How does VO2 test determine % calories from fat?

I just had a VO2 test done. The tester put a gas analyzer mask on me
that measured O2 in and CO2 out. We mounted my bike on a cycletrainer
which measured real-time power output and cadence. I wore an HRM.

Now I'm looking at the data log. The data listed includes:

abs VO2 - absolute value of volume of oxygen consumption [ml/kg/min]
rel VO2 - relative val. of volume of oxygen consumption [ml/min]
VO2 predicted - predicted VO2 value in regards to sex and age [%]
VCO2 - value of volume of CO2 produced [ml/min]
RQ - ratio of volume of CO2 produced / volume of O2 consumed
HR - HeartRate [bpm]
VE STPD - expired ventilation for standard temperture pressure dry [L]
Fat - fat calories burned at each point of the exercise [Kcal/min]
Kcal - total calories burned at each point of the exercise [Kcal/min]
Power - [watts]

What I don't understand is this: given the test measurement I
described above, how can the system determine my fat calories burned?

I understand how total calories burned can be computed from power
integrated over time. I can also see how total aerobic calories can
be computed based on volume of O2 and CO2. But how can the system
differentiate calories that were generated by burning fat versus those
generated by burning glycogen/glucose?

Dave

David Wuertele writes:

I just had a VO2 test done. The tester put a gas analyzer mask on me
that measured O2 in and CO2 out. We mounted my bike on a cycletrainer
which measured real-time power output and cadence. I wore an HRM.

Now I'm looking at the data log. The data listed includes:

abs VO2 - absolute value of volume of oxygen consumption [ml/kg/min]
rel VO2 - relative val. of volume of oxygen consumption [ml/min]
VO2 predicted - predicted VO2 value in regards to sex and age [%]
VCO2 - value of volume of CO2 produced [ml/min]
RQ - ratio of volume of CO2 produced / volume of O2 consumed
HR - HeartRate [bpm]
VE STPD - expired ventilation for standard temperture pressure dry [L]
Fat - fat calories burned at each point of the exercise [Kcal/min]
Kcal - total calories burned at each point of the exercise [Kcal/min]
Power - [watts]

What I don't understand is this: given the test measurement I
described above, how can the system determine my fat calories burned?

I understand how total calories burned can be computed from power
integrated over time. I can also see how total aerobic calories can
be computed based on volume of O2 and CO2. But how can the system
differentiate calories that were generated by burning fat versus those
generated by burning glycogen/glucose?

Carbohydrates are called that because their chemical formula looks
like (CH2O)n In other words, carbohydrates are composed of "hydrated"
carbon atoms, every carbon is associated with two hydrogens and an
oxygen.

Fats, on the other hand, are more complicated, with a typical formula
being (CH2O)3(CH2)3n(CO2H)3, with n=17 for a typical fat. The key
point is that there is much less oxygen in fats then in carbohydrates.

When either a fat or a carbohydrate "burns" in the body, it combines
with oxygen to form carbon dioxide and water. Because carbohydrates
have more oxygen in them, it takes less oxygen from the atmosphere to
produce a given quantity of carbon dioxide than when fats are burned.

So, by measuring the quantity of carbon dioxide produced relative to
the amount of oxygen consumed, it is possible to tell what portion of
the calories being burned are coming from fat. This quantity,
VCO2/VO2 is know as the "respiratory quotient." If you body is
burning purely carbohydrates, the respiratory quotient is 1.0, if you
are burning purely fat it is about 0.66 A typical value for someone
just hanging out is about 0.80.

To complicate matters, your body can also burn protein for energy.

"Jim Smith" wrote in message
...
David Wuertele writes:


snip


What I don't understand is this: given the test measurement I
described above, how can the system determine my fat calories burned?

I understand how total calories burned can be computed from power
integrated over time. I can also see how total aerobic calories can
be computed based on volume of O2 and CO2. But how can the system
differentiate calories that were generated by burning fat versus those
generated by burning glycogen/glucose?

Carbohydrates are called that because their chemical formula looks
like (CH2O)n In other words, carbohydrates are composed of "hydrated"
carbon atoms, every carbon is associated with two hydrogens and an
oxygen.

Fats, on the other hand, are more complicated, with a typical formula
being (CH2O)3(CH2)3n(CO2H)3, with n=17 for a typical fat. The key
point is that there is much less oxygen in fats then in carbohydrates.

When either a fat or a carbohydrate "burns" in the body, it combines
with oxygen to form carbon dioxide and water. Because carbohydrates
have more oxygen in them, it takes less oxygen from the atmosphere to
produce a given quantity of carbon dioxide than when fats are burned.

So, by measuring the quantity of carbon dioxide produced relative to
the amount of oxygen consumed, it is possible to tell what portion of
the calories being burned are coming from fat. This quantity,
VCO2/VO2 is know as the "respiratory quotient." If you body is
burning purely carbohydrates, the respiratory quotient is 1.0, if you
are burning purely fat it is about 0.66 A typical value for someone
just hanging out is about 0.80.

To complicate matters, your body can also burn protein for energy.

What a well thought out, reasoned, response. What I can't figure out is
what it's doing on a rec.bicycles.* newsgroup!