120 METABOLISM AND NUTRITION 



In old age the metabolism estimated by the square meter of surface is 

 appreciably less than in middle life. While the C0 2 -excretion in men be- 

 tween the ages of twenty-two and fifty-six was found to be 11.16 g. per square 

 meter per hour, between the ages of seventy and seventy-seven years it was 

 only 9.18 g. ; for women between the ages of seventeen and forty it was found 

 to be 11.75 g., between the ages of seventy-one and eighty-six, 9.79 g. (Magnus- 

 Levy and Falk). The results of Ekholm on metabolism in the aged agree 

 perfectly with these. This author found the mean of ten experiments on in- 

 dividuals between sixty-eight and eighty-one years of age to be 902 Cal. per 

 square meter of body surface per day, while in resting men of middle age 

 it amounts to 1,071 Cal. 



We can say, therefore, that the age of the individual is one of the factors 

 determining the intensity of metabolism, it being greater per square meter of 

 body surface during the period of growth (with the exception of infancy) 

 than in middle life, and greater in middle life than in old age. 



8. RETENTION OF PROTEID IN THE BODY 



In our study of the decomposition of proteid in the body, supplied with 

 meat alone, we found that but a few days elapse before the body places itself 

 in N-equilibrium. The retention of proteid with such a diet continues there- 

 fore for only a short time and cannot reach any considerable amount. The 

 greatest quantity of " flesh " which Voit was able to lay-on in dogs kept on a 

 pure meat diet was 1,365 g. (= 46.5 g. N). On the average he was unable 

 to bring about a deposit of more than 500 g. (= 17 g. N). With meat alone 

 one may keep an animal in a uniform condition of proteid nutrition which 

 has been attained previously in some other way, but once it is lost he cannot 

 restore this condition with an exclusively proteid diet, nor bring about a 

 " deposit of flesh." 



From his experiments on an exclusive meat diet Voit drew the further 

 conclusion that under circumstances otherwise the same, proteid is stored to 

 a greater extent and for a longer time before N-equilibrium sets in, if the 

 animal be already fat than if he be lean. In other words, the fat already 

 deposited in the body saves the proteid fed from being destroyed and thereby 

 permits a greater retention. 



We have already seen that the N-output equals the N-intake even with a 

 mixed diet composed of proteid and nonnitrogenous substances. But the 

 destruction of proteid is reduced to a certain extent by the presence of these 

 N-free substances; hence we might expect that a more extensive storage of 

 proteid continuing for a longer time could be brought about by feeding non- 

 nitrogenous substances with proteid. 



Volt's many experiments with such combinations show this to be true. It 

 is evident from these same experiments, however, that the saving of " flesh " is 

 not much greater with a rich supply of meat than with a small supply. With 

 2,000 g. of meat and 250 g. of fat the daily sparing was 186 g.; with 1,000 g. 

 meat and 300 g. fat it was 167 g., while with 1,800 g. meat and 250 g. fat it 

 was 140 g., and with 1,500 g. meat and 150 g. fat, only 70 g. It is impossible to 



