Fitness

Oxygen is used to get energy from glucose/fat, so oxygen consumption is directly related to calorie burn. One nice way to think of it is that carbon atoms, the main atom in food, leaves our body by o2 (oxygen) going in and co2 (carbon monoxide) going out, not via pee/poop as many people might think, so heavier breathing during exercise to get more oxygen in is also an indication of more carbon out/calories burned.