It is preferable to focus on one goal at a time (either gaining muscle or losing fat) rather than trying to do both simultaneously, because these processes are physiologically contradictory.

noli's read

Traditional fitness theory does indeed suggest that muscle gain requires a caloric surplus and fat loss a deficit, which makes body recomposition difficult. However, modern research strongly nuances this point: body recomposition (losing fat while gaining muscle) is entirely possible, especially in beginners, individuals who are overweight, or after a long break from exercise, as shown by several meta-analyses (e.g., Barakat et al., 2020, RCT/systematic review evidence). What is exaggerated here is the idea that these two processes are strictly mutually exclusive. Although maximizing hypertrophy or fat loss is indeed faster when isolating phases, the body can utilize fat stores to provide the energy required for protein synthesis. The advice is therefore a useful simplification for managing effort and motivation, but it does not reflect a biological impossibility.

the sources

1. 1Hall KD, Heymsfield SB, Kemnitz JW, et al. 2012
2. 2Rolls BJ 2009