254 VITAMIN D GROUP 



termination of the requirements of any species for vitamin D is done under 

 conditions which exclude the actinic rays. 



The amount and ratio of calcium and phosphorus in the diet also in- 

 fluence the need of the animal for vitamin D. As discussed previously, it 

 is impossible to produce rickets in rats and other rodents which have been 

 studied unless the diet is low in either phosphorus or calcium. However, 

 even with larger mammals and fowls which develop rickets on diets con- 

 taining ample amounts of calcium and phosphorus, the demand for vitamin 

 D is increased if the ratio of calcium to phosphorus is far removed from 

 that required by the animal, and/or the calcium or phosphorus in the diet 

 is deficient. 



The availability of phosphorus (and also calcium) is another factor in 

 the reciuirements of animals for vitamin D. Most of the common ricketo- 

 genic diets contain approximately 0.3 to 0.4 % of phosphorus. If this were 

 all available, these diets would not produce rickets in rats, but they are 

 composed largely of cereals which contain considerable phytic acid. The 

 phosphorus of phytic acid is not available until the acid is hydrolyzed by 

 the phytase of the intestinal secretions. When a large amount of calcium 

 is added to the diet, the phytic acid is precipitated as the calcium salt. 

 The acid cannot be hydrolyzed, and the phosphorus cannot be made avail- 

 able. Mellanby^ discussed the chemistry of phytic acid and its salts and their 

 relation to rickets. If the diet contains inorganic phosphates, it is probable 

 that here again the high calcium makes some of the phosphorus unavailable 

 by precipitating it as an insoluble calcium phosphate. 



Still another factor which influences the reciuirements of animals for vi- 

 tamin D is the acidity of the diet. As the acidity is increased, the less rick- 

 etogenic the diet becomes.^ The effect, in all probability, is due to an in- 

 creased solubility of the calcium and phosphorus. Shohl et al.,^ however, 

 found little difference among acid, neutral, and alkaline phosphates in 

 their effects on rickets in rats. 



In the following discussion on the requirements of farm animals, it is 

 assumed that the animals are away from direct sunlight, but that the sup- 

 ply of calcium and phosphorus is adequate, and the ratio of these two ele- 

 ments to each other is not far from that demanded by the animals. 



Recently the Committee on Animal Nutrition of the National Research 

 Council has issued several reports giving the recommended nutrient allow- 



1 E. Mellanby, A Story of Nutritional Research. Williams and Wilkins Co., Balti- 

 more, 1950. 



2 T. F. Zucker, W. C. Johnson, and M. Barnett, Proc. Soc. Exptl. Biol. Med. 20, 

 20 (1922-1923). 



3 A. T. Shohl, H. B. Bennett, and K. L. Weed, J. Biol Chem. 78, 181 (1928). 



