Changes in Body-Weight. 109 



when the subjects were lying quietly at rest, and for the most part were in 

 profound sleep. Since the bodily activities were at the lowest point in this 

 period, it is highly probable that experiments with the same individual, or 

 experiments with different individuals can better be compared during this 

 period than during any other period in the day. Consequently, it seems advis- 

 able to have two separate discussions for each factor of metabolism, one, for 

 the metabolism during sleep, and the other, the metabolism when awake. This 

 method of treating the subject is followed in all the discussions in this report, 

 with the exception of the discussion of the water vaporized from lungs and skin. 



Changes in Body-Weight. 



To the layman one of the most obvious indices of the value of a given diet 

 would be the changes in body-weight, for it is commonly believed that any 

 increase in body-weight indicates ample, if not excessive, nourishment, and 

 that a decrease is evidence of insufficient nourishment. For experiments of 

 long duration, such as are commonly made on domestic animals when feeding 

 or fattening for market, this is a remarkably good index. With man, however, 

 the experiments must of necessity be of short duration, since a routine diet 

 can not be adhered to for so long a time as with animals, and fluctuations in 

 body-weight are, therefore, by no means a proper index of gain or loss of 

 body-material. 



CHANGE IN BODY-WEIGHT AS INDEX OF BODY CONDITION. 



Factors involved in change of body-weight. An increase in body-weight is 

 a resultant of a number of factors. There may have been actual additions to 

 protein, fat, carbohydrate, and water, these four being the principal ingredients 

 of the body, or there may have been an increase in two or three of these com- 

 pounds with an actual decrease in the others, and there may have been losses 

 from three of them compensated by a gain in the fourth. It is evident, there- 

 fore, that a gain in body- weight is of real service only when it indicates increase 

 of material other than water, and yet a large majority of the fluctuations in 

 the body-weight can be attributed to material changes in the water-content of 

 the body. When it is considered that some 60 per cent of the total weight of 

 the body is water, and this water is easily lessened or increased, it can be seen 

 that the gain or loss of a few pounds by the body may be very largely due to 

 fluctuations in water-content and in no wise gives a true idea of the addition 

 to or loss from the store of organic body-material. This fact has frequently 

 been the source of considerable error in experiments of short duration, in that 

 a diet manifestly inadequate for maintenance has actually been partaken of for 

 3 or 4 days while the body-weight of the subject of the experiment has remained 

 practically unchanged. As a result of this apparent constancy in body-weight, 

 a diet obviously deficient has been thought ample for the needs of the body. 



