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Efficacy of Testosterone plus NASA Exercise Countermeasures during Head-Down Bed Rest


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Medicine & Science in Sports & Exercise: September 2018 - Volume 50 - Issue 9 - p 1929-1939
doi: 10.1249/MSS.0000000000001616


Exposure to microgravity during long-duration space flight results in a number of physiological impairments including loss of muscle mass and strength (1). Despite consistent advances over several decades of human space flight (2), exercise countermeasures alone are still not fully effective at preventing or reversing the functional decrements in skeletal muscle that occur in response to space flight (3,4). With the length of space missions increasing, additional approaches are necessary to bolster the positive influence of exercise countermeasures and fully combat the deleterious physical, functional, and physiological alterations that occur with long-duration space flight.

Losses of skeletal muscle mass and strength are widely recognized adverse consequences of human space flight. Preservation of preflight muscle mass and strength is a primary requisite for successful countermeasures (5). Skeletal muscle serves highly diverse roles, being both necessary for mobility and stability and is critical in numerous processes including hemodynamics, energy balance, thermoregulation, lipid oxidation, and amino acid handling (6,7).

The operational goal of NASA is to prevent muscle atrophy during space flight rather than treatment after the losses have occurred (8,9). Current space flight strategies rely heavily on inflight exercise to keep losses of muscle mass at a minimum (10), followed by physical rehabilitation upon landing aimed at reversing the consequential losses in function that have occurred. The protection offered by current countermeasures during approximately 6 months of space flight narrowly keep the losses in lean body mass (LBM) at bay, ranging from decreases of ~0.42 kg with the use of the interim resistive exercise device to gains of ~0.77 kg following the transition to the advanced resistive exercise device. Despite these small margins and improved inflight exercise regimens (11), astronauts have continued to lose lower body strength (~14.2% loss in calf strength with interim resistive exercise device and ~11.6% loss with advanced resistive exercise device). Thus, there is a clear need for the development of simple, yet reliable preventative strategies that work synergistically to augment the effects of exercise or that can be used as a standalone countermeasure in the event inflight exercise is not an option (i.e., exercise hardware failure, injury).

The endogenous male sex hormone testosterone is protective against muscle atrophy, especially among populations where regular exercise may be difficult to implement as a first line approach (i.e., disease, disability, injury) (12–14). The clinical use of testosterone in men has steadily increased over the recent years and low-dose testosterone replacement therapy has been shown to be very safe (15). Testosterone is both anabolic and anticatabolic to skeletal muscle protein (16,17), making it an attractive countermeasure option. Our previous research has demonstrated benefits of androgens on muscle anabolism in several populations with diverse comorbidities and levels of habitual activity including healthy older men (18), older women (19), severely burned male patients (20), and cancer patients (12). We recently showed that intermittent (cycled) periods of testosterone administration have similar anabolic benefits to uninterrupted administration (21).

During the integrated NASA 70-d head-down bed rest (HDBR) campaign (see Cromwell et al. [22] in this issue for detailed overview), we sought to determine whether intermittent low-dose testosterone would act synergistically with the mechanical stimuli provided by exercise to promote both muscle mass and strength during 70 d of HDBR in men representative of the astronaut population. Testosterone was administered intermittently (2 wk of testosterone or placebo followed by 2 wk no treatment), offering a low-risk pharmacologic approach to prevent losses in skeletal muscle mass and strength that may occur during long-duration space flight. We hypothesized that low-dose testosterone would safely and effectively augment the effects of NASA exercise countermeasures on skeletal muscle mass and strength during HDBR.



Subjects were recruited and medically screened through the National Aeronautics and Space Administration (NASA) test subject screening facility at the Johnson Space Center in Houston, TX. NASA Human Research Project standard criteria were followed for screening and conduct of the study (22) with the addition of exclusion criteria specific to the testosterone countermeasures (see Table, Supplemental Digital Content 1, exclusion criteria, The study complied with the Declaration of Helsinki and was approved by The University of Texas Medical Branch (UTMB) and NASA institutional review boards. Written informed consent was obtained from all subjects. The study was conducted at the Flight Analogs Research Unit (FARU) at UTMB in Galveston, TX. Testing was conducted at UTMB and NASA/Johnson Space Center. The integrated NASA study was registered with (Identifier: NCT00891449).


