NET AVAILABLE ENERGY—MAINTENANCE. 429 
It scarcely seems possible to draw any well-founded conclusions 
regarding the net availability of the several nutrients from such 
widely divergent results as those tabulated above, even if the ex- 
treme and obviously incorrect figures be discarded. Two things, 
however, seem worthy of remark. 
First, in but few cases does the net availability of the food reach 
100 per cent., and most of those results relate to cane-sugar or rham- 
nose; that is, to cases in which some of the gain of carbon which is 
computed as fat may have been in the form of a carbohydrate. It 
would seem fairly safe to conclude, therefore, that no such complete 
substitution of the heat resulting from digestive work for that re- 
sulting from the general metabolism took place as Rubner’s hypoth- 
esis supposes. Apparently, under the conditions of these experi- 
ments, there was, in most cases at least, a material loss of energy 
in digestive work. 
Second, there is no clear indication of a smaller loss of energy 
below than above the maintenance ration, although the wide range 
of the results renders a definite conclusion upon this point hazardous. 
This question, however, may be more properly considered in con- 
nection with a study of the utilization of the net available energy of 
the food. 
Finally, it is to be said that if the validity of the conception of 
a critical amount of food, as developed on p. 409, be admitted— 
that is, of an amount of food below which the heat resulting from 
the work of digestion and assimilation is substituted for that pro- 
duced by the general metabolism, while above it no such substtu- 
tion takes place—a very important element is lacking for the 
interpretation of the above experiments, except, perhaps, those on 
timothy hay, in which the uniformity of the results with varying 
amounts seems to show clearly that all the rations supplied more 
than the critical amount of food. If that conception is correct, 
to determine the real availability of the energy of a food it is 
necessary to compare the effects of two quantities both of which 
are greater than the critical amount. On the other hand, the 
complete substitution of energy supposed by Rubner could only 
be demonstrated by comparing quantities less than the critical 
amount, while a comparison of quantities below the latter 
amount (including, of course, fasting) with those exceeding it 
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