The Comparative Value of Different Food Proteins for Reproduction and Lactation in the Rat

Abstract

In connection with the study of the relative value of beef muscle, liver and kidney for reproduction and lactation in the rat, a similar study was also made of milk and egg and the results compared with those secured on the three meats. As in the case of the meats, the egg was fed in both cooked and raw forms. Milk, however, was fed only in dried form. The rations were planned to contain 15 and 20 per cent protein, and various supplements were used depending on the particular food constituent for which the test was being conducted. The percentage of fat was adjusted, when necessary, by the use of lard, but lard was omitted after its destructive effect on vitamin E was noted. In the later rations, the destructive effect of cod liver oil was prevented by adding the oil daily or omitting it entirely. The results secured from feeding rations, containing 15 and 20 per cent of protein with 20 per cent of fat, indicated that egg is superior to milk for both reproduction and lactation. When a special salt mixture containing some of the rarer minerals was used in place of the Osborne and Mendel mixture, reproduction on milk was greatly improved, indicating that the difference in results on milk and egg rations containing the Osborne and Mendel salt mixture was due partly to a difference in ash content. Lactation on both the milk and egg rations was improved by the use of the special salt mixture. While it is impossible to make any definite statement as to the relative value of milk and egg proteins as compared to those of liver and kidney, it appears that those of dried egg and kidney are superior to the other two. Muscle protein is of lower value than that of milk, egg or the glandular meats. When the difference in the quality of the proteins was largely ruled out by feeding them at a 20 per cent level, the vitamin E content of the five dried products was found to decrease in the following order; egg, milk, round, liver, kidney. Even skim milk contains considerable vitamin E in the absence of destructive agencies such as lard and cod liver oil. The statement of Sure (1926) that more vitamin E is needed for lactation than for reproduction was confirmed. Success in lactation on the different rations was found to be especially dependent on their content of vitamin B(B₁). An alcoholic extract of ether-extracted wheat germ, which is rich in this vitamin, caused an improvement in lactation on both milk and beef round rations. Additional wheat germ oil and yeast also caused improvement in many cases. The superiority of dried egg over the other protein foods for lactation was no doubt partly due to its content of vitamin E. In fact, lactation on all of the rations would probably have been improved by an additional source of this vitamin. When fed at a 20 per cent protein level and adequately supplemented with minerals and vitamins, it is probable that all of the cooked and dried protein foods tested would be adequate for both reproduction and lactation. Of the five, egg would seem to require the fewest supplements. In the raw state, kidney appears to be the most adequate. Raw egg, like raw beef round, is poorly assimilated.