It's strange to think about a cell in your body breathing, but when each individual cell converts food into energy, that's what it is doing. Your blood carries glucose and oxygen to every cell in your body. The cell “inhales” the sugar and oxygen and “exhales” carbon dioxide and water, sending those two byproducts to the lungs and kidneys where they are expelled. The remaining molecule -- adenosine triphosphate, or ATP -- is the energy that powers all cellular activity, and by extension, every move you make.
When you take in calories, your body, with the aid of insulin, converts that energy into glucose and transports it through the blood stream. The glucose molecule passes through cell walls and is converted to pyruvic acid in the cytoplasm, the cell body contained within the membrane. Just two molecules of ATP result from this reaction, but the pyruvic acid is then sent to the mitochondrion, the cell's power plant, for more processing.
The two pyruvic acid molecules are converted into acetyl CoA inside the mitochondrion before they begin the Krebs cycle. The mitochondrion, with the help of free oxygen atoms, processes acetyl CoA into waste products CO2 and sugar. Four more molecules of ATP result from this process, and the CO2 is “exhaled” through the cell wall. The electrons from the stripped hydrogen atoms pass through the electron transport train resulting in the biggest energy payoff of the cellular respiration process, or 32 more molecules of ATP, all from just the one molecule of glucose.
ATP synthesis occurs 24 hours per day, every day of your life. The calories you consume only indirectly provide your body with the energy it needs. They actually provide the energy to produce the high-energy bonds of the ATP molecule that then provides power to the muscles and energy to the brains electrochemical responses. When you take in fewer calories than you need in a given day to run these systems, the body turns to fat stores, and to a lesser degree protein from existing muscle, to convert carbon compounds into ATP through cellular respiration.
Oxygen is toxic to biological molecules and cellular material. Biologists refer to this as the “oxygen paradox” because you can’t live without it, but it ultimately damages cells while it keeps you alive. Oxygen molecules used in ATP production in the mitochondria produce free radicals, or unbound electrons. These electrons tear through cell walls and eventually wear out the cell’s energy factory. This “oxidative stress” interferes with cell division which can result in rogue, mutated cells massing together to form tumors, according to Life Extension Magazine.
For decades, rodent studies have shown conclusively that calorie restriction dramatically extends life expectancy. The process by which this happens has eluded researchers, and trials searching for the effect on human longevity have been inconclusive. A March 2007 study by Anthony E Civitarese, et al, published in the journal PLoS Medicine, demonstrated a correlation between restricted calories and cellular health. The researchers concluded that calorie restriction, even short-term, resulted in more efficient mitochondrial reactions during cellular respiration, which lowered oxidative stress and revealed measurable reductions in DNA damage.