Answer is: eat food.
Chemical reaction of cellular respiration (convert biochemical energy):
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
glucose + oxygen → carbon dioxide + water + energy .
This reaction is exothermic (energy is released).
Carbon dioxide and water are released during cellular respiration.
There are 18 oxygen atoms (six in glucose and twelve in six molecules of oxygen) in left side of chemical reaction and also 18 oxygen atoms (twelve in six molecules of carbon dioxide and six in six molecules of water) at the right.