Environmental Engineering Reference
In-Depth Information
the basic energy sources of human nutrition. Then I sur-
vey the basics of human thermoregulation, the energy
needs of common work and leisure activities, and the
physical limits of human performance. This chapter
closes with a concise review of humans as simple hetero-
trophs, foraging societies whose only source of mechani-
cal energy was human muscles.
hydrates are ingested as polysaccharides (starches, glu-
cose polymers) and as simple sugars. These include two
monosaccharides (fructose and glucose present in plants)
and three disaccharides: lactose, natural milk sugar; su-
crose in refined sugar; and maltose, produced by enzy-
matic degradation of starch and used as a sweetener.
Proximate analysis of foods commonly finds the energy
value of carbohydrates indirectly as a residual, after sub-
tracting the analyzed amounts of proteins, lipids, water,
and ash.
Dietary proteins have much more important qualita-
tive roles than supplying energy. Human growth is im-
possible without concurrent digestion of nine essential
amino acids in infants, eight in children and adults (Smil
2000b). Amino acids contain 15%-18% of N; 16% is
commonly used as the average value. These acids are pre-
cursors of structural and functional proteins in muscles,
bones, and internal organs; in enzymes, hormones, neu-
rotransmitters, antibodies; and in metabolically active
compounds. Additional N is also needed to replace small
but constant protein losses caused by the breakdown and
reutilization of the compounds, excretions of N (in urine,
feces, and sweat), shedding of skin, and cutting of hair
and nails (Pellett 1990).
Unavoidable (obligatory) N losses through excretion
remain fairly constant in adulthood (41-69 mg/kg,
average 53 mg/kg); other losses add about 8 mg/kg.
Complete proteins, with more than adequate shares of
all essential amino acids, are available only in foods of an-
imal origin (meats, fish, eggs, dairy products) and mush-
rooms, whereas all plant foods have incomplete proteins,
with one or more amino acids relatively deficient (cereal
grains are deficient in lysine, leguminous seeds in methio-
nine and cysteine). Lipids contain two essential fatty acids
that must be present in all healthy diets, linoleic acid and
5.1 Energy Sources and Basal Metabolism
Like all other heterotrophs, humans must consume three
classes of nutrients available in plant and animal foods.
These essential nutrients include preformed organic com-
pounds of the three kinds of macronutrients as well as
two distinct kinds of micronutrients, vitamins and min-
eral elements. Digested nutrients are absorbed in the
small intestine and distributed by the blood in order to
reproduce, grow, be active, and adapt. Energy-yielding
macronutrients—carbohydrates, proteins, and lipids—
are consumed at rates of 10
1
(proteins and lipids) to
10
2
(carbohydrates) g/day. The intakes of micronu-
trients range from 2.5 g/day for K and Na to a mere 3
mg/day for vitamin B-12, but their adequate presence is
imperative for healthy growth and activity. Deficiencies
of Ca, P, Mg, and Zn impede normal growth, and inad-
equate vitamin intakes disrupt the functioning of essen-
tial metabolic and maintenance processes, leading, for
instance, to gastrointestinal disturbances with folacin
shortages and epithelial hemorrhaging with low ascorbic
acid (best known as scurvy, common on long sea voyages
before the late eighteenth century).
Food energy requirements are not simply a matter of
sufficient quantities of macronutrients. Most of the bio-
sphere's carbohydrates (lignin, cellulose, and hemicellu-
lose, including those in cell walls of ingested plant
foods) cannot be digested by humans. Digestible carbo-
