FUNCTIONS OF THE NUTRIENTS 187 



animal's food is used for the production of energy, and it 

 is physiologically economical that this energy be largely 

 supplied by the non-nitrogenous nutrients, particularly 

 carbohydrates. If the proteins are broken down to 

 supply energy, there is always a definite proportion of 

 urea and uric acid residue that must be eliminated through 

 the kidneys. An unnecessarily heavy protein diet bur- 

 dens these organs and floods the system with these nitrog- 

 enous wastes. On the other hand, the carbohydrates, 

 when not stored as fat, are completely oxidized to the 

 simplest compounds, carbon dioxid and water, which are 

 eliminated through the lungs and skin, part of the water 

 so formed acting as a solvent of the urinary compounds. 

 Investigation seems to prove conclusively that the 

 animal body has a physiological preference for carbo- 

 hydrates over the fats or other nutrients as a source of 

 energy. After the free ingestion of sugar, the respiratory 

 quotient in certain experiments has become 1.00 when 

 just previously it was much less than 1.00. This demon- 

 strates that while fat was being oxidized before the sugar 

 was taken, the oxidation immediately changed wholly to 

 the sugar. This indicates the physiological adaptability 

 of starches and sugars for maintaining muscular activity. 

 271. Protein-sparers. The carbohydrates and fats 

 are sometimes classed as "protein-sparers." This means 

 that, with an adequate supply of these bodies in the food, 

 protein destruction may be reduced to the lowest pos- 

 sible limit. To illustrate, if a man doing moderate work 

 were maintaining an energy balance when eating of 

 digestible nutrients 218 grams of protein, 400 grams of 

 carbohydrates, and 56 grams of fat, and 100 grams of 

 digestible carbohydrates were added to the daily food, 

 approximately 100 grams of digestible protein could 



