The impact of weight loss on fat-free mass, muscle, bone and hematopoiesis health: Implications for emerging pharmacotherapies aiming at fat reduction and lean mass preservation.

Similar to bariatric surgery, incretin receptor agonists have revolutionized the treatment of obesity, achieving up to 15-25 % weight loss in many patients, i.e., at a rate approaching that achieved with bariatric surgery. However, over 25 % of total weight lost from both surgery and pharmacotherapy typically comes from fat-free mass, including skeletal muscle mass, which is often overlooked and can impair metabolic health and increase the risk of subsequent sarcopenic obesity. Loss of muscle and bone as well as anemia can compromise physical function, metabolic rate, and overall health, especially in older adults. The myostatin-activin-follistatin-inhibin system, originally implicated in reproductive function and subsequently muscle regulation, appears to be crucial for muscle and bone maintenance during weight loss. Activins and myostatin promote muscle degradation, while follistatins inhibit their activity in states of negative energy balance, thereby preserving lean mass. Novel compounds in the pipeline, such as Bimagrumab, Trevogrumab, and Garetosmab-which inhibit activin and myostatin signaling-have demonstrated promise in preventing muscle loss while promoting fat loss. Either alone or combined with incretin receptor agonists, these medications may enhance fat loss while preserving or even increasing muscle and bone mass, offering a potential solution for improving body composition and metabolic health during significant weight loss. Since this dual therapeutic approach could help address the challenges of muscle and bone loss during weight loss, well-designed studies are needed to optimize these strategies and assess long-term benefits. For the time being, considerations like advanced age and prefrailty may affect the choice of suitable candidates in clinical practice for current and emerging anti-obesity medications due to the associated risk of sarcopenia.