Explaining fat loss and muscle gain by only discussing the calorie balance model would be overly simplistic. A deficit doesn’t guarantee 100% fat loss. You can be in a deficit and lose muscle, not just fat. A calorie surplus doesn’t guarantee 100% muscle gain – as we all know too well, you can be in a surplus and gain fat, not just muscle.
The key is to partition as much of that surplus into lean tissue and have as much of the energy deficit withdrawn from fat tissue as humanly possible. Anabolic and catabolic hormones are primary players in the partitioning of energy and nutrients.
Hormones are like traffic cops, directing energy and nutrients down one road into cells or down another road out of cells, and these endocrine factors are influenced by weight training as well as lifestyle, including sleep, stress, diet composition, state of health and other factors.
The same concept applies to hormones and energy partitioning as with energy balance: you have anabolic hormones, which are responsible for the depositing of energy into tissues or the building of tissues, and you have catabolic hormones that are responsible for the mobilization of energy from tissues or the breakdown of tissues.
Because these hormones are antagonistic to one another, you don’t release them at the same time, so building processes and breakdown processes are not occurring simultaneously.
For example, glucagon is antagonistic to insulin. Cortisol is antagonistic to growth hormone and testosterone (the testosterone to cortisol ratio is often used as a measure of the anabolic status of your body.
Furthermore, while in an aggressive caloric deficit and a dieted-down state, you are far more likely to be releasing predominantly catabolic hormones. So while muscle gain and fat loss are clearly not dictated by calories alone, your body is simply not set up to gain muscles in a deficit or to put on muscle and lose fat at the same moment in time.