Diagnostic Value of Phosphate Metabolism. 413 



If RI is positive the rat has rickets and if negative the rat is 

 normal. For example, if a rat increases in weight 2,000 mg. per 

 day, 2,000 X 0.0022 = 4.4. If its P retention is 4 mg. per day, 

 4 — 2=2 and RI = 4.4 — 2 =2.4, therefore, this rat has rick- 

 ets. If the P retention is 7 mg. RI = — 0.6 and this rat is normal 

 The magnitude of RI, if it is positive, denotes the severity of 

 rickets provided the metabolism has been determined over long 

 enough periods and with sufficient accuracy, but the skeleton 

 grows on a "maintenance" diet and this tends to disturb the 

 applicability of the formula. Rats usually retain ^ to ^ of the 

 P of the diet. Since a rat's body contains about 0.5 per cent. 

 P and the normal rate of growth is about 2 grams per day they 

 should retain about 10 mg. P per day or 5 mg. per gm. increase 

 in body weight, and this is found in normal rats of the age studied. 

 As pointed out by McCollum, the rate of growth influences the 

 degree of rickets. I found that during loss of weight, 0.12 per 

 cent. P in the diet prevented rickets whereas during normal growth 

 0.4 per cent, in the diet may be insufficient to prevent rickets. 

 Every growth promoting substance has a rachitic influence when 

 it promotes growth, therefore, every antirachitic substance that 

 promotes growth may have two opposing actions. In my rach- 

 itic rats RI varies from o to 4 and in my normal rats RI varies 

 from — 1 to — 5 due to storage of P in excess of the "minimum" 

 requirement. The use of RI in diagnosing rickets has the advan- 

 tage that it can be done without killing or injuring the animal. 



The Ca retention is low in rickets as well as in osteoporosis. 

 If the P intake is lowered the P retention is lowered and the Ca 

 retention is lowered (rickets). If the Ca intake is lowered the 

 Ca retention is lowered but the P retention remains normal (os- 

 teoporosis). This was found true for levels of P content of the 

 diet varying from 0.2 per cent, to 0.4 per cent. The Ca content 

 was reduced from 0.8 per cent, in the diet to 0.2 per cent, and the 

 Ca retention became less than \ the normal. At the end of a 

 month on this diet, the thickness of the bones and the thickness 

 of their walls was reduced. The total volume of the wall sub- 

 stance of the shafts of the long bones was reduced to 53.7 per 

 cent, of the normal. 



