Exercise: Oxygen Consumption & Carbon Dioxide Output
Indirect Calorimetry, Respiratory Exchange Ratio and Determination of Fuel Type

Humans can briefly extract energy from fat and carbohydrate without
needing oxygen in a process called anaerobic metabolism, but this
process can only continue for a very short time before it causes so much
lactic acid to accumulate in tissue that further metabolism becomes
impossible. We need oxygen to do it right and ultimately our ability to
burn fat and carbohydrate is totally dependent upon oxygen. This
dependency requires that oxygen consumption be directly proportional to
calories burned in exercise such that if we can accurately measure
oxygen consumed, we can determine calories burned. This is known as
indirect calorimetry.

Another useful parameter is carbon dioxide output. When fat or
carbohydrate is "burned" it ultimately invloves combining carbon atoms in
the fuel with oxygen to produce carbon dioxide that is then breathed out
from the lungs. In a very general way therefore, carbon dioxide output is
proportional to oxygen consumption, but with a caveat (and this is VERY
important): Carbon dioxide output per liter of oxygen consumed is slightly
greater when we are burning carbohydrate than when we are burning fat
. This difference means that if we carefully measure how much oxygen is
going into and how much carbon dioxide is coming out of a person who is
exercising, we can, from the ratio of carbon dioxide out and oxygen in,
get a pretty good idea of exactly what percent of the calories burned are
coming from fat and what percent from carbohydrate. This ration of
carbon dioxide-out to oxygen-in is called the "Respiratory Exchange Ratio"
or RER.
Armed with indirect
calorimetry through
oxygen consumption
and the RER, we can
then set out to
determine what
types and intensities
of exercise work
best to burn the
most calories and
fat. Instead of
guessing or
theorizing, we can
use real data to get
real answers. We'll
explore that data
and those answers
in the next section.
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