I have to start this post with a biology lesson. You probably already know that there are three macronutrients: carbohydrates, protein, and fat. These are our bodies’ sources of fuel, but they are not all treated equally.
The article I linked to above explains this quite well, and in more detail than I’m going to, but the short version is that carbohydrates are your body’s primary fuel source, largely because they are the easiest to break down. While your body can and will break fats and proteins down into energy, it takes longer than breaking down carbs and therefore, as a less efficient process, is not preferred by your body in most cases.
Fat contains the most energy potential per gram, so it is also commonly used by the body as a source of fuel – either in low intensity exercise where it has the time to break the fat down, or during endurance exercise in an attempt to preserve glycogen (sugar), which comes from carbs and is used by your body to create energy.
Finally, the least preferred source of fuel is protein. It is used in late stages of endurance exercise, when glycogen (sugar) runs really low and the body is forced to break protein into amino acids for fuel. However, this is not ideal because protein is much better used for other purposes – specifically muscle build and repair functions.
So that brings us to how low carb diets work.