Environmental Engineering Reference
In-Depth Information
a-linolenic acid, which must be digested preformed to
become precursors of prostaglandins (to regulate gastric
function and smooth-muscle activity and to release hor-
mones) and parts of cell membranes. Food lipids also
carry fat-soluble vitamins (A, D, E, and K).
The energy content of ingested food is measured accu-
rately after complete combustion yielding CO
2
and H
2
O.
Part of the ingested (gross) energy is lost in feces and in-
testinal gases (H
2
,CH
4
) produced by microbial fermen-
tation of unabsorbed carbohydrates in the colon. A small
part of digestible energy (DE) is voided in urine and
lost through the skin. A fraction of the remaining metab-
olizable energy (ME) is lost as the heat of microbial
fermentation and as the heat due to dietary-induced ther-
mogenesis (specific dynamic action of food, which ele-
vates the BMR of individuals on a mixed diet by about
10%). A very small fraction of net metabolizable energy
(NME) is lost to thermogenesis due to hormones, drugs,
or effects of cold or stimulants, leaving the net energy for
maintenance, which sustains basal metabolism and is
available for physical activity (fig. 5.1) (FAO 2003).
The gross energy (GE) of carbohydrates is 17.3 kJ/g.
These compounds have been the dominant energizers
of human evolution. In the modern world their main
sources are staple cereals (predominantly rice, wheat,
corn, and millet, eaten as whole grains or milled into
flours to make breads, pastas, and pastries), tubers (white
and sweet potatoes, cassava), leguminous grains (beans,
soybeans, peas, lentils), and sugar refined from cane and
beets. Complex carbohydrates are preferable to the sim-
pler varieties, especially when consumed as whole seeds
(beans, peas, lentils) or whole-grain products containing
indigestible dietary fiber whose regular intake (50-
100 g/day) is a critical part of proper nutrition (FNB
2005).
5.1 Energy cascade in human nutrition, from gross energy of
foodstuffs to net metabolizable energy. From FAO (2003).
The gross energy of proteins is 23 kJ/g. The highest
concentration in plant foods is found in legumes (partic-
ularly soybeans), cereals, and nuts; tubers, vegetables,
and fruits have only trace amounts. Fats have a gross
energy more than twice as high as carbohydrates and
proteins (39 kJ/g) and are present in relatively high con-
centrations in plant seeds, fatty meat, fish, and dairy
products. The average consumption of fats in preindus-
trial societies was very limited. Now they constitute as
much as 40% of total food energy intake in rich nations,
a dietary excess and a mark of unbalanced nutrition.
Finally, alcohol (ethanol) is a peculiar energy source of
