HISTORY OF DIFFERENT FOODSTUFFS IN BODY 1041 



to the proteins, does not serve as a permanent building stone, but 

 merely as a temporary acquisition which is destined to yield energy. 



Work and heat are derived in largest amounts from the muscles, 

 and we have seen that their contractions require the presence of glyco- 

 gen and sugar, and that each gram of sugar furnishes as much as four 

 calories of the latter form of energy. Considering the preponderating 

 mass of muscle-tissue and the high heat-value of sugar, it may, therefore, 

 be concluded that the oxidation of this substance constitutes a safety 

 mechanism by means of which the body is enabled to protect its real 

 building stones, the proteins. Fat plays a similar role, but we shall 

 see later on that it occupies an intermediate position and serves merely 

 as an accessory means of safeguarding the protein substratum of the 

 body. Thus, an animal may be retained in nitrogen-equilibrium if it 

 continues to ingest a small amount of protein material to make up its 

 ordinary loss in tissue-proteins, and if it continues to take in a suffi- 

 cient quantity of carbohydrates (or fats) to make up for the energy 

 requirements of the body. Inasmuch as the carbohydrates merely 

 play the part of oxidizing substances and sparers of the tissue-proteins, 

 it will be seen that they alone cannot keep the body in nitrogen- 

 equilibrium. Consequently, an animal fed exclusively on carbohy- 

 drate food must eventually lose its tissue-proteins, and starve to death 

 in spite of its abundant intake of carbohydrates. On the other hand, 

 if an animal is in nitrogen-equilibrium to begin with, the abundant inges- 

 tion of carbohydrates first gives rise to a storage of glycogen and subse- 

 quently to a synthesis of the superfluous sugar into fat. The latter is 

 held in reserve as an accessory substance to be employed for future 

 oxidations. This "carbohydrate-fat" differs somewhat in its consist- 

 ency from the ordinary tissue-fat. 



The final product of the oxidation of sugar is carbon dioxid and 

 water, and its principal excretory channel the lungs. Thus, we find 

 that the increased output of energy which accompanies muscular 

 exercise is characterized by a greater outgo of carbon dioxid and a 

 greater consumption of oxygen. This respiratory change immediately 

 suggests an increased metabolism of the carbohydrates and fats. But, 

 while the final stage of the oxidation of the carbohydrates is quite evi- 

 dent, much diversity of opinion prevails regarding the intermediary 

 transformations of this foodstuff. The initial change is a hydrolytic 

 cleavage which liberates some chemical energy, and the final stage an 

 oxidation and evolution of that large amount of energy, which after 

 all is the purpose of the reduction of this foodstuff. As an interme- 

 diary stage is usually mentioned the production of lactic acid, through 

 the preliminary formation of glyceric aldehyde and methylglyoxal. 

 It has been shown that lactic acid is present in the body chiefly as the 

 dextro-rotatory variety or sarcolactic acid, and as the optically inactive 

 variety. Whether all of this lactic acid is derived from the sugars is 

 still in doubt, although it must be admitted that this is its principal 

 source. 



66 



