Your body is a construction worker
*All credit for this goes to T-Nation. I had to re-post because it’s too good not to share. It’s such a simple way of understanding the different roles of all the components to building muscle*
“Imagine that your muscles are like a house you're trying to build.
The bricks used to build the house represent the amino acids – from the ingestion of protein – while the money you're paying the workers – so that they'll do the work – represents the carbs and fat you eat.
Finally, the workers represent the factors involved in the protein synthesis process (testosterone mainly) and the truck bringing the bricks to the workers represent insulin, which plays a capital role in transporting the nutrients to the muscle cells.
If you don't give the workers enough bricks (protein) they won't be able to build the house as fast as they could. So in that regard, an insufficient protein intake will slow muscle growth.
If you don't pay your workers enough total nutrition they won't be as motivated to work hard. As a result, the house won't be built quickly.
And if you really cut the workers' pay, they might even get mad, go on strike, and start demolishing the house (catabolism due to an excessively low caloric intake). So in that regard, not consuming enough protein or calories to support muscle growth will lead to slower gains.
What would happen if you started to send more bricks (increase protein intake) to the workers? They'll be able to build the house more rapidly because they aren't lacking in raw material. But at some point, sending more and more bricks won't lead to a faster rate of construction.
The workers can only perform so much work in any given amount of time. For example, if your crew can add 1,000 bricks per day to the walls, giving them 2,000 bricks per day will be useless: it exceeds their work capacity. So the excess bricks will go to waste.
If you increase your workers' salary (increase caloric intake) chances are their motivation will rise, and as a result they'll build the house faster. Just like with bricks, there comes a point where increasing the workers' salary won't have any effect on the house-building rate: the workers will reach their physical limit. Once they do, you can increase their salary all you want, but they won't be able to add bricks to the house any faster.”