Testosterone suppression does not exacerbate disuse atrophy and impairs muscle recovery that is not rescued by high protein.

Androgen deprivation therapy (ADT) decreases muscle mass, force, and physical activity levels, but it is unclear whether disuse atrophy and testosterone suppression are additive. Additionally, conflicting reports exist on load-mediated hypertrophy during ADT and if protein supplementation offsets these deficits. This study sought to determine the role of testosterone suppression and a high-protein diet on ) immobilization-induced atrophy and ) muscle regrowth during reloading. Eight-week-old male Fischer 344 rats underwent sham surgery (Sham), castration surgery (ORX), or ORX and a high-casein diet supplemented with branched-chain amino acids (BCAA) (ORX+CAS/AA) followed by 10 days of unilateral immobilization (IMM) and 0, 6, or 14 days of reloading. With IMM, body mass gains were 8% greater than ORX and ORX+CAS/AA that increased to 15% during reloading (both < 0.01). IMM reduced muscle mass by 11-34% (all < 0.01) and extensor digitorum longus and soleus (SOL) force by 21% and 49% (both < 0.01), respectively, with no group differences. During reloading, castration reduced gastrocnemius mass (12%) at 6 days and SOL mass (20%) and SOL force recovery (46%) at 14 days relative to Sham (all < 0.05). Specific force reduced castration deficits, indicating that muscle atrophy was a key contributor. IMM decreased SOL cross-sectional area by 30.3% ( < 0.001), with a trend for reduced regrowth in ORX and ORX+CAS/AA following reloading ( = 0.083). Castration did not exacerbate disuse atrophy but may impair recovery of muscle function, with no benefit from a CAS/AA diet during reloading. Examining functional outcomes in addition to muscle mass during dietary interventions provides novel insights into muscle regrowth during ADT. Low testosterone levels during skeletal muscle disuse did not worsen declines in muscle mass and function, although hypogonadism may attenuate recovery during subsequent reloading. Diets high in casein did not improve outcomes during immobilization or reloading. Practical strategies are needed that do not compromise caloric intake yet provide effective protein doses to augment these adverse effects.