200 THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [VI, II
There is current in animal nutrition a method of assessing the energy value of
rations that is far in advance of any method used in human nutrition. According
to this method, introduced by Armsby some thirty years ago, the final value of a
ration as a source of energy in metabolism is obtained by deducting from its gross
energy (heat of combustion) all the losses of energy incident to its utilization.
The metabolizable energy is the gross energy minus indigestible energy (gross
energy of faeces and intestinal gases) and unoxidized energy (gross energy of urine).
The final, or net energy value is equal to the metabolizable energy minus the increase
in the heat production incident to the consumption and utilization of the ration.
This latter increment consists largely (in farm animals) or entirely (in humans) of
the " specific dynamic effect " of food. The net energy of a unit weight of a ration,
expressed as a percentage of the contained metabolizable energy, measures the net
availability of the latter.
With these definitions in mind, the first implication of the above defined
conception of nutritive balance in a ration or diet is that, except for differences in
digestibility, the net energy value of all perfectly balanced rations is the same under
the same conditions of feeding, or, somewhat more precisely, the net availability
of the metabolizable energy of all perfectly balanced rations is maximal for any
imposed conditions of feeding. When the net energy conception was developed
by Armsby, it was his idea that each food material had its own fixed characteristic
net energy value and that the net energy value of a ration was the weighted mean
of the net energy values of the constituent foods. All his investigations at Penn-
sylvania State College were based upon this simple hypothesis, which was not
inconsistent with any of the theories of energy utilization prevalent at that time.
Forbes and his associates have been impelled to depart from this hypothesis of
Armsby, first, because of experimental evidence to the effect that the net energy
value of a ration or feed is not constant, but depends upon the conditions of feed-
ing and second, because other evidence indicated that the net energy value of a
ration bore no simple relation to the net energy value of the costituent foods.
However, Forbes' recently announced "law of maximum normal nutritive value," 2
although it advocates the use of completely balanced rations in determinations of
net energy values, does not state nor imply that the net availability of the meta-
bolizable energy of such rations will be maximal and identical. In fact, the state-
ment that " an individual foodstuff expresses its normal and most characteristic
nutritive value
only as it is a part of a ration which is qualitatively com-
plete and quantitatively sufficient .. " seems opposed to this deduction, which, if
true, would lead one to suppose that the most characteristic nutritive value of a food
would be observed only when it is fed alone. When properly balanced with other
foods, its distinctive nutritive properties would be entirely submerged in a resultant
optimal combination that would be no better nor worse than that of many other
2
Science. 77 : 306, 1933.