exercise), and Phase 2 TRT (three months of TRT and exercise). Lastly, body composition and basal metabolic rate were assessed via bioelectrical impedance analysis at baseline and throughout the TRT period. Results indicated a substantial increase in lean muscle mass and a reduction in body fat during the TRT phases. Lean muscle mass increased by 6% during Phase 1 TRT and continued to rise by 3.8% in Phase 2 TRT, while body fat percentage decreased by 1.7% and 1.3% in TRT Phase 1 and TRT Phase 2, respectively. The participant's basal metabolic rate also improved, with a 4.5% increase during Phase 1 TRT and a further 3.2% rise in Phase 2 TRT. The time spent in HR Zones 1-5 significantly (P ≤ 0.007) shifted throughout the study. While time in moderate-to-high-intensity zones (HR Zones 3-5) increased in Phase 1, a trend toward more time accumulated in lower-intensity exercise (HR Zones 1-2) emerged in Phase 2, suggesting a potential adaptation in cardiovascular efficiency. Despite these shifts, overall exercise duration and average and maximal HR responses remained stable across the phases, indicating consistent cardiovascular demand. Combining TRT with regular aerobic and strength exercise greater than 60 minutes, at least four to five times per week, enhances lean muscle mass and reduces body fat, while the exercise intensity varies from phase to phase. The increase in lean mass was dose-dependent, with larger gains observed early in the TRT supplementation period compared to no TRT. Additionally, the use of wearable technology provided valuable insights into the participant's HR responses to training. These findings highlight the exercise approaches and HR responses potentially required for significant body recomposition and improved metabolic health with TRT. Further research with larger samples is needed to confirm these results and explore long-term health outcomes.