Abstract
Whether or not rickets develops on standard “rickets-producing” dietaries may depend on the stock of rats which is used. In our experiments rats from six different sources were tested. It was found that four of these stocks regularly developed rickets when fed from the age of four to eight weeks on the Sherman-Pappenheimer diet; one stock developed rickets but showed a definite tendency to spontaneous calcification of the bones, and one absolutely failed to develop rickets. This refractory group comprised 50 rats four weeks of age. They failed to develop rickets either on the low phosphorus and high calcium diet (No. 84) or on the low calcium and high phosphorus diet (No. 85 C). On the dietary employed by McCollum and his associates, which contains 3 per cent. of calcium carbonate, rachitic lesions developed in some animals but not in others. It is evident, therefore, that the term “rickets-producing dietary” cannot be applied unreservedly but rather in relation to definite stocks of animals.
The divergence in susceptibility is, to some extent, associated with a variability in the percentage of inorganic phosphate of the blood. There is, however, no strict parallelism in this respect. The blood of rats four weeks of age has been found to vary in this constituent from 6.5 mg. to 12.0 mg. per cent.; the calcium has ranged from 6.1 mg. to 8.2 mg. per cent. The refractory rats had the highest percentage of inorganic phosphate in the blood.
The resistance of this stock is not to be attributed mainly to peculiarity of strain or breed hut to previous diet, for it was overcome by modification of the dietary. Pregnant rats of this strain were obtained and fed the stock laboratory dietary as soon as they had given birth to young and throughout the lactating period.
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