Environmental Engineering Reference
In-Depth Information
relatively high density; its gross energy is 29.3 kJ/g. It
can be utilized only by the liver at hourly rates usually
not surpassing 0.1 g/kg body weight (@190 kJ/h, or
53 W, for a 65-kg person).
Because ME or NME is appreciably lower than GE, it
is necessary to use appropriate food energy conversion
factors to express the actual energy available to humans
(FAO 2003). The oldest solution (and still the standard)
is the Atwater general factor system, introduced by W. O.
Atwater and his collaborators at the U.S. Department
of Agriculture in the late 1890s (Atwater and Woods
1896). Atwater factors correct for losses from the three
macronutrients during digestion, absorption, and urinary
excretion, and do not distinguish among foodstuffs.
Their precise values are 16.7 kJ/g for carbohydrates and
proteins, 37.3 kJ/g for lipids, and 28.9 kJ/g for ethanol,
but these numbers are commonly rounded to, respec-
tively, 17, 37, and 29 kJ/g. Food composition tables
consider only actually available energy, which means that
digestion in healthy people on balanced diets has a very
high efficiency: 99% for carbohydrates and ethanol, 95%
for fats, and 92% for proteins (more than 20% of dietary
protein, about 5.2 kJ/g, is lost through urine).
The extensive general factor systems modifies and
extends Atwater factors by adding separate values for
monosaccharides (16 kJ/g) and for dietary fiber (8 kJ/
g, assuming@70% of it is fermentable). The Atwater spe-
cific factor system refines the basic approach by assigning
different, food-specific factors to macronutrients. The
differences arise from different amino acid or fatty acid
shares as well as from the way individual foodstuffs have
been processed. Specific Atwater factors for protein range
from 17 kJ/g for high-extraction wheat flour to 10.2
kJ/g for vegetables; for carbohydrates, from 17.4 kJ/g
for white rice to 10.4 kJ/g for lemons; and for lipids,
from 37.7 kJ/g for eggs and meat to 35 kJ/g for grain
products (Merrill and Watt 1973).
All these conversions refer to ME, but because not all
of that is available for the production of ATP to energize
metabolism, yet another conversion is made using NME
factors. There is no difference between ME and NME
factors for carbohydrates. The NME vs. ME values (in
kJ/g) are, for ethanol, 26 vs. 29; for dietary fiber, 6 vs.
8; and of great practical significance, for protein, 13 vs.
17, a 24% reduction (FAO 2003). Using different meth-
ods to compare the energy content of actual diets shows
that modified Atwater factors are reduced from standard
ME valuations by no more than 2%-5% and that NME
factors add another 2%-5%; consequently, the total dif-
ference between the classic Atwater values and the com-
bined ME and NME factors can range from minor, about
7.5% for the UK urban population, to considerable, with
the highest published value of nearly 20% for Australian
Aborigines (FAO 2003).
Basal metabolism accounts for the largest share of daily
energy needs in all but highly active individuals. It ener-
gizes the body's essential life processes, including cell
functions, the synthesis of enzymes, the constant work
of internal organs (heartbeat, breathing, peristalsis), and
the maintenance of body temperature (homeothermy).
The basal metabolic rate (BMR) in humans is measured
in the supine position in a thermoneutral environment
(no heat-generating or heat-dissipating responses) and
after 10-12 h fasting (to eliminate metabolic response
to food, which peaks @1 h after a meal and can boost
BMR up to 10%). The share of metabolizing tissue is
highly age-dependent, and gender is a key determinant
of fat (adipose tissue), which accounts for most of human
dimorphism (Bailey 1982). The differences in body
composition are very small at birth but increase with