Healthy male volunteers (24–55 yr) were recruited and randomly assigned to one of three HDBR groups: nonexercising control + placebo (CON), exercise + placebo (PEX), and exercise + testosterone (TEX). Placebo versus testosterone treatment assignments were blinded (CON) or double-blinded (PEX vs TEX). A testosterone-alone group (nonexercising group) was originally planned for inclusion in this NASA Campaign, but was beyond the cost and schedule allocated for this study. In addition, because all astronauts are required to perform inflight countermeasure exercise, the testosterone-alone group provided the least relevance to space flight and was thus eliminated in the planning phase of the 70-d NASA Campaign.

The study design assumed eight subjects per group would provide at least 80% power (α = 0.05) to detect sufficient changes in muscle mass and strength. To complete 24 subjects (8 per group), a total of 27 men were enrolled in this protocol. Three men were released from the study, and their data were not included in final analyses. Of these three men, one was dismissed after the first pre–bed rest vision examination showed elevated intraocular pressure in the right eye and myopic disc findings in the fundus photographs. One subject withdrew for nonmedical reasons after 16 d of pre–bed rest because of difficulties performing exercise on the equipment. A third subject was released for noncompliance after the sixth week of the bed rest phase of the study.

Bed Rest Protocol

A simplified schematic of the bed rest timeline is provided as supplemental figure (see Figure, Supplemental Digital Content 2, study timeline,

Pre–bed rest phase (BR − 21 to BR − 1)

Subjects were housed in the FARU bed rest facility at UTMB and remained ambulatory for a period of 2 wk for CON (BR − 14 to BR − 1) and 3 wk for PEX and TEX (BR − 21 to BR − 1, including exercise familiarization). All PEX and TEX subjects participated in a 3-wk ambulatory pretraining program where they began the training, attended familiarization sessions, and practiced the exercise prescription. NASA standard measures (i.e., physiological and psychological testing, diet stabilization, etc., required for all NASA bed rest and flight conditions [22]) were started during this phase, including diet stabilization and sleep/wake cycle. Pre–bed rest data were collected to establish baseline information.

Bed rest phase (BR0 to BR70)

Subjects were confined to 6° HDBR for 10 wk (BR1–BR70). Data collection continued during the bed rest phase.

Post–bed rest recovery phase (BR + 0 to BR + 14)

Subjects returned to ambulatory conditions and received rehabilitation while remaining at the FARU for 2 wk (BR + 0 to BR + 13). During this phase, post-HDBR recovery data were collected. NASA provided 24-h·d−1 subject monitors to ensure subject compliance and to record data to ensure accuracy. The FARU is also equipped with 24-h video monitoring of the subjects overseen by the unit nursing staff.


Dietary intake was based on the NASA space flight nutritional requirements and was monitored throughout the study (22). Meals were prepared at the FARU metabolic kitchen under guidance of a trained dietitian. Dietary intake was standardized using the Harris–Benedict equation for each individual, using activity levels of 1.3 for CON and 1.6 for PEX and TEX subjects. Macronutrient composition of the diet was targeted at 55% carbohydrate, 30% fat, and 15% protein. To prevent undernutrition of subjects during HDBR, pre–bed rest measurements of body weight were used in the Harris–Benedict equation when body mass declined. However, increases (but not decreases) in body mass during HDBR were carried into the calculations to allow for improvements in LBM, which was considered a primary outcome.

