The objective of this study is to determine the effects of a single dose of testosterone undecanoate during and in recovery from simulated operational stress.

This is a double-blind, randomized, placebo controlled trial in 32 physically active men exposed to 20 complete days (days 8-27) of simulated operational stress followed by 20 complete days of recovery (days 29-48). After completing baseline testing (Phase 1), participants will be randomized to receive either a single intramuscular injection of testosterone undecanoate or an isovolumetric placebo (Day 8). The 20-day simulated operational stress (Phase 2) will be highly controlled (live-in study) and consist of 4 successive cycles of undulating stress, starting with 2 consecutive days of low stress followed by 3 consecutive days of high stress. Low- and high-stress days will result from low and high militarily-relevant exercise-induced energy expenditures, adequate and restricted sleep (8 hours vs. 4 hours daily), and diet restriction to produce energy deficits. After completing Phase 2, participants will be released to resume their habitual physical activity routines and will be provided a controlled diet to consume (Phase 3), to assess recovery from sustained, severe operational stress.

Inclusion Criteria:

• Men aged 18-35 years
• Ability to understand verbal or written instructions/testing materials in English
• Physically active (as determined by accelerometry and review of a physical activity log)
• Not taking any prescription medications and/or willing to refrain from all medication use prior to and throughout the entire study period, unless provided/approved by the study physician
• Willing to refrain from alcohol, smoking, e-cigarettes or use of any nicotine product, caffeine, and dietary supplement use throughout the entire study period
• Willing to live on the Pennington Biomedical Research Center inpatient unit for 20 consecutive days
• Meets age-specific US Army body composition standards according to Army Regulation 600-9, which includes estimates of percent body fat based on height, weight, and circumference measures (neck and waist)
• Total testosterone concentration is within the normal physiological range (300-1,000 ng/dL)

Exclusion Criteria:

• Musculoskeletal injuries that compromise exercise capability
• Diagnosed cardiometabolic disorders (i.e., hypertension, hyperlipidemia, kidney disease, diabetes, etc.)
• Allergies or intolerance to foods, vegetarian practices, or history of complications with lidocaine
• Anabolic steroid, human growth hormone, or nutritional testosterone precursor-like supplement use within the past 6 months
• Will not refrain from smoking (any nicotine product), alcohol, caffeine, or any other dietary supplement during the study
• Adults unable to consent
• Women
• Prisoners
• Sedentary or engages in insufficient quantities of physical activity per week (aerobic and/or resistance training as determined by accelerometry and review of a physical activity log)
• Exceeds age-specific US Army body composition standards according to Army Regulation 600-9
• Previous history of kidney stones unless otherwise approved by the medical investigator
• Systolic blood pressure > 150 or diastolic blood pressure > 95 mmHg
• Previous history of breast or prostate cancer
• Previous history of Chronic Obstructive Pulmonary Disease or Obstructive Sleep Apnea
• Prostate-Specific Antigen (PSA) > 3ng/ml, Hematocrit > 50%, or positive urine drug screening
• Based on the investigative team's clinical judgment, a subject may not be appropriate for participation in the study

Adverse event data was collected from the time of the screening visit until study completion, up to 4 months.