Many times we talk about the metabolism without really knowing what it is, as something that if it breaks down makes us get fat or makes us sick. But do we know what the metabolism is and how it really works?
The human being, like any other living being, needs energy to be able to live, and maintain its vital structures and functions.
The body derives its energy from metabolism that is inside our cells. There are chemical reactions that transform the nutrients of food into energy and this is what we call metabolism. 
The basal metabolism is the energy that we need to maintain our vital functions.

How Does the Metabolism Work and How It Can Affect Our Muscle Mass? 

When we eat food, some molecules of the digestive system called enzymes break down proteins into amino acids, fats into fatty acids and carbohydrates into simple sugars such as glucose.
These compounds are absorbed by the blood, which takes them to the cells where other enzymes accelerate or regulate the chemical reactions necessary for them to be metabolized, they are processed so that the energy is released or stored.
The energy that the food provides is measured in calories. And when the amount of calories ingested exceeds what we need for our daily activity, this excess accumulates in the form of fat.
Depending on the final result of the metabolic reactions, two different processes will occur:
Catabolism: Also called destructive metabolism. It is the process by which complex substances and or large molecules, such as carbohydrates, fats or proteins, are decomposed to obtain energy. This process allows us to contract the muscles to move, warm the body, as well as carry out the anabolic reactions. When we do not have enough energy because we have not ingested the sufficient amount of macronutrients, or because we have spent too much, there is a destruction of muscle tissue.
Anabolism: Also called constructive metabolism. This is the process by which simple molecules, through the application of energy, are transformed into larger and more complex molecules (proteins, carbohydrates and fats) and new cells that will contribute to the maintenance of body tissues and creation of new fabrics, as well as the creation of energy reserves to be used in the future. To the people who do exercise, we want to encourage anabolism at all costs, because it is the process that will allow us to create muscle mass or tissue, through nutrition that allows us to do so.
There are many factors and hormones involved in these processes, but there are a number of facilitators, which I have explained below.

Anabolic Facilitators:

Food: We have to give enough energy to the body so that it performs its functions well. When I refer to energy, I do not mean just the number of kcal. The origin of the same also matters, and much. In the case of muscular construction, a sufficient supply of good quality protein is essential. Proteins and amino acids are the pillars and bricks of our body construction.

Energy after Exercise: Once our energy reserves (muscle glycogen) have been exhausted, after intense exercise, it will be necessary to refill them, to prevent our body from taking our muscle mass as our energy source. Some complex carbohydrate is indicated for this moment.

Good Fats: Fats are necessary to maintain a good state of health. but be careful, not all are the same. Fats such as extra virgin olive oil, avocado, nuts, coconut oil, fats from blue fish, etc. Here I also want to mention the amount. They are good but they have a lot of calories, so be careful with the quantities.

Hydration: Our body is formed in 70% of water, so it is important to drink enough or eat foods that provide more hydration. I am not going to enter the 2litres norm, because the requirements of each person are different, but drink enough.

Rest: During the resting hours the body regenerates itself. Try to sleep between 7 and 8 hours. You will notice the difference.

Catabolism Facilitators:

Excess of Cardio: Cardivascular exercise allows us to gain resistance as well as using lipids (fat) as an energy source, but it does not favor muscle growth, moreover, it destroys it after a certain threshold.

High intensity of exercise without sufficient reserves of glycogen: Glucogen is obtained in the first instance of carbohydrates. If we do not have enough reserves, the body to generate energy, will resort to muscle tissue. Be careful with this, I do not want to say that we should eat extra carbohydrates if we are not going to need them. We are talking about high intensity.

Stress: when we are subjected to a lot of stress, the body generates a hormone called cortisol. The high levels of cortisol maintained over time, favor muscle catabolism and fat accumulation.

Overtraining:  More is not better. If we train and do not respect the processes of recovery and super compensation, we run the risk of entering into overtraining. This causes fatigue, poor performance, discomfort and even greater risk of injury. Schedule your workouts according to your goals and respect the times.

Conclusion

There is no infallible and universal formula or method for the creation of muscle mass. The key is to find the individual balance between the nutrients we need, the type and intensity of exercise we perform, the emotional state in which we find ourselves, the hours of rest and a good planning of the training.