Production of Heat and Work. 87 



production of muscular energy, then the body fat is next dra'-^Ti 

 upon for this purpose. If this is insufficient in amount or is much 

 diminished by continued work, then as the last resort the protein 

 tissues or muscles will be called upon to furnish the needed energy. 



110. Relative value of nutrients.— Investigations by Zuntz and his 

 associates have clearly shown that the value of each of the differ- 

 ent classes of food nutrients for the production of work depends 

 upon the total energy it contains. In one experiment^ the diet of 

 a man turning a wheel consisted, during separate periods, chiefly 

 of either fat, carbohydrates, or protein. For 1 kgm. of work the 

 following amounts of energy were expended : 



Energy expended 

 Period Nutrient eaten per kgm. of work 



Cal. 



I Protein 11.92 



II Carbohydrate 11.54 



III Fat 9.53 



IV Protein 10.78 



V Fat. 9.25 



It is shown that approximately the same fuel rations were re- 

 quired to produce a given amount of work whether the fuel was 

 protein, carbohydrates, or fat. It will be noticed that the energy 

 expended was less in the last trials on account of the proficiency 

 which had been attained in the work. 



111. Energy requirements for work.— The total energy required 

 to produce a certain amount of external work depends upon many 

 factors. Experiments by Zuntz- with the horse show that an in- 

 crease in the speed at which work is performed results in an in- 

 creased expenditure of energy per unit of work. Practice in per- 

 forming a certain work lessens the energy expenditure for that par- 

 ticular form of labor. In experiments upon himself Gruber^ found 

 that in climbing a tower the amount of carbon dioxid exhaled and 

 hence the energy expended was decreased by 20 per ct. after train- 

 ing for 2 weeks. In experiments by Lowy* on himself, and by 

 Zuntz^ upon horses, fatigue caused an increase of from 14 to 41 per 

 ct. in the amount of energy expended in performing a given amount 

 of work. This increased expenditure of energy is largely due to 

 the fact that with increasing fatigue the muscles normally called 

 into use, which are the most efficient in performing the given work, 



^ Arch. Physiol. (Pfliiger), 83, 1901, p. 564. 



= Landw. .Jahrb., 27, 1898, Sup. III. 



' Ztschr. Biol., 38, 1891, p. 466. 



♦ Arch. Physiol. (Pfliiger), 49, 1891, p. 413. 



^ Loc. eit. 



