Energy Metabolism 43Energy requirements in physically
active groups
The DLW technique has been used to assess energy
requirements in highly physically active groups of
people. The most extreme case is a study that assessed
the energy requirements of cyclists performing in the
3 week long Tour de France bicycle race. The level of
total energy expenditure recorded (PAL factor of
5.3, or approximately 35.7 MJ/day) was the highest
recorded sustained level in humans. In another study
involving young male soldiers training for jungle
warfare, energy requirements were 19.9 MJ/day
(PAL factor of 2.6). The total energy expenditure of
four mountaineers climbing Mount Everest was
13.6 MJ/day (PAL 2.0–2.7), which was similar to
energy expenditure during on-site preparation prior
to climbing (14.7 MJ/day). Total energy expenditure
in free-living collegiate swimmers was almost 16.8 MJ/
day in men and 10.9 MJ/day in women. In elite female
runners previously performed studies of energy
intake suggested unusually low energy requirements.
However, in a study in nine highly trained young
women, free-living energy expenditure was 11.9 ±
1.3 MJ/day, compared with the reported energy intake
of 9.2 ± 1.9 MJ/day. This study suggests that elite
female runners underreport true levels of energy
intake and confi rms the absence of energy-saving
metabolic adaptations in this population.
Regular participation in exercise is traditionally
thought to elevate energy requirements through the
additional direct cost of the activity, as well as through
an increase in RMR. However, in some situations
energy requirements are not necessarily altered by
participation in regular physical activity. For example,
in a study of an elderly group of healthy volunteers,
there was no signifi cant change in total energy ex-
penditure in the last 2 weeks of an 8 week vigorous
endurance training program. The failure to detect
an increase in total energy expenditure occurred
despite a 10% increase in RMR (6703.2 ± 898.8 to
7404.6 ± 714 kJ/day), as well as an additional 630 kJ/
day associated with the exercise program. These
increases in energy expenditure were counteracted by
a signifi cant reduction in the energy expenditure of
physical activity during nonexercising time (2.4 ± 1.6
versus 1.4 ± 1.9 MJ/day). The lack of increase in total
energy expenditure in this study is probably explained
by a compensatory energy-conserving adaptation to
this vigorous training program leading to a reduction
in spontaneous physical activity and/or a reduction
in voluntary physical activities, similar to that
observed in several animal studies. Thus, it should
not automatically be assumed that energy require-
ments are elevated by participation in activity
programs, and the ultimate change in energy
requirements may be dictated by the intensity of the
training program and the net sum of change in
the individual components of energy expenditure.
An important area of research is to identify the
optimal program of exercise intervention in terms of
exercise mode, type, duration, and intensity that can
have optimal effects on all components of energy
balance.Energy requirements in pregnancy
and lactation
Pregnancy and lactation are two other examples in
which energy metabolism is altered in order to achieve
positive energy balance. The specifi c changes in
energy requirements during pregnancy are unclear
and the various factors affecting this change are
complex. Traditional government guidelines suggest
that energy requirements are raised by 1.3 MJ/day
during pregnancy. This fi gure is based on theoretical
calculations based on the energy accumulation asso-
ciated with pregnancy. However, these fi gures do not
include potential adaptations in either metabolic effi -
ciency or PAL during pregnancy. In a study that per-
formed measures in 12 women every 6 weeks during
pregnancy the average increase in total energy
expenditure was 1.1 MJ/day. The average energy cost
of pregnancy (change in total energy expenditure plus
change in energy storage) was 1.6 MJ/day. However,
there was considerable variation among the 12
subjects for the increase in average total energy
expenditure (264.6 kJ/day to 3.8 MJ/day) and the
average energy cost of pregnancy (147 kJ/day to
5.2 MJ/day).
Metabolic adaptations during lactation have been
examined in well-nourished women using the DLW
technique. The energy cost of lactation was calculated
to be 3.7 MJ/day. Just over half of this energy cost was
achieved by an increase in energy intake, while the
remainder was met by a decrease in physical activity
energy expenditure (3.2 MJ + 873.6 kJ/day at 8 weeks
of lactation compared with 3.9 + 1.1 MJ/day in the
same women prior to pregnancy).