458 ANIMAL PHYSIOLOGY. 
lean as upon twice the quantity of lean flesh alone. It is plain, 
then, that the metabolism is actually slowed by a fatty diet. 
When an animal is given but little fat, none whatever is laid 
up, but all the carbon of the fat can be accounted for in the 
excreta, chiefly as carbonic anhydride. Again, the fatty por- 
tion remaining constant, it has been found that increasing the 
proteid leads not to a storage of the_carbon of the proteid ex- 
cess, but to an increased consumption of this element. It is 
then possible to understand how excessive consumption of pro- 
teids may lead, as seems to be the case, to the disappearance of 
fat and loss of weight, so that a proteid diet increases not only 
nitrogenous but non-nitrogenous metabolism. That carbohy- 
drates mixed with a due proportion of the other constituents 
of a diet do increase fat formation is well established; though 
there is no equally well-grounded explanation of how this is 
accomplished. Upon the whole, it seems most likely that fat 
can be directly formed from carbohydrates, or, at all events, 
that they directly give rise to fat if they are not converted 
themselves into that substance. 
Comparative.—It is found that there are relations between 
the food used and the quantity of carbonic dioxide expelled 
which are instructive. The formula following show the amount 
of oxygen necessary to convert a starch and a fat into carbonic 
anhydride and water: 
1. CHO; + On = 6(CO.) + 5(H.0). 
2. CsrHisOc + Ow = 57(COs) + 52(H,0). 
It will be observed that in the first case the oxygen used to 
oxidize the starch has all reappeared as CO,, while in the sec- 
ond only 114 parts out of 160 so reappear. As a matter of fact, 
more of the oxygen used does in herbivora reappear as COs, 
and less as water, while the reverse holds for the carnivora, the 
proportion being, it is estimated, as from 90 to 60 per cent. 
This is to be explained by the character of the food in each 
instance, for this relation no longer holds during fasting, when 
the herbivorous animal becomes carnivorous in the sense that 
it consumes its own tissues. 
To most persons the carbohydrates are more digestible than 
fats, though they have less potential energy, as will shortly 
be seen. 
The Effects of Salts, Water, etc., in the Diet—We have already 
considered how salts in the form of condiments may beneficially 
influence the digestion; but, when we come to inquire as to the 
