In order to change your body composition you need to successfully build muscle. You need a properly designed workout program that signals growth and a diet that supplies your body with what it needs to make that growth happen.
On the diet side, one of those required supplies is a caloric surplus, which means consuming more calories than your body need; to maintain its current state.
Why is this surplus needed? Simply put it’s because your body simply needs some amount of additional calorie to synthesize new muscle tissue as well as support the recovery and training performance needed to make progressive overload happen.
Calorie partitioning (also called "nutrient partitioning") refers to where these extra calories and nutrients will go (or how they will be "partitioned" in your body) when you consume them. It also refers to where they will come from when you're in a deficit and consuming less of them (meaning, your body can potentially bum stored body fat or muscle tissue for energy).
Now obviously, in a surplus, we want every single calorie we eat to go toward muscle growth only, but it turns out there are actually three places for them to go:
1. Toward new muscle.
This is what we want.
2. Toward new body fat.
This is what no one wants, but it's what every fat person got. Like I explained earlier, this is how every fat person in the history of mankind ended up getting fat. They consumed too many calories and a "surplus" existed. Taaadaaa! Combine this with the fact that A) there was no muscle-growth-signaling workout program in place, and B) the surplus in these cases was often both excessive and long lasting, and you get a common scenario where most/all of the extra unused (surplus) calories consumed were stored as fat.
3. Toward a combination of both.
This is another one we don't want, but it's something most of us are quite familiar with getting. For every X pounds of muscle we gain, it seems Y pounds of fat always come along with it. (The exact amounts for X and Y will vary significantly for reasons I'll explain shortly.)
Now even though you can’t control everything (i.e. your genetics), there are quite a few key factors that you CAN control that will GREATLY influence calorie partitioning, your ratio of muscle to fat gains, and your overall ability to make muscle growth (and a nice body recomp) happen.
To add to that, having balanced hormones is also crucially important. A better progesterone/estrogen ratio tends to have positive partitioning effects (more muscle, less fat) while chronically high levels of estrogen (estrogen dominance) and the stress hormone cortisol have the opposite effect (less muscle, more fat).
Thyroid and nervous system activity affect not only metabolic rate but also fat burning; optimized thyroid and nervous system levels mean better fat burning, which means less muscle loss when you diet. It also means less fat gain when you overeat. This is why having a healthy metabolism & thyroid will positively affect all of this.
So, while there are some uncontrollable factors which are always going to be there in the background putting some sort of natural limit on our progress, the key to achieving superior muscle growth is going to be optimizing every single one of these controllable factors to the best of our ability.
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