Energy Metabolism 37gives a more accurate estimate of energy expenditure
and RQ.
Step 1
First, the contribution of protein oxidation to oxygen
consumption (V
.
O 2 ) and carbon dioxide production
(V
.
CO 2 ) is estimated based on the knowledge that the
nitrogen content of protein is 1/6.25:
V.
O2(prot) = n × 6.25 × 0.97
V.
CO2(prot) = n × 6.25 × 0.77where V is volume, 0.97 and 0.77 are liters of O 2 con-
sumed and CO 2 produced by the biological oxidation
of 1 g of protein, respectively, and prot is protein.
Step 2
Next, nonprotein V
.
O 2 (V.
O2(nonprot)) and nonprotein
V
.
CO 2 (V.
CO2(nonprot)) are calculated:
V.
O2(nonprot) = V.
O 2 − V.
O2(prot)
V.
CO2(nonprot) = V.
CO 2 − V.
CO2(prot)
V.
O2(nonprot) = C × 0.828 + F × 2.03
V.
CO2(nonprot) = C × 0.828 + F × 1.43where C and F are grams of oxidized carbohydrate
and fat, respectively, and can be found by solving the
two equations with two unknowns; O 2 and CO 2 pro-
duced by the combustion of 1 g of carbohydrate is
0.828 liters, whereas the combustion of 1 g triglycer-
ide consumes 2.03 liters O 2 and produces 1.43 liters
CO 2. The protein oxidation (P) is n × 6.25 g.
Step 3
The RQ is defi ned as:
V.
CO 2 /V.
O 2Nonprotein RQ (RQ(nonprot)) is calculated by the
equation:
RQ(nonprot) = V.
CO2(nonprot)/V.
O2(nonprot)Step 4
Next, energy expenditure can be calculated:
Energy expenditure (kJ/min)
= [19.63 + 4.59 (RQ(nonprot) − 0.707]
× V.
O2(nonprot) + 18.78 × V.
O2(nonprot)or
Energy expenditure (kJ/min) =
17 × P + 17.5 × C + 38.9 × Fwhere 17, 17.5, and 38.9 are the heat produced (kJ)
by the combustion of 1 g of protein, glycogen, and
triglyceride, respectively.
The equations are produced by the insertion of the
heat equivalent for carbohydrate and fat, and are
valid even though there is a quantitative conversion
of carbohydrate to lipid (de novo lipogenesis) or
glyconeogenesis.
The caloric equivalent for O 2 is similar to the three
main substrates: 21 kJ/l O 2 for carbohydrate, 19 kJ/l
O 2 for fat, and 17.8 kJ/l O 2 for protein (which con-
tributes only modestly to energy expenditure). Energy
expenditure can therefore be calculated with reason-
able accuracy by the equation:
Energy expenditure (kJ/min) = 20 kJ/l × V.
O 2 (l/min)
With pure fat oxidation the RQ is 0.707, with pure
carbohydrate oxidation it is 1.0, and with pure protein
oxidation it is approximately 0.8.Step 5
Oxidation of protein (P), carbohydrate (C), and fat
(F) can be calculated by the following equations,
where n is the unit g/min:
P (g/min) = 6.25 × n
C (g/min) = 4.55 × V.
CO 2 − 3.21 × V.
O 2 − 2.87
F (g/min) = 1.67 × V.
O 2 − 1.67 × V.
CO 2 − 1.92 × n3.4 Factors that infl uence
energy expenditureResting metabolic rate
Each of the components of energy expenditure is
determined by various factors. RMR is highly variable
between individuals (±25%), but is very consistent
within individuals (<5%). Since RMR occurs pre-
dominantly in muscle and the major organs of the
body, the main source of individual variability in
RMR is an individual’s amount of organ and muscle
mass. Thus, fat-free mass (FFM; the total mass of the
body that is not fat, i.e., predominantly organs and
muscle) explains 60–80% of the variation in RMR
between individuals. This concept can be explained
using the woodstove analogy; the larger the wood-
stove (or FFM), the larger the amount of heat pro-
duction (or the larger the RMR). Since FFM is a het-
erogeneous mixture of all nonfat body components,
the metabolic rate associated with each kilogram of