Oxygen Required for Metabolism
Metabolism is a generic term for all the chemical reactions that break down or "burn" food to provide energy for the operation of an organism. The word "burn" is used advisedly, because the energy yield from a food in the human metabolic process is comparable to the energy obtained by actual combustion. The energy available from a food is commonly stated in dietary Calories, and the Calorie rating of a food may actually be obtained by burning it in a pure oxygen atmosphere in a calorimeter to measure the energy yield from this combustion.
Like ordinary combustion, the metabolism of food requires a supply of oxygen and produces carbon dioxide as a combustion product. For various foods one can state a representative energy yield, an amount of oxygen required, and an expected amount of carbon dioxide released. Here are some values from Nelson.
Note that the amount of energy produced for the four types of food is roughly proportional to the amount of oxygen use, so that the metabolic rate can be measured by measuring the rate of oxygen consumption. However, the amount of carbon dioxide produced by the four types of food is different, so the ratio CO2/O2 provides some information about the type of food being utilized.
The average for the three types of food in the above table is 4.7 kcal of energy release for each liter of oxygen consumed. On the average, an adult at rest consumes about 16 liters of oxygen per hour. This gives a nominal basal metabolic rate of 75 kcal/hr which translates to 87 watts.
Crucial to the metabolic process is the molecule adenosine triphosphate (ATP), considered by biologists to be the energy currency of life. Almost every process in the body that uses energy gets it from ATP, and in the process converts it to ADP. The energy from the oxidation of food (metabolism) is used to convert the ADP back to ATP, making the energy available for body processes. One of the main pathways for doing so is in the oxidation of glucose.