Studies on unidentified growth factors

Abstract

1. Weanling rats were given diets containing all the nutrients known to be required, with L-amino acids in place of protein. Dietary supplements were added isonitrogenously.2. Torula yeast (50 g/kg diet) and dried brewers' yeast (50 g/kg) significantly improved the rats' growth rate. Part of this activity was attributed to the minerals present in the yeasts, zinc, iodine, iron, copper and manganese all being implicated.3. Marmite (20 g/kg) and a basic fraction (6.6 g/kg) obtained from it by cation-exchange chromatography significantly improved growth. The ash component of the basic fraction was inactive.4. Growth was increased by an extra supplement of vitamins and by treating the rats with neomycin sulphate and tetracycline. The growth stimulus due to Marmite was additive with that due to the antibiotics, but not with that due to the vitamin supplement.5. The growth rate with torula yeast (50 g/kg) was maintained at about 30% greater than on the basal diet during a test lasting 80 d. At the end of that period six control and five supplemented male rats sired normal litters. Post-mortem examination, including histology, of others showed no abnormalities due to factor G deficiency.6. Fresh ox liver (100 g/kg diet) significantly improved the growth rate, but this improvement could be attributed to the effects of the water and minerals contained in the liver.7. These results confirm the discovery by Schwarz, Smith & Oda (1966) that yeasts contain an organic growth-promoting factor (factor G) for rats receiving amino acid diets. Factor G is not likely to be one of the vitamin B group.