Smithsonian National Museum of Natural History

What Happens to Your Lunch After You Eat It?

You are an aerobic organism. All aerobic organisms use oxygen to release energy from the proteins, fats, and carbohydrates in food. This process is called metabolism. Metabolism in your body’s cells also creates water and carbon dioxide, which your body excretes.

We inherit this ability to use oxygen from ancient organisms that evolved ways to survive increasing amounts of oxygen in earth’s atmosphere. Without this adaptation, the oxygen would have poisoned these early organisms.