STROM HM I\ MAN AND BKAST 



in milk wliich in Western civilizations is the main source of dietary calcium. 

 Recent work in England at the Agricultural Research Council Field Station 

 at Compton- is in line with jjreviously reported studies from the U.S.A. 

 In si.x days dairy cows excreted into the milk 0-17 to 3-H per cent (mean 

 0-67 per cent) of a single dose of ®®Sr administered with the feed: the excre- 

 tion fell ofT rapidly thereafter. The amount excreted seemed to be related 

 to the yield of milk and the time-course of excretion of ^"Sr was closely 

 parallel to that of ■'^Ca when the two nucleides were administered together. 



With some of this background, experiments on man were started some 

 years ago at the Medical Research Council Radiobiological Research Unit 

 by Harrison. At that time it was not felt to be justifiable to administer the 

 fission-products ®^Sr or ^"Sr to normal subjects, and it was our special aim 

 to investigate these products in the physiological state of man. (In the U.S.A. 

 ^•'Sr has been given to human patients in the terminal stages of malignant 

 disease. We felt, however, that the status of such individuals with regard to 

 calcium and its chemical analogues might well be atypical because of 

 wasting and confinement to bed.) Harrison et al.'^ therefore adopted the 

 policy of giving pharmacological doses of stable strontium to their volunteers 

 and by comparing the intake and output of this stable strontium (assayed 

 by neutron-activation-analysis), showed that the retention must have been 

 comparable with that of most laboratory experimental animals. The route 

 of excretion in man was identified as predominantly in urine for absorbed 

 strontium, although naturally much unabsorbed strontium passed through 

 the gut to appear in faeces. 



More recently with the y-ray emitting ^^Sr and the A.E.R.E.-whole-body- 

 y-counter^ both available, Harrison (personal communication) has felt 

 justified in giving two volunteers the small dose of 0-5 [xC intravenously. 

 Thus the excretion could be followed for a period of some months and the 

 retention calculated and compared with the observed retention as recorded 

 by the whole-body-counter. The results of this exercise are not yet satis- 

 factorily interpreted. Suffice it to say that they indicate the turnover of 

 strontium in man to be considerably faster than the somewhat pessimistic 

 value of some ten years as the biological half-life adopted by the Inter- 

 national Commission on Radiological Protection. 



Since in general the laboratory animal appears to be a not unrepresenta- 

 tive model for man, it is fair to consider the strontium-calcium relationships 

 so far determined and to discuss them. 



The Absorption and Excretion of Calcium 

 The mammal has remarkable powers for controlling its calcium metabolism 

 to maintain either balance in the adult or regulated growth in infancy and 

 adolescence. The complete understanding of this mechanism is far from 

 being realized. 



It is known, however, that the gut plays some part in the regulation. 

 Balance of calcium can be maintained in the adult on a wide range of diets 

 of varying composition. On diets with a marginal amount of calcium the 

 percentage absorption of calcium is high. With increasing excess over this 

 threshold the percentage absorption of calcium is reduced so that the abso- 

 lute amount absorbed does not run pari passu with the amount in the diet. 



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