120 O. J. Malm, R. Nicolaysen and L. Skjelkvale 



turning positive. Anyhow, it seems reasonable in view 

 of the undoubtedly established adaptation of absorption in 

 rats, to assume that an increase of the power of absorption 

 is responsible for most of the adaptation of the absorption. 



Persons above fifty years of age show on an average no less 

 ability to adapt their absorption than younger persons. 

 The results of Bogonoff et al. (1953) in old men on very low 

 Ca diets strengthened this view. 



Urinary Ca and its adaptation 



The quantitative aspects have recently been discussed 

 (Nicolaysen et al., 1953), and the linearity within individuals 

 as well as the variability between individuals has been 

 stressed. 



In a few persons no reduction of the urinary output follows 

 halving of the intake, but in most persons some reduction 

 follows, and in some a substantial reduction is responsible 

 for quite a large part of the quick achievement of balance 

 following the reduction of Ca intake. 



However, when two consecutive periods on a low level of 

 intake are compared, the general impression is that absorption 

 improves in nearly all, whereas the urinary Ca level mostly 

 remains horizontal. 



Ca requirement 



The validity of the figures rests entirely upon the method 

 of study. The material here presented reveals the number of 

 weaknesses inherent in such a study of arbitrarily chosen 

 subjects of mostly unknown previous dietary habits and 

 status of nutrition as regards Ca and vitamin D. Judged 

 from the number of persons able to retain substantial amounts 

 of Ca it may be said that they did not represent a population 

 well nourished as regards nutrients essential to maximal 

 skeletal calcification. On the other hand such a contention 

 would lend support to the hypothesis that the limit of adapta- 

 tion may have been reached in some. 



