Energy Metabolism 41energy balance, the energy needs or energy require-
ments of the body to maintain energy balance must
be equal to total daily energy expenditure. Total daily
energy expenditure is the sum of the individual com-
ponents of energy expenditure as discussed previ-
ously, and represents the total energy requirements of
an individual that are required to maintain energy
balance. Until recently, there was no accurate way to
measure total energy expenditure or energy needs of
humans. The DLW technique has provided a truly
noninvasive means to measure accurately total daily
energy expenditure, and thus energy needs, in free-
living humans. Before DLW, energy requirements
were usually assessed by measurement or prediction
of RMR, the largest component of energy require-
ments. However, since the relationship between RMR
and total energy expenditure is highly variable because
of differences in physical activity, the estimation of
energy needs from knowledge of RMR is not that
accurate and requires a crude estimate of physical
activity level. Nevertheless, reasonable estimates can
be made to estimate daily energy budgets for indi-
viduals (Table 3.4).
Following the validation of DLW in humans, this
technique has been applied to many different popula-
tions. Total energy expenditure is often compared
across groups or individuals using the ratio of one’s
total energy expenditure to RMR, or physical activity
level (PAL). Thus, for example, if the total energy
expenditure was 12.6 MJ/day and the RMR was
6.3 MJ/day, the PAL factor would be 2.0. This value
indicates that total energy expenditure is twice the
RMR. The PAL factor has been assessed in a variety of
types of individual. A low PAL indicates a sedentary
lifestyle, whereas a high PAL represents a highly active
lifestyle. The highest recorded sustained PAL in
humans was recorded in cyclists participating in the
Tour de France road race. These elite athletes could
sustain a daily energy expenditure that was up to
fi ve times their RMR over extended periods. Smaller
animals, such as migrating birds, have a much higher
ceiling for achieving higher rates of total energy expen-
diture, which can reach up to 20 times their RMR.
Factors such as body weight, FFM, and RMR
account for 40–60% of the variation in total energy
expenditure. Total energy expenditure is similar
between lean and obese individuals after taking into
account differences in FFM. Thus, fatness has small,
but important, additional effects on total energy
expenditure, partly through RMR, as discussed previ-
ously, but also by increasing the energetic cost of any
physical activity.
With regard to age, some studies suggest that only
a limited change in total energy expenditure (relative
to RMR) occurs from childhood to adulthood, but
that a decline occurs in the elderly. Recent data also
suggest a gender-related difference in total energy
expenditure, in addition to that previously described
for RMR. In a meta-analysis that examined data from
a variety of published studies, absolute total energy
expenditure was signifi cantly higher in males than in
females by 3.1 MJ/day (10.2 ± 2.1 MJ/day in females,
13.3 ± 3.1 MJ/day in males), and nonresting energy
expenditure remained higher in men by 1.1 MJ/day.Table 3.4 Typical daily energy budgets for a sedentary and a physically active individual of
identical occupation, body weight, and resting metabolic rate of 6.0 MJ/day (4.2 kJ/min)Activity Activity indexMinutes per day MJ per day
Sedentary Active Sedentary Active
Sleep 1.0 480 480 2.0 2.0
Daily needs 1.06 120 120 5.3 5.3
Occupational 1.5 480 480 3.0 3.0
Passive recreation 2.0 360 300 3.0 2.5
Exercise 12.0 0 60 0 3.0
Total 1440 1440 8.6 11.1
PA L = 1.4 PAL = 1.8Thus, the sedentary individual would need to perform 60 min of vigorous activity each day at an
intensity of 12.0 to increase the physical activity level (PAL) from a sedentary 1.4 to an active and
healthy 1.8.