Effect of Small Quantities of a Yeast Preparation on the Recovery of Appetite in Sheep

In his classical experiments on accessory food factors, Hopkins (1912) showed that not only milk but also protein-free and salt-free extracts of yeast contain 'organic complexes' without which rats failed to grow and often died, if fed on mixtures of purified casein, fat, carbohydrate and salts.Since then it has been established that yeast is a rich source of the members of the vitamin B complex (for a recent review see Van Lanen & Tanner, 1948), and as such it has been used not only as a therapeutic agent, but also as a starting material for the detection and isolation of new members of the vitamin B complex.Furthermore, the value of yeast as the sole source of protein in the diet of rats was demonstrated by Osborne & Mendel (1919); later Macrae, El-Sadr & Sellers (1942) confirmed the high biological value of yeast protein for rats and showed that a small addition of dried yeast to a diet containing only maize protein greatly increased the value of the latter in the rearing of pigs.Ritzman (1945) found yeast protein highly digestible for cows.However, the experiments of Braude, Kon & White (1943, 1944) on the rachitogenic effect of rations containing a relatively high proportion of yeast, and on the adverse effect of yeast on the storage of vitamin A in the liver of pigs, suggest that as a major dietary supplement for farm animals yeast should be used with caution.These, and other aspects of the nutritive value of food yeast, have been discussed in an excellent survey prepared by the Accessory Food Factors Committee 191z), or of some other minor constituent of the diet, which directly, or indirectly through microbial activities in the rumen, might affect the appetite of the animal.T o test this hypothesis a yeast preparation was made, which, on introduction into the rumen of the sheep, without otherwise changing the diet, led not only to the recovery