Introduction to Human Nutrition

Energy Metabolism 37

gives 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 × 0.77

where 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 × 1.43

where 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 2

Nonprotein 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 × F

where 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 × n

3.4 Factors that infl uence

energy expenditure

Resting 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