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age. Average percentages of fat in total body composition
are, newborns, 14%; young Western men, 15%; men
aged 60, 23%; young Western women, 27%; and women
aged 60, 36%. Women store fat at a lifetime average rate
of 0.3-0.4 kg/a, men at an average 0.15-0.25 kg/a. In
2005 the global anthropomass of 6.5 billion people
equaled about 2 EJ.
The insidious storage of fat—at about 6-16 MJ/a
(0.2-0.5 W) drawing away no more than 0.5% of daily
food energy input—has its counterpart in the loss of
lean body mass. Muscles compose only about 20% of a
newborn's mass but average 52% of weight in young
men and 40% in young women. After the third decade,
the male's greater lean mass is lost more rapidly (2-3
kg/decade) than the female's (@1.5 kg/decade), and
autopsies show people over 70 years with about 40% less
muscle than they had as young adults. This loss repre-
sents a change 1 OM smaller than adipose tissue gain, as
little as 600 kJ/a, or 20 mW, but it is an inexorable sign
of aging even for those who avoided a significant fat
increase.
Rapid buildup of the metabolizing lean body mass in
childhood and adolescence and its later gradual decline
are reflected in changed BMR. The extraordinarily high
human encephalization quotient (actual/expected brain
mass for body weight) is slightly over 6, compared to
values between 2 and 3.5 for hominids and primates. At
about 350 g, the neonate brain is twice as large as that of
a newborn chimpanzee, and by age 5 it becomes more
than three times as massive as the brain of our closest pri-
mate species (Foley and Lee 1991). As a result, the new-
born brain (10% of total body weight) accounts for about
50% of BMR (fig. 5.2). The adult brain (2% of body
mass) claims about 16% of BMR, and yet there is no sig-
5.2
Partitioning of basal metabolic rates in adults and infants.
Based on data in FAO (1985).
nificant correlation between relative BMR and relative
brain size in humans and other encephalized mammals.
We do not have more of metabolically expensive
tissues (internal organs and muscles) than would be
expected for a primate of our size. Aiello and Wheeler
(1995) suggested that the only way to support larger
brains without raising the overall metabolic rate is to re-
duce the size of another major internal organ. With rela-
tively little room left to reduce the mass of liver, heart,
and kidneys, the gastrointestinal tract was the only meta-
bolically expensive tissue whose size could be reduced
with a better diet. The expensive-tissue hypothesis (well-
supported by relative organ sizes) thus maintains that a
higher-quality diet (richer in animal foods) allowed for
the development of a relatively smaller gut and freed
more energy for a larger brain.
As with animals, human BMRs have been measured
mostly by converting the oxygen uptake into energy
equivalents (average 20 kJ/L). Systematic larger-scale
measurements of BMRs began in Western Europe and
North America before World War I, and by the mid-
1980s about 11,000 individual results were available for
healthy individuals of both sexes and all ages (Schofield,
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