Effect of Variable Intake on Body Composition 131 



commonly realized. The shortcomings of the balance method 

 are functions of such items as the effects of variable caloric 

 intake, quality and quantity of protein intake and mineral 

 ratios on the fat content of the body, the distribution of body 

 water and the relative size of body components such as 

 skeleton and muscle. These problems have been most com- 

 pletely explored in relation to evaluation of the problem of 

 protein adequacy (Mitchell, 1944; Allison, 1954; Calloway and 

 Spector, 1953; Spector and Calloway, 1953). It is also little 

 appreciated that systematic errors occur in the calculation of 

 apparent retentions that are cumulative in a positive direc- 

 tion, the magnitude of the cumulative error being in direct 

 proportion to the magnitude of the intake. This makes 

 difficult the comparison of retentions at variable intakes. The 

 relevance of this criticism with regard to calcium retentions 

 has been discussed by Mitchell and Curzon (1939) and by 

 Mitchell and co-workers (1945). 



Examination of the composition of growth increments by 

 direct body analysis has shown that, once chemical maturity 

 is reached, the composition of the fat-free body with regard 

 to protein and ash is nearly constant, regardless of any pro- 

 cedures taken to modify weight gain (Moulton, 1923; Moulton, 

 Trowbridge and Haigh, 1922; Pickens, Anderson and Smith, 

 1940). As determined by direct body analysis the body com- 

 position of rats growing on mineral-poor diets shows little 

 change except for a deficit of calcium (Light et at., 1934). 



The concept of variable cellular composition of the body is 

 difficult to reconcile with the knowledge of the composition 

 of tissues. All of the individual tissues of the albino rat have 

 been analysed for their water, protein, fat and mineral con- 

 tent by many investigators. All of these data show a mono- 

 tonous constancy when calculated on a fat-free basis. This 

 occurs despite almost infinite variation in the rations fed to 

 the animals. Unless special experimental conditions are 

 imposed, individual tissues seem to hold fast to their chemical 

 composition. The principle variation occurs with age (Lowry 

 et al., 1942). At any given age composition is constant. Even 



