section 5.5
Energy Metabolism
77
the kinetic theory of reaction rates. From the Arrhenius
equation, it is apparent that the larger the value of
Es
(or
Ea),
the smaller the value of
k.
Hence, the basis for
kinetic irreversibility is the same as that for thermody-
namic irreversibility; the more energy a process takes (the
more positive the value of AG or
Ea),
the less likely the
reaction is to occur.
Although values for A are not well understood, the
Arrhenius equation can, on rearrangement, be used to
calculate
Es:
Es
—R
In
k =
------
R
In A.
T
The slope of a plot o f
—R
In
k
versus (1/7/) equals
Es,
and
A
Ha = Es —
RT.
5.5 Energy Metabolism
In order to carry out the body’s essential functions (e.g.,
growth and repair, pregnancy, lactation, physical activ-
ity, maintenance of body temperature), food must be con-
sumed and utilized and body constituents synthesized.
The term “metabolism” encompasses the numerous chem-
ical transformations that occur within the human body.
Metabolism comprises
anabolism
and
catabolism.
An-
abolism is concerned with the synthesis of new molecules,
usually larger than the reactants, and is an energy-requiring
process. Catabolism is concerned with degradation pro-
cesses, usually the breaking down of large molecules into
smaller ones, and is an energy-yielding process.
Inter-
mediary metabolism
refers to all changes that occur in a
food substance beginning with absorption and ending with
excretion.
Two main sources of energy for metabolism are carbo-
hydrates and fats (lipids). Proteins have less importance as
an energy supplier. Knowledge of the total energy (A
H)
content of the major body fuels is necessary to understand
how energy requirements are met by different fuels.
The reactions for representative food fuels, glucose and
palmitic acid, will be used to indicate differences in A
H
values. The subscripts s,
£,
and g (solid, liquid, and gas, re-
spectively) indicate the phase of the material under
1
atm,
the pressure at which the reaction is carried out, and at
the given temperature. The A
H°
values given are
stan-
dard heats of formation,
since all reactants are in their
standard (natural) state for the temperature given.
Oxidation of 1 mol of glucose
(a carbohydrate) to
carbon dioxide and water:
C6H120 6(S) + 602(g) —►
6C02(g) + 6H20(£)
At 20°C,
AH
= —673 kcal/mol or —2816 kJ/mol.
Oxidation of 1 mol of palmitic acid (a fatty acid) to
carbon dioxide and water:
Ci6H3
2
0 2(S) + 2302(g)—>16C02(g) + 16H20(i)
At 20°C,
A
H°
= 2380 kcal/mol
or
-9.96 MJ/mol
(1 MJ = 1 megajoule = 1000 kJ).
The average
A H
values in kilocalories per gram are
as follows: carbohydrate, 4.1; lipid, 9.4; and protein, 5.6.
These values vary within each class of food depending
on chemical structure. For example, the value for starch
is 4.1 kcal/g, whereas for glucose it is 3.8. Furthermore,
in humans the end product of protein metabolism is urea
instead of C 0
2
and nitrogen, which are the end products
obtained after complete oxidation in the bomb calorime-
ter. Therefore, a realistic estimate of energy derived from
protein in the body, taking into account incomplete oxida-
tion and specific dynamic action, is 4.1 kcal/g. In human
nutrition, the commonly used values for energy yield in
kilocalories (or kilojoules) per gram are as follows: car-
bohydrate, 4 (16.7); lipid, 9 (37.7); and protein, 4 (16.7).
During the metabolism of food substances in the body,
oxygen is consumed and C 0
2
is produced. The molar ra-
tio of C 0
2
produced to oxygen consumed is known as
the
respiratory quotient
(RQ) and is characteristic of a
given substrate. For example, in the complete oxidation of
glucose presented above,
volume of C 0
2
produced
6C0
2
RQ = ------------------— ----------= -------
-
= 1.0.
volume of
0
2
consumed
602
For the corresponding oxidation of palmitic acid,
RQ =
16C02
2302
= 0.7.
The RQ for protein is difficult to measure because it is not
oxidized completely in the body and part of its carbon, hy-
drogen, and oxygen is lost in the urine and feces. However,
the RQ is usually taken to be 0.8. The measurement of RQ
thus provides a means for assessing the type of food that
is being metabolized. An overall RQ of 0.85 is obtained in
a normal adult who consumes a mixed diet. This value is
enhanced by carbohydrate feeding and decreased by lipid
feeding. Heat production can be calculated from oxygen
consumption and RQ as follows. For glucose oxidation:
C6Hi20 6 + 60 2 = 6C02 + 6H20
180 g
192 g
(134L)
Since the energy yield of 1 g of glucose is 3.8 kcal, 180 g
of glucose yields 180 x 3.8 or 684 kcal. The energy yield
per liter of oxygen consumed equals (180 x 3.8)/134 =
5.1 kcal (21.3 kJ). Again, for palmitate oxidation,
Ci6H3
2
0
2
+ 2302
256 g
236 g
16C02 + 16H20.
