108 PHYSIOLOGY: F. G. BENEDICT 
This, of itself, points strongly towards distinct differences in the intensity 
of cellular activity. 
While the larger proportion of individuals studied were in youth, 
i.e., 20 to 30 years of age, a few were under 17 and a number over 40. 
On studying the plots showing the variations with normal individuals, 
both for men and women, a distinct tendency may be noted for the older 
people of both sexes to have a somewhat decreased metabolism. The evi- 
dence is equally as clear that in youth the metabolism is considerably 
increased, thus pointing towards an increased cellular activity in early 
youth and a decreased cellular activity or possible atrophy of active 
protoplasmic tissue with increasing age. 
One of the most noticeable factors influencing metabolism is that of 
sleep. It has been commonly assumed that sleep per se does not affect 
metaboHsm, but it has been found in a long study of a fasting man that 
there were numerous metabolic planes during the day, showing that the 
stimulus to cellular activity must have varied considerably. With this 
fasting man neither the mass of protoplasmic tissue nor the surface area 
of the body could have altered materially in the course of 24 hours, yet 
we find that if we give a value of 100 to the basal metabolism during the 
night, when the subject was sound asleep, the value when the subject 
was lying awake in the morning, with complete muscular repose, would 
be 114, and in the late afternoon under the same conditions the value 
would be 122. 
Our laboratory data include experiments with a considerable number 
of normal men covering several months, and in some cases, several 
years. A study of these data shows that the metabolism of 35 subjects 
on whom experiments were made 5 days or more apart and, on the 
average, 'several months apart, varied not far from 14%, although 
in all cases the subject was in complete muscular repose and in the 
post absorptive condition. Since during this period there was no ma- 
terial alteration in the body weight and consequently in the body surface, 
the variation in the metaboKsm must be ascribed to a difference in the 
stimulus to cellular activity. 
In the fasting experiment not only was there a difference in the meta- 
bolic level noted at different times of the day due to the condition of being 
awake and asleep, and in the late afternoon after the experimental 
program of the day, but, as the 31-day fast progressed, the heat produc- 
tion per square meter of body surface varied from 859 calories on the 
first fasting day to 668 calories on the twenty- third fasting day. Al- 
though the loss in body weight was material, apparently the skin shrank 
