Macronutrients are groups of nutrients we need in large quantities to provide the body with energy. The 3 macronutrients are Carbohydrates, Proteins & Fats. The fact we need them in large quantities is where the term macro comes from. There are recommended ranges of each to consume but in the end it will come down to personal circumstances.
Macronutrients contain energy in the form of calories, carbs are the main energy source but other sources can be used too.
The calorie content of each macronutrient:
Carbs - 4 calories per gram
Proteins - 4 calories per gram
Fats - 9 calories per gram
It is important we eat a variety of foods to obtain enough of each macronutrient. A lot of foods contain sources of multiple or all macronutrients but there are also a lot of foods that contain mainly just a high proportion of just one macronutrient.
Each of the 3 macronutrients have a specific function in the body:
Carbs - broken down into glucose and are the primary energy source, stored as glycogen in muscles for energy use later on.
Proteins - digested into amino acids, used to help create new proteins and build and repair muscles.
Fats - broken down into fatty acids and glycerol, stored around your body as an energy source, insulate the organs and an important component of cell membranes.