Exercise Countermeasure

All PEX and TEX subjects exercised 6 d·wk−1 following an investigational NASA exercise protocol (SPRINT) based on high-intensity aerobic training combined with resistive strength exercise (see Ploutz-Snyder et al. [23]). Three intensities of resistance (light, moderate, and heavy) and aerobic (short, medium and longer intervals) training sessions were performed. These intensities were rotated among various exercises and days so that each was performed once per week. All exercise sessions were carefully controlled and monitored for safety and intensity. Supine aerobic exercise was performed using the Standalone Zero Gravity Locomotion Simulator vertical treadmill and a supine cycle ergometer, and resistance exercise was performed on a horizontal squat device, a horizontal leg press (for leg press and calf raise exercise), and a prone leg curl machine. High-intensity interval aerobic exercise and continuous aerobic exercise were performed on alternating days. Resistance exercise was performed 3 d·wk−1 on the same day as the continuous aerobic exercise, separated by 4–6 h.

Testosterone Countermeasure

The efficacy of an intermittent (cycled) testosterone enanthate treatment approach was previously demonstrated and compared with traditional continuous weekly administration of 100 mg testosterone enanthate in healthy older men with endogenous testosterone in the low-normal physiological range (21). The treatment cycle was adjusted to 2 wk on/off to fit a 70-d bed rest protocol. Starting on BR − 1, placebo or testosterone enanthate injections (100 mg·wk−1, intramuscular) were administered in 2-wk intervals (weekly testosterone enanthate for 2 wk, followed by 2 wk off, etc.) for the duration of the 70-d bed rest period. Thus, injections occurred immediately before bed rest (BR − 1) and periodically during bed rest (BR7, BR28, BR35, BR56, and BR63).

Blood Measures

All blood samples were drawn in the morning under fasting conditions before personal hygiene and any scheduled testing or treatment. Serum and plasma were separated and stored at −80°C. Testosterone was measured weekly during bed rest by the UTMB Clinical Laboratory. Lipid profiles and prostate-specific antigen (PSA) were measured at BR14, BR42, and BR70 by the UTMB Clinical Laboratory. Additional measures including hematocrit were collected and analyzed by the NASA clinical and nutritional biochemistry laboratories. Other blood measures were analyzed in batches samples with a high-throughput continuous random-access immunoassay analyzer (Immulite 2000; Siemens, Deerfield, IL) after completion of the study.


Plasma samples were assayed using a Human Cytokine Milliplex Map kit (Millipore Corporation, Billerica, MA). Measurements were performed using a Bio-Plex® 200 SystemsReader (Bio-Rad Laboratories, Hercules, CA).

Body Composition


Total body and regional body composition was determined by dual-energy x-ray absorptiometry (iDXA; GE Lunar, Waukesha, WI) as previously described (12). All scans were analyzed by the same trained technician.

Muscle Function

Muscle strength

Isometric and isokinetic muscle strength and endurance tests were conducted twice during pre–bed rest and twice in post–bed rest. Muscle performance testing was conducted using a Biodex isokinetic dynamometer on selected muscle groups before HDBR on BR − 12 and BR − 5 and again after HDBR on BR + 2 and BR + 12 (see Table, Supplemental Digital Content 3, performance protocol, A standard protocol for warm-up before testing was followed for each muscle group. Testing was performed using the right limb.

Perceptual fatigue

Fatigue was assessed by asking the subject to rate their current level of fatigue (weariness, tiredness) on a scale from 0 (no fatigue) to 10 (as bad as you can imagine). This question was asked and recorded by the trainer immediately before each exercise session, at the halfway point during exercise, and immediately after completion of exercise.

Statistical Analyses

Data were gathered and organized using Excel. All figures were prepared using GraphPad Prism. Two-way repeated-measures ANOVA was conducted on changes from baseline using GraphPad Prism. In case of missing values, mixed-model analyses were conducted. In contrast to repeated-measures ANOVA, linear-mixed model repeated-measures analyses allow for small numbers of missing values in the data sets. Linear-mixed model analyses and bivariate correlations were conducted using SPSS. Bonferroni post hoc tests were conducted when analyses revealed statistical significance (P < 0.05). All measures are presented as mean ± SD unless specified otherwise.


Subjects and Bed Rest Protocol

The 24 subjects that successfully completed the study were evenly randomized over the treatment groups (n = 8 each) resulting in mean ages of 38 ± 8 (CON), 33 ± 5 (PEX), and 33 ± 10 (TEX) yr; mean heights of 177 ± 5 (CON), 179 ± 6 (PEX), and 181 ± 7 (TEX) cm; and mean body mass index of 26 ± 2 (CON), 24 ± 3 (PEX), and 23 ± 4 (TEX) kg·m−2. There were no differences in measured baseline characteristics between the groups.


Dietary intakes before and after bed rest were similar between the groups. Mean daily energy intakes during HDBR were higher for PEX (2777 ± 104 kcal) and TEX (2874 ± 337 kcal) compared with CON (2488 ± 267 kcal, P < 0.02), with no differences between PEX and TEX.

Exercise Countermeasure

Subjects attended 100% of the planned sessions, adherence to the SPRINT exercise prescriptions was excellent, and the number of adverse events related to exercise was minimal (23). Exercise intensities and load progressively increased during bed rest with no differences between the PEX and TEX groups.

Testosterone Countermeasure and Safety Profile

Adherence to the injection schedule was 100%, and no doses were missed for any of the subjects. Testosterone levels at the scheduled blood draws remained below the specified maximum normal cutoff of 1200 ng·dL−1 for all subjects (Table 1) and no dose adjustments were made. Effects of time and time–treatment interactions were significant for testosterone concentrations. Post hoc analyses identified no group differences at any time points despite a trend for lower values in TEX compared with CON and EX on BR49. There were no testosterone-related adverse events. Lipid profiles and PSA levels remained within normal levels (Table 2). Cholesterol was lower in TEX at BR14 when compared with CON. Triglycerides and VLDL significantly declined and hematocrit increased during HDBR (i.e., effects of time), and there were no differences between the treatment groups.

Blood measures.
Safety measures.

Blood Measures


There was a significant effect of time on estradiol, luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin, insulin-like growth factor- 1, insulin-like growth factor–binding protein 3, insulin, C-peptide, and high-sensitivity C-reactive protein (hsCRP; Table 1). Time–treatment interactions were significant for LH, FSH, and hsCRP. Patterns of LH and FSH in TEX followed the treatment schedule with lowest values observed after each cycle of testosterone injections (BR14, BR42, BR70). Both LH and FSH returned to basal levels during periods when no testosterone treatment was received. A significant effect of treatment was observed for hsCRP due to increases in the PEX group during the final days of bed rest.


Among the cytokines measured, interleukin (IL) 6 increased in PEX when compared with CON at BR70. IL-2 and IL-7 tended to be higher at baseline in TEX. Although cytokines tended to decline in TEX overall (Table 3), only changes in IL-5 and IL-7 were considered significantly different in this group when compared with CON.


Body Composition


Effects of time and time–treatment were significant for leg LBM (see Table, Supplemental Digital Content 4, body composition, Significant effects of time were noted for total and trunk LBM. Total LBM, leg LBM, and trunk LBM consistently increased in TEX and decreased in CON, with little or no changes in PEX (Fig. 1). There were no significant changes in arm LBM in any of the groups.

Changes in body composition. Total LBM (A) and FM (B), leg LBM (C) and FM (D), and trunk LBM (E) and FM (F). CON, red circles with solid line; PEX, green triangle with broken lines; TEX, blue squares with solid lines. Group IDs next to time points signify treatment effects. P < 0.05, two-way ANOVA with Bonferroni post hoc test. Values are mean ± SD.

Fat mass

Effects of time and time–treatment were significant for total, trunk, and leg fat mass (FM) (see, Table, Supplemental Digital Content 4, body composition, Total FM, leg FM, and trunk FM consistently decreased in TEX and increased in CON and PEX (Fig. 1). There were no significant changes in arm FM in any of the groups.

