Vitamin B12 Deficiency in the Rat Fed High Protein Rations

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Abstract
Experiments were conducted in an effort to ascertain the reason for the decreased growth response obtained in weanling rats fed vitamin B12-deficient rations as the protein level was raised from 25% to 65% with vitamin B12-deficient casein. The intensified growth depression with the higher protein levels appeared related to the catabolism of the excess protein but did not seem to involve impaired protein digestion or difficulty in handling the end products of nitrogen metabolism. Rather, the vitamin appeared to be concerned in the metabolic disposal of the carbon skeletons of some or all of the amino acids comprising the added protein. As a result of tests in which the individual amino acids were added singly to the 25% vitamin B12-deficient ration in amounts equivalent to those in the 40% added casein, the amino acids involved were identified as valine, isoleucine, threonine and serine. proline tended to have a counteracting effect. The amino acids related to methylmalonate metabolism — valine, isoleucine, and, to a large extent, threonine — apparently contributed to the growth depression on the high protein ration to a much greater extent, relatively, than the one related to formate metabolism — serine.