The Law of Conservation of Energy states that energy and matter exist in a closed system in the universe, and no energy or matter can be entirely destroyed or newly created. All energy and matter can do is change form -- matter-energy, or energy-matter, but it continues to exist in the same ratio as before.
Cellular respiration is the basic metabolic process of cells in the body. Cells take in nutritional material, convert it to energy, and expel waste. The conversion of nutrition to energy is an example of Conservation of Energy; the matter that is taken into the cells becomes usable energy in the form of ATP, the basic energy unit of cellular activity. Conversion to ATP causes the creation of waste heat, which is released by the cell into the bloodstream, which cools the body as it flows between the heart, skin, and lungs.
ATP conversion always results in less ATP energy than original nutritional material, because of the heat produced; this heat is also a form of energy potential in the nutritional material, and so the total ATP and heat equals the energy potential in the original nutrition. This allows Conservation of Energy to remain constant; the matter of nutrition is never destroyed, but simply converted into forms of energy.