A Resting Metabolic Rate test is designed to measure how many calories you are actually burning at rest. I can then factor in your activity level to give you a highly accurate calorie requirement each day. This is then the starting point for your journey.
A Resting Metabolic Rate test uses a method called Indirect Calorimetry. This means that it collects and measures your breath in order to measure how much oxygen is in it after you breathe out. The machine has sampled the air, so it knows how oxygen rich the air you are breathing in is, and by looking at the difference between oxygen in the air breathed in, and oxygen in the air breathed out, it can measure how much oxygen your body has absorbed from that breath. By adding these all up, it is able to see what your body is taking on at rest over time, and it converts this to a calorie figure.