THE PROTEIN REQUIREMENTS OF MANKIND 105 



during starvation. It was thought that in the amount of 

 nitrogen excreted during fasting experiments there would be 

 obtained a measure of the quantity of protein necessary to make 

 good the loss from tissue disintegration. However, it is found 

 that excretion of nitrogen during hunger or fasting is modified 

 by a variety of circumstances, particularly the amount of 

 "labile" or circulating proteins and the condition of the body 

 with regard to presence of fat. 



For the first few days after the withdrawal of food the excre- 

 tion of nitrogen is directly dependent upon the protein intake 

 and decomposition during the preceding days. The amount of 

 labile protein present in the body varies with the level of protein 

 metabolism during the period immediately before the fast com- 

 menced. If this level beforehand is high, the nitrogen output 

 during the first few days is large ; and if low, the output is corre- 

 spondingly small. 



According to Voit, about 8 per cent, of the stable protein 

 and 70 per cent, of the labile protein of the organism are decom- 

 posed during starvation, the latter being completely used up 

 within three days. From the fourth and fifth day onwards only 

 stable tissue protein remains for oxidation, the labile having 

 disappeared. Similarly with regard to the glycogen stored in 

 the body, which can act as a protein sparer. It is practically 

 exhausted by the third day of fasting, and, as a result, the 

 protein metabolism, as measured by the nitrogen excretion, 

 shows a temporary rise. After some initial variations, the 

 nitrogenous katabolism in man gradually decreases during the 

 course of starvation. In the first ten days the nitrogen excre- 

 tion seldom falls below 10 grammes, but later on falls still 

 lower.* 



An average of the results obtained from different fasting 

 experiments shows that the nitrogen excreted in the urine from 

 the fifth day of starvation onwards is about 5 grammes per day. 

 This amount is, however, not sufficient to supply the energy 

 required by the body. 



It is estimated that only about 10 per cent, of the total energy 

 is derived from tissue disintegration, the remaining 90 per cent, 

 being supplied by the large store of fat present in the body when 

 in an ordinary condition of nutrition. So long as this store of 

 fat lasts the protein tissues will be drawn on mainly to meet the 

 * v. Noorden, " Physiology of Metabolism." 