Bone mineral density

There was a significant effect of time on leg bone mineral density (BMD) and a time–treatment interaction effect on pelvic BMD (see Table, Supplemental Digital Content 4, body composition, Pelvic BMD declined in CON but not in PEX or TEX during the final weeks of HDBR. These changes in pelvis BMD in the CON group (−2.8% change from baseline compared with −0.1% and +0.6% for PEX and TEX, respectively) correspond very well with other bed rest and space flight–induced changes in this anatomical region (24). The decline in pelvis BMD occurred suddenly after approximately 6–8 wk of bed rest.

Muscle Function

Muscle strength

Knee extension strength (KES) and knee flexion strength (KFS) were measured before and after HDBR (see Table, Supplemental Digital Content 5, muscle strength, Isometric KES declined during HDBR (lower at BR + 2 vs BR − 12 and BR − 5) in all groups. Isokinetic KES (60°·s−1 and 180°·s−1) measures were lower for CON compared with EX after HDBR, and there was a trend for higher KFS in the TEX group (P = 0.056 compared with PEX, measured at 60°·s−1). Analyses of the pre–post changes in these strength measures revealed different responses between the groups during HDBR. KES and KFS (measured at 0°, 60°, and 180°) were protected similarly in TEX and PEX compared with CON (P < 0.05, linear-mixed model with Bonferroni post hoc comparison).

Peak voluntary knee extension and flexion strength by contraction velocity after HDBR were similar between the groups (see Figure, Supplemental Digital Content 6, isokinetic strength, Concentric (APFC) and eccentric (APFE) ankle plantar flexion strength at 30°·s−1 was measured before and after HDBR. The greatest strength losses observed were in the calf among the CON group (ankle plantar flexion at 30°·s−1) with changes of −24% for APFC and −30% for APFE strength immediately after bed rest when compared with the average pre–bed rest measures (Fig. 2). By contrast, the losses in PEX (−11% change for concentric and −14% change for eccentric) were blunted during HDBR, with the most robust protection occurring in TEX (−3% change for concentric and −3% change for eccentric).

Changes in calf strength. APFC (A) and APFE (B) strength at 30°·s−1 expressed as percent difference from the average of two pre–bed rest sessions. Two-way ANOVA with Bonferroni post hoc test. Values are mean ± SD.

Muscle strength correlations

Strength data from sessions closest to the start and end of bed rest were compared with leg LBM data from the DXA measures. Pre- to post-HDBR changes in KES, KFS, APFC, and APFE correlated strongly with individual pre–post changes in leg LBM (Fig. 3). Post-HDBR leg LBM correlated with post-HDBR KES and KFS, and pre-HDBR leg LBM correlated strongly with post-HDBR KFS but not KES (data not shown). Post-HDBR measures of total body mass correlated with post-HDBR KES (data not shown).

Changes (%) in leg LBM correlated with changes (%) in isokinetic leg strength measures. A, Knee extension (180°·s−1). B, Knee flexion (180°·s−1). C, APFC (30°·s−1). D, APFE (30°·s−1). Red circles, CON; green triangles, PEX; blue squares, TEX.

Muscle quality

Muscle quality was estimated by calculating the ratios between KES or KFS measurements versus total leg LBM and the DXA measures obtained nearest the respective exercise session (i.e., BR − 12 and BR + 2 exercise). There were significant negative effects of time on some approximations of muscle quality (KES0°·s−1·leg LBM−1 and KES60°·s−1·leg LBM−1) without differences between the groups. The KES180°·s−1·leg LBM−1 score in the PEX group was higher than the CON group, but there was no significant effect of time (see Table, Supplemental Digital Content 7, muscle quality,

Perceptual fatigue

Self-reported fatigue during exercise increased incrementally in both the PEX and TEX groups during bed rest (see Figure, Supplemental Digital Content 8, perceptual fatigue, Self-reported fatigue immediately before or after exercise did not change during bed rest.


