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

Abstract

In this comparative study, beef round, liver and kidney, in both cooked and raw forms, were fed to rats in order to determine the relative efficiency of these foods as sources of protein and accessories, primarily for reproduction and lactation. The rations contained sufficient amounts of the meats being tested to supply 15 and 20 per cent protein. Various supplements were used, depending on the particular food constituent being tested for. Variations in the proportions of protein and fat in the ration involved changes in the amount of lard used as a source of added fat. Rations in which lard was omitted entirely, proved better for reproduction than those which contained even a small amount. Cod liver oil was also found to have a detrimental effect on reproduction when mixed into large quantities of the rations. Therefore, in all of the later rations this oil was either added daily to small amounts of the rations or omitted entirely and partly replaced by radiation. When no supplements of vitamin E were made, the meats themselves being relied upon as a source of this vitamin, the rations, containing only 15 per cent of protein with lard and cod liver oil, thus contained two variable factors since these added fats had a destructive effect on the vitamin. This made the interpretation of results as to the relative value of the proteins, as such, rather difficult. When, however, vitamin E was supplied in adequate amounts, the superiority of kidney protein seemed to be clearly indicated. Liver protein ranked next, and muscle last. 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 dried meats was found to decrease in the following order: Round, liver, kidney. It seems probable that 5 per cent of yeast does not provide enough vitamin B (or B₁) for highly successful lactation in those rations containing dried meats. This was indicated by the fact that lactation on ration 120 (muscle) was distinctly improved by addition of the alcoholic extract of ether-extracted wheat embryo, which is rich in vitamin B (or B₁). Additional yeast, together with wheat germ oil, also caused improvement in lactation on rations 136 (liver) and 146 (kidney). The superiority of liver over the other meats for this function was no doubt due to its content of vitamin B, particularly the heat-labile fraction. It is thus difficult to say which of the three meats is best for reproduction and lactation, unless it is specified in what form and amount the food is to be fed, and with what supplements. If fed in the dried form at a 20 per cent protein level in a ration containing no substances which would have a destructive effect on vitamin E, beef round probably requires supplements of minerals and vitamins A, B, and D. Beef liver and kidney probably require in addition a supplement of vitamin E. In the raw state beef round is not easily assimilated by rats, but liver and kidney appear to have a higher food value when raw than when cooked. It is not known whether cooking has any detrimental effect on the proteins of meat, but a large part of the vitamin B (antineuritic) must be destroyed. Of the raw meats, kidney would seem to require the fewest supplements for successful reproduction and lactation. The author wishes to acknowledge her indebtedness to Professor Henry A. Mattill for helpful advice throughout the work.