In this connection, it is a significant fact that with _fasting_ animals, where the store of carbohydrate material is more or less used up, severe muscle work may be accomplished without any appreciable increase in nitrogen output, thus showing that proteid material is not involved and clearly pointing to fat as the source of the muscular energy. Thus, in an experiment referred to by Leathes, a dog on the sixth and seventh day of starvation was made to do work in a treadmill equivalent to climbing to a height of 1400 meters, yet the output of nitrogen was increased from six to only six and a half grams. Obviously, not much of the energy of this muscle work could have come from the breaking down of proteid, but it must have been derived mainly from the oxidation of fat. There is abundant evidence that fat can be used as a source of energy by muscles, as well as carbohydrates and proteids, and there is every reason for believing that the yield of work for a given amount of chemical energy in the form of fat is as good as in the case of either of the other two substances. In fact, the observations of Zuntz show that fat can be used just as economically by the body for muscle work as either carbohydrates or proteid. Thus, in one experiment,[47] he determined the oxygen-consumption and respiratory quotient in a man resting and working on three different diets--one principally fat, one principally carbohydrate, and the other principally proteid--and found that slightly less oxygen and energy were required to do work on the fat diet than on the others. This is clearly shown in the following table: [47] Quoted from Leathes: Problems in Animal Metabolism, p. 100. +------------+------------------+------------------+------+----------------+ | | | | | Per | | | Resting. | Working. |Kilo- | Kilogram-meter | | | | |gram- | of Work. | | Diet +--------+---------+--------+---------+meters+------+---------+ |Principally.| Oxygen |Respira- | Oxygen |Respira- | of |Oxygen|Calories.| | |Used per| tory |Used per| tory | Work | Used.| | | | Minute.|Quotient.| Minute.|Quotient.| Done.| | | +------------+--------+---------+--------+---------+------+------+---------+ | | c.c. | | c.c. | | | c.c. | | |Fat | 319 | 0.72 | 1029 | 0.72 | 354 | 2.01 | 9.39 | |Carbohydrate| 277 | 0.90 | 1029 | 0.90 | 346 | 2.17 | 10.41 | |Proteid | 306 | 0.80 | 1127 | 0.80 | 345 | 2.38 | 11.35 | +------------+--------+---------+--------+---------+------+------+---------+