This study is the first of its kind to demonstrate that low-dose, intermittent (cycled) testosterone combined with a formal exercise and diet regimen safely and effectively increases LBM compared with exercise alone and bed rest alone. FM increased in CON and PEX, whereas body composition was significantly improved in TEX, with lean mass increasing and FM staying at—or below—baseline (Fig. 1). KES and KFS were maintained in both exercise groups, whereas the control group exhibited significant losses in muscle mass and function. Both concentric and eccentric plantar flexion strength decreased 20% below baseline in CON—losses deemed unacceptable by NASA standards (11) (Fig. 2). Calf strength is notoriously difficult to protect with exercise countermeasures during space flight (11). Exercise alone did not fully prevent loss in calf strength during HDBR but did blunt the losses similarly to results obtained with exercise countermeasures during ~6 months of space flight. The addition of testosterone to the exercise countermeasures further improved calf concentric strength demonstrating the efficacy and utility of testosterone as an adjunct pharmacologic agent to augment NASA (SPRINT) exercise countermeasures.

In the relative absence of mechanical or gravitational loading, such as during long-term HDBR, moderately high-dose testosterone supplementation has been shown to improve body composition, but this is not necessarily associated with increased muscle strength (25). During a 28-d HDBR study, it was demonstrated that weekly, continuous administration of 200 mg testosterone resulted in increased serum concentrations and increased LBM in bed rest subjects (25). However, a dose of 200 mg administered weekly in a continuous fashion is a dose higher than the standard of care dose recommended (and cumulatively four times higher than the dose administered in the current study), and the study did not incorporate an exercise regimen, which is considered a standard countermeasure in all space flight programs. Despite the robust dose of testosterone, muscle strength reportedly did not increase in the absence of an exercise regimen. Physical activity, particularly high-load resistance exercise, is a potent anabolic stimulus, and mechanical stimulation is likely required to elicit the full benefits of testosterone therapy on muscle function. By comparison, the supplementation of ambulatory older men with half this dose of testosterone, or less, improved both muscle mass and strength over 5 months of treatment (21). However, controlling for inadvertent changes in habitual activity during testosterone studies in free-living populations has proved challenging and, although habitual changes are typically discouraged, compliance is difficult to confirm. There have been many studies showing functional benefits of androgens on LBM and strength, although mechanistically these studies were not designed to discern whether the anabolic effects were direct or indirect—perhaps by altering vigor, resistance to fatigue, or habitual levels of activity. In the present study, physical activity was included but tightly regulated, and the exercise program was kept identical between the subjects receiving testosterone and those receiving placebo. The increased lean mass and decreased FM in the subjects receiving testosterone were therefore not secondary to differences in habitual activity or work performed during the scheduled bouts of exercise. Other factors that could potentially be confounds such as diet, sleep/wake, stress, and others were also tightly controlled in this unique study setting.

An important finding from this study is that the anabolic effects of the intermittent low-dose testosterone countermeasure were demonstrated without chronically raising testosterone levels. This is an important clinical finding because testosterone replacement is often prescribed to men with the intention to raise endogenous testosterone levels to within normal physiological levels to achieve functional benefits (18,26). In contrast to reports elsewhere (27), high-intensity exercise countermeasures did not chronically repress testosterone during HDBR in the present study. However, it is unknown whether physical activity acutely affected testosterone availability in the exercising subjects because blood samples were not collected surrounding the exercise sessions. This study provided no information regarding the changes in testosterone levels immediately after injections in TEX subjects. Endogenous testosterone production is pulsatile and follows diurnal patterns regulated by feedback inhibition through the hypothalamic–pituitary–gonadal (HPG) axis. This diurnal pattern is most robust in younger adults and becomes blunted with age (28). Nadir total testosterone measured 7 d after treatment remained unaltered in the CON and PEX groups, and were near or below basal endogenous concentrations in all subjects receiving exogenous testosterone (TEX). Conversely, dips in testosterone concentrations were observed in TEX subjects, likely contributable to HPG feedback responses and reductions in endogenous testosterone production. This suggests that the HPG axis remained fully functional in the TEX subjects as LH and FSH secretion decreased in TEX after testosterone treatment and the HPG axis returned to baseline between each of the treatment cycles (Table 1). Thus, this study demonstrates that the anabolic actions of testosterone can be realized when administered cyclically without chronically raising testosterone levels or disrupting the HPG axis in otherwise healthy men. This is important when considered in light of our previous findings in older men with endogenous testosterone in the lower half of the normal range who were treated in a monthly cycled fashion with weekly injections of testosterone for 4 wk, alternated with 4 wk of placebo (21). Although the acute doses of testosterone administered in that study were identical to the doses used in the present study (i.e., 100 mg testosterone enanthate), total testosterone concentrations were increased from low-normal to mid-normal range in older men when measured 7 d after the last dose of each 4-wk cycle. Whether the observed differences in circulating testosterone between these studies are consequences of differences in dosing regimen, exercise regimen, confinement to bed rest, subject age, endogenous testosterone, or HPG axis function remains unknown.

In the history of human space flight, some cases of hypogonadism have been demonstrated in astronauts (29) and advances in countermeasures may be credited for improvements in hormonal balance observed in most current explorers (30). Despite improvements in countermeasures and maintenance of astronaut health during flight, a complete prevention of the deleterious effects of space flight with NASA exercise countermeasures has not yet been realized. There is still little knowledge regarding the effect of space flight on endocrine function and whether acute or chronic alterations in hormonal status influence astronaut physiology and performance. Our current report indicates that, at least in the absence of frank deficiencies, measurements of testosterone concentration per se may be of limited prognostic value for the assessment of the role of this hormone on functional status without more knowledge on other metabolic regulatory factors. There were no associations between circulating levels of testosterone and improvements in either body composition or muscle strength in this study (data not shown), despite one group clearly receiving exogenous testosterone. Targeted studies involving hormone stimulation tests to determine the sensitivity to changes in hormone levels could be considered if the involvement of the endocrine system in space flight related alterations is to be fully assessed.

There is increasing consensus on the safety and efficacy of the responsibly administered testosterone treatment in populations with various comorbidities (15,31,32). The data from this study support the use of cycled, low-dose testosterone as a safe treatment option in individuals with no underlying comorbidities. Although it is acknowledged that this investigation involved a small group of healthy, exercising subjects, there were no testosterone-induced adverse events or changes in PSA, blood lipid profiles, or chronic alterations in circulating testosterone or related hormones. Furthermore, testosterone treatment did not contribute further to the bed rest induced elevations in hematocrit. This cycled testosterone approach offers several advantages over a continuous administration regimen: 1) the total amount of testosterone administered per person is greatly reduced compared with continuous treatment; 2) it minimizes the potential side effects that may occur during high-dose or chronic administration (32); 3) it allows for a period of normalization for the HPG axis during the “off” periods; and 4) it allows for repeated periods of increased protein anabolism during the “on” periods.

Notably, the acute-phase protein CRP was noticeably elevated in four of eight subjects in the PEX group at BR70 (18.6, 17.4, 2.22, 1.55, 0.620, 0.346, 0.308, and 0.199 mg·L−1 for the respective PEX subjects; see Table 1 for mean values). The cause for these elevations is not clear but may be related to stress before ambulation. Among the cytokines measured, only IL-6 was increased in PEX at BR70. Although the significance of these elevations is unclear, adipose tissue–derived IL-6 is known to induce elevated production of CRP and has been associated with diabetes, cardiovascular disease, sleep disturbance, and health-related quality of life (33,34). In contrast, skeletal muscle–derived IL-6 is known to also play an anti-inflammatory role and is released in acute response to muscle contractions (35). However, the role for contraction-induced increases in IL-6 is somewhat obscure and may occur as a response mechanism to muscle damage. Although testosterone therapy has been shown to be anticatabolic (17) and anti-inflammatory (36), it is unclear whether testosterone treatment directly prevented increases in stress or inflammatory signals in TEX or whether the group differences were secondary to alterations in body composition or other factors.

NASA has long recognized the importance of protecting musculoskeletal mass from space flight–induced losses and more recently has identified the need for research focusing on pharmacologic or hormonal/growth factor approaches that increase the effectiveness of exercise countermeasures (37). An intriguing question that remains from this study is whether cycled, low-dose intermittent testosterone may aid in the development of personalized countermeasures or provide an operational effect such as influencing exercise requirements during space flight. Although the exercise program followed in the current study was largely successful in maintaining LBM during 70 d of HDBR (although this did not coincide with prevention of FM accumulation), testosterone plus exercise resulted in actual gains of LBM during HDBR. Arguably, gaining LBM will not be an objective for most healthy astronauts until losses have occurred during the mission. Therefore, the operational benefits of an adjuvant countermeasure may be most relevant in the design and optimization of minimum exercise requirements or as a contingency/recovery strategy. Indeed, the exercise program followed by the subjects in the present study was considered intense (nine exercise sessions per week many at nearly maximal effort) and much higher than the exercise routine for most individuals (38). Self-reported fatigue during exercise increased with time, although subjects were able to complete the exercise program without signs of overtraining. In anticipation of very long and remote space exploration such as Mars transit, it will be important to understand how to periodize exercise and retain motivation and therefore sustainability. Adjuvants such as testosterone may be useful for personalizing countermeasures as well. There are operational scenarios where high-load resistance exercise may not be feasible such as equipment failure/limitation, scheduling constraints, illness, injury or even psychological factors related to motivation, adherence, mood, or stress. In addition, understanding the intersection of exercise and testosterone dosing will have important clinical applications. Most bedridden patients would be unable to perform intense exercise prescriptions but could benefit from other muscle-sparing treatments such as testosterone, especially if combined with some form of mechanical loading.

In summary, the results from this study demonstrate that cycled testosterone treatment in conjunction with regular aerobic and resistive exercise is safe and effective as a countermeasure against HDBR-induced changes in body composition of healthy men. The exercise prescription was protective against HDBR-induced declines in LBM and strength; however, the addition of cycled, low-dose intermittent testosterone treatment was necessary to promote gains in LBM and protect against increases in FM. Furthermore, changes in leg LBM correlated directly with changes in lower body strength. The anabolic effects were achieved without chronically raising circulating testosterone, PSA, hematocrit, or blood lipids in any of the subjects receiving testosterone treatment. Understanding the unique synergy between exercise dosing and testosterone supplementation could have important implications for both clinical outcomes and spaceflight operations.

This work would not have been possible without the dedication of the volunteers that participated. We thank Mr. John M. Quisenberry, Mr. James P. Lynch, Mrs. Rosalinda Riviera, and Dr. Astrid M. Horstman for their assistance during the study procedures and laboratory analyses. We thank the NASA Flight Analogs Project and the UTMB Clinical Research Center staff at the FARU for planning and implementation support during this collaborative study.

This study was funded by the National Aeronautics and Space Administration (No. NNX10AP86G, R. J. U./M. S. M.) and conducted with the support of the Institute for Translational Sciences at the University of Texas Medical Branch, supported in part by a Clinical and Translational Science Award (UL1TR000071) from the National Center for Advancing Translational Sciences, National Institutes of Health.

Dillon, Sheffield-Moore, Ploutz-Snyder, and Urban designed the research; Dillon, Ploutz-Snyder, Ryder, Danesi, Randolph, and Gilkison performed the research; Dillon, Durham, Danesi, and Randolph analyzed the data; Dillon, Sheffield-Moore, Durham, and Urban wrote the article.

The authors have no conflicts of interest. The results of the present study do not constitute endorsement by the American College of Sport Medicine. The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.


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