Ibutamoren (also known as MK-677) is a selective agonist of the ghrelin receptor and a growth hormone secretagogue, meaning it stimulates the secretion of growth hormone (GH) and insulin-like growth factor 1 (IGF-1). Several clinical studies have demonstrated that Ibutamoren can increase circulating levels of GH and IGF-1 in both healthy adults and specific patient populations. These hormones play crucial roles in muscle growth, recovery, and metabolism, which form the scientific rationale for its use as an athletic and exercise aid.

Research has shown that Ibutamoren can increase lean body mass in both elderly individuals and young adults without significantly affecting fat mass. For example, a study published in "The Journal of Clinical Endocrinology & Metabolism" (1998) reported increased lean mass and improved nitrogen balance in healthy volunteers. Other studies have reported improvements in bone mineral density and potential benefits in physical function, particularly in older adults.

However, there is limited direct evidence that Ibutamoren improves actual athletic performance, exercise endurance, or strength in healthy athletes. Most studies have focused on clinical populations or surrogate endpoints such as hormone levels and body composition. Furthermore, potential side effects (like increased appetite, transient edema, or insulin resistance) and its status as a prohibited substance by WADA limit its practical application in sports. Thus, while there is scientific rationale and some supporting evidence for its ability to enhance muscle mass, robust evidence for improvements in athletic performance remains limited.