Caloric surplus means that we consume more calories than we burn. The excess calories will be stored in the body as fat, and our weight will increase.

Caloric deficit means that we burn more calories than we consume. If the deficit is too big, the body will compensate and use energy that is stored in the body, either as fat or muscle tissues. An exaggerated deficit can also lead the body to enter an emergency mode, in which the metabolism slows down.

In the food that we eat, there are 3 major groups: carbohydrates, fats, and proteins.  The main purpose of carbohydrates and fats is to provide energy, whereas the proteins mainly build the body and the muscles. Our body uses energy from all these 3 sources of energy throughout the day (including when we sleep).

When we are physically active we use mainly carbohydrates and some fat as energy sources, and sometimes proteins as well. When we don’t have sufficient carbohydrates our body will break down muscle tissues and convert the released amino acids into glucose, which will be used for energy.

When we sleep our body uses mostly fat as energy source.

In one hour of sleep, we’ll consume about 60 calories, out of which 50 come from fat.

In one hour of running we’ll consume about 400 calories (depends on the intensity) out of which about 200 come from fat.

In one hour of a vigorous-intensity activity (such as racing) we’ll consume about 1,000 calories, out of which about 300 come from fat.

We can draw 2 conclusions from these facts:

1. The more vigorously intensive the workout, the more calories we consume.
2. In low-intensity activities we use a larger percentage of fat, however, the total amount of fat is larger in high-intensity activities.

Therefore we should try to aspire to work out at the highest intensity that we tolerate. It’s also important to remind new trainees to always start slow and easy.