NUTKITION INVESTIGATIONS AND THEIR RESULTS. 361 



as a result of studies carried on by different investigators, and data 

 are now so abundant that studies of proximate composition of food 

 materials no longer constitute one of the lines of work followed in the 

 cooperative nutrition investigations of the Office. Dietary studies — 

 that is, studies of the kinds and amount of food purchased, eaten, and 

 wasted— were early recognized as of great importance, and a large 

 number have been made in private families, schools, colleges, public 

 institutions, and elsewhere under a variety of conditions and in widely 

 separated regions. 



No matter what its composition, food is of no use to the body 

 unless it is digested, and it is natural that experiments should have 

 been undertaken with a variety of food materials to learn how 

 thoroughly they were assimilated by the body and to ascertain the 

 effect of various methods of preparation and combination upon 

 thoroughness of digestion. ' Furthermore, it is supposable that the 

 occupation in which the subject is engaged, whether active or seden- 

 tary, may have an influence upon the work of the digestive tract, 

 and this question has also been studied. Many questions regarding 

 the thoroughness of assimilation may be investigated with the aid 

 of ferments under conditions which approximate those in the body, 

 and a large number of such artificial digestion experiments have been 

 carried on, particularly in studying ease and rapidity of digestion, a 

 question which is very different from thoroughness of digestion, 

 though the two are often confused in popular discussions of the 

 subject. 



Variations in the excretion of nitrogen have long been regarded as 

 indications of changes taking place in the body, and it has been a 

 general custom of physiologists to study the balance of income and 

 outgo of nitrogen. Such studies have formed a part of the nutrition 

 investigations of the Department. Much more useful as a means 

 of studying the food requirements of the body and other questions 

 are determinations of the balance of carbon, oxygen, and hydrogen, 

 as well as nitrogen, and determinations of the balance of income and 

 outgo of energ3^ iSuch studies necessitate special apparatus, and a 

 respiration calorimeter has been devised which is admirably adapted 

 to the purpose for which it is designed and which, it seems fair to 

 say, is so far the most perfect instrument of its type. ^The respiration 

 calorimeter is of such a size that a man may remain in comparative 

 comfort in the respiration chamber for a number of hours or days, 

 and the measurements of income and outgo of matter and energy 

 may be made mth great accuracy. The determination of energy 

 values of food and excretory products necessitates some special 

 apparatus for measuring the heat of combustion of these materials, 

 and m connection with the nutrition investigations a bomb calorimeter 

 has been perfected which has proved very satisfactory. 



