Energy Metabolism 39digest, process, and metabolize the contents of the
meal. The thermic effect of feeding is higher for
protein and carbohydrate than for fat. This is because,
for fat, the process of energy storage is very effi cient,
whereas, for carbohydrate and protein, additional
energy is required for metabolic conversion to the
appropriate storage form (i.e., excess glucose con-
verted to glycogen for storage, and excess amino acids
from protein converted to fat for storage). In addition
to the obligatory energetic cost of processing and
storage of nutrients, a more variable facultative ther-
mogenic component has been described. This com-
ponent is mainly pertinent to carbohydrates, which
through increased insulin secretion produce a dipha-
sic activation of the sympathoadrenal system. The
initial phase is an insulin-mediated increase in sym-
pathetic activity, which produces a β-adrenoceptor-
mediated increase in energy expenditure. The second
and later phase occurs when a counter-regulatory
increase in plasma epinephrine is elicited by the
falling blood glucose. This increase in epinephrine
has a similar slight stimulatory effect on energy
expenditure. As a result of the mediation by β-
adrenoceptors the thermic effect of carbohydrate-rich
meals can be slightly reduced by pharmacological β-
adrenoceptor antagonists.
Energy expenditure related to physical
activity
Physical activity energy expenditure encompasses all
types of activity, including sports and leisure, work-
related activities, general activities of daily living, and
fi dgeting. The metabolic rate of physical activity is
determined by the amount or duration of activity
(i.e., time), the type of physical activity (e.g., walking,
running, typing), and the intensity at which the par-
ticular activity is performed. The metabolic cost of
physical activities is frequently expressed as metabolic
equivalents (METs), which represent multiples of
RMR. Thus, by defi nition, sitting quietly after a 12
hour fast is equivalent to 1 MET. Table 3.3 provides
MET values for other typical physical activities.
The cumulative total daily energy cost of physical
activity is highly variable both within and between
individuals. Therefore, physical activity provides the
greatest source of plasticity or fl exibility in the energy
expenditure system, and is the component through
which large changes in energy expenditure can be
achieved.Total energy expenditure: measurement
by doubly labeled water
The integrated sum of all components of energy
expenditure is termed total energy expenditure. Until
recently, there was no good way to measure total
energy expenditure in humans living under their
habitual conditions. Total energy expenditure can be
measured over 24 hours or longer in a metabolic
chamber, but this environment is artifi cial and is not
representative of the normal daily pattern of physical
activity. The DLW technique can be used to obtain
an integrated measure of all components of daily
energy expenditure over extended periods, typically
7–14 days, while subjects are living in their usualTable 3.2 Simple equations for estimating resting metabolic rate
(RMR) from body weight according to gender and age
RMR (kJ/day)Age (years) Equation for males Equation for females
0–3 (60.9 × wt) − 54 (61.0 × wt) − 51
3–10 (22.7 × wt) + 495 (22.5 × wt) + 499
10–18 (17.5 × wt) + 651 (12.2 × wt) + 746
18–30 (15.3 × wt) + 679 (14.7 × wt) + 496
30–60 (11.6 × wt) + 879 (8.7 × wt) + 829
60 (13.5 × wt) + 487 (10.5 × wt) + 596
wt, body weight (kg).
Table 3.3 Examples of metabolic equivalent
(MET) values for various physical activitiesActivity MET
Basketball 8.0
Chopping wood 6.0
Cleaning house 2.0–4.0
Cycling for pleasure 8.0
Gardening 5.0
Kayaking 5.0
Mowing lawn (power mower) 4.5
Painting house 4.0–5.0
Playing musical instrument 2.0–4.0
Running slowly (8–11 km/h) 8.0–10.0
Running quickly (14–16 km/h) 16.0–18.0
Soccer 7.0–10.0
Strength training 6.0
Stretching 4.0
Tennis 6.0–8.0
Skiing 7.0–14.0
Swimming laps 6.0–12.0
Walking 3.0–5.0
Water skiing 6